<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE language SYSTEM "klayout_doc.dtd">

<doc>

  <title>The Application API</title>

  <h2-index/>

  <p>
  This section covers the basic application API. The application API consists of the main application class
  and several classes that represent the user interface. This sections presents a selection of classes 
  that make up the application API. These classes provide the main entry points into the application API. 
  Further classes are documented in <link href="/code/index.xml">RBA Class Index</link>.
  </p>

  <p>
  All classes discussed herein are contained in the RBA namespace. In you code you either have to use
  qualified names (i.e. RBA::Application) or include the RBA module in you macro's namespace.
  </p>

  <h2>The Application class</h2>
  <keyword name="Application"/>

  <p>
  The Application class is documented in detail in <class_doc href="Application"/>. It represents the application
  and because there is just one application, there also is just one instance of the application object.
  That instance can be obtained through the "instance" class method: 
  </p>

  <pre>Application::instance</pre>

  <p>The application object is the main entry point into the API. It offers several methods and attributes. In particular:</p>

  <ul>
    <li><class_doc href="Application#application_data_path"/>: returns the user-local storage path. This is where KLayout saves
      user-specific files, for example the configuration file.
    </li>
    <li><class_doc href="Application#execute"/>: runs the application. Normally, this method is called implicitly when the application
      is started. It is possible to use KLayout as a Ruby interpreter by supplying a Ruby script on the command
      line with the "-r" option. Such scripts must run the application explicitly if they want to.
    </li>
    <li><class_doc href="Application#exit"/>: exits the application. This method unconditionally terminates the application in a clean
      way.
    </li>
    <li><class_doc href="Application#get_config"/> and <class_doc href="Application#set_config"/>: read and write the configuration database. 
      The configuration database
      is a storage of name/value pairs which is stored in the configuration file. These methods can be used
      to manipulate that storage. Use the <class_doc href="Application#get_config_names"/> method to retrieve the names of the configuration
      parameters stored inside the configuration database. Use the <class_doc href="Application#commit_config"/> method to activate 
      settings that have been made with "set_config".
    </li>
    <li><class_doc href="Application#inst_path"/>: returns the installation path. That is where the executable is located.
    </li>
    <li><class_doc href="Application#is_editable?"/>: returns true, if KLayout runs in editable mode.
    </li>
    <li><class_doc href="Application#klayout_path"/>: returns the KLAYOUT_PATH value. This is the search path where 
      KLayout looks for library files or macros. This method delivers the application data path and 
      can be used to look up files required by the macro.
    </li>
    <li><class_doc href="Application#main_window"/>: delivers the MainWindow object which represents the application's main window.
      See below for a description of that class.
    </li>
    <li><class_doc href="Application#process_events"/>: process pending events. If that method is called periodically during 
      long operations, the application will be able to process events and thus handle clicks on a "Stop"
      button for example. Please note that calling this method is not safe in every context, because
      not every execution context is reentrant.
    </li>
    <li><class_doc href="Application#read_config"/> and <class_doc href="Application#write_config"/>: reads
      and writes the configuration database from a file.
    </li>
    <li><class_doc href="Application#version"/>: delivers KLayout's version string. This string can be used to switch 
      the implementation of a script depending on KLayout's version.
    </li>
  </ul>

  <h2>The MainWindow class</h2>
  <keyword name="MainWindow"/>

  <p>
  The MainWindow class is documented in detail in <class_doc href="MainWindow"/>. It represents the main application
  window. The main window instance can be obtained with:
  </p>

  <pre>Application::instance.main_window</pre>

  <p>
  The main window object is the entry point to all user-interface related objects. 
  It offers a couple of methods. In particular:
  </p>

  <ul>
    <li><class_doc href="MainWindow#cancel"/>: cancels any pending operation (i.e. dragging of an object in move mode) and resets the
      mode to the default mode (Select). Use this method to establish a known user interface state.</li>
    <li><class_doc href="MainWindow#close_all"/> and <class_doc href="MainWindow#close_current_view"/>: close all or the current tab.</li>
    <li><u>MainWindow#cm_*</u>: these are methods which are bound to the menu items in the menu bar. They can be used 
      to trigger a menu function from a script.</li>
    <li><class_doc href="MainWindow#create_layout"/>: create a new layout and load it into a layout view. This method has a parameter
      that controls whether the layout is shown in a new tab, replaces the layout in the current tab or adds to
      the current tab.</li>
    <li><class_doc href="MainWindow#create_layout"/>: creates a new, empty layout and loads it.</li>
    <li><class_doc href="MainWindow#create_view"/>: creates a new, empty tab.</li>
    <li><class_doc href="MainWindow#current_view"/>: returns a LayoutView object (see below) which represents the current tab.</li>
    <li><class_doc href="MainWindow#current_view_index"/>: returns the index of the current tab. Some methods like "select_view" operate
      with view indexes. The view index is the number of the tab (0 is the leftmost one). The "view" method
      allows obtaining the LayoutView object from a view index. The same way <class_doc href="MainWindow#current_view_index="/> 
      selects the view with the index given in this call.</li>
    <li><class_doc href="MainWindow#grid_micron"/>: gets the global grid in micrometer units.</li>
    <li><class_doc href="MainWindow#initial_technology"/> and <class_doc href="MainWindow#initial_technology="/>: gets or sets the name of the technology to use to new layouts
      or for layouts loaded without an explicit technology specification.</li>
    <li><class_doc href="MainWindow#load_layout"/>: loads a layout into a tab. There are various variants of this method, offering 
      various levels of configuration. All these variants have a parameter
      that controls whether the layout is shown in a new tab, replaces the layout in the current tab or adds to
      the current tab.</li>
    <li><class_doc href="MainWindow#menu"/>: provides access to the menu object of class AbstractMenu (see <class_doc href="AbstractMenu"/>). 
      This object provides access to the main menu, the toolbar and various context menus. With this object it 
      is possible to manipulate the menu.</li>
    <li><class_doc href="MainWindow#message"/>: show a message in the status bar.</li>
    <li><class_doc href="MainWindow#reader_options"/>: gets the current layout reader options and allows modification of the latter. 
      The reader options configure the reading of layouts.</li>
    <li><class_doc href="MainWindow#save_session"/> and <class_doc href="MainWindow#restore_session"/>: save or restore a session. 
      Sessions contain a window settings and information about the layouts loaded. 
      Sessions allows storing and restoring of the state of the main window.</li>
    <li><class_doc href="MainWindow#select_view"/>: switches to the given tab. This equivalent to using <class_doc href="MainWindow#current_view_index="/>.</li>
    <li><class_doc href="MainWindow#view"/>: gets the <class_doc href="LayoutView"/> object for a given tab index.</li>
    <li><class_doc href="MainWindow#views"/>: gets the number of tabs.</li>
  </ul>

  <p>
  The MainWindow supplies three events.  
  See <link href="/programming/events.xml"/> for details about events. 
  These are the events:
  </p>

  <ul>
    <li><b>on_current_view_changed</b>: 
    This event is triggered when the current tab changes. The signal is available with an integer parameter: this is the index of the 
    previous tab. The new tab is already the current tab when this event is triggered.
    </li>
    <li><b>on_view_created</b>: 
    This event is triggered when a new view is created. The signal is available with an integer parameter: this is the index of the 
    new tab.
    </li>
    <li><b>on_view_closed</b>: 
    This event is triggered when a new view is closed. The signal is available with an integer parameter: this is the index of the 
    tab that was closed.
    </li>
  </ul>

  <p>
  In addition, the MainWindow class features many parameterless methods starting with "cm_...". These methods are identical with 
  the methods called when the respective menu functions are triggered. They are of use when menu events need to be emulated 
  in code, for example to implement special key bindings.
  </p>

  <h2>The LayoutView class</h2>
  <keyword name="LayoutView"/>

  <p>
  The LayoutView class is documented in detail in <class_doc href="LayoutView"/>. It represents one layout tab in the 
  main window. A single layout view can show multiple layouts. The <class_doc href="CellView"/> objects represent one
  layout loaded into a view. They specific the layout loaded plus the cell selected for drawing.
  Each LayoutView has a list of CellView objects corresponding to the layouts shown in the same panel.
  </p>

  <p>
  A LayoutView object can be obtained from the main window either by reading the current view or by getting
  the view object for a tab by the tab index:
  </p>

  <pre># Current view:
Application::instance.main_window.current_view
# or short:
LayoutView::current

# By index:
Application::instance.main_window.view(index)

# Note: the index of the current view is
Application::instance.main_window.current_view_index
# and the number of views is
Application::instance.main_window.views</pre>

  <p>
  The index is 0 for the first tab. Note that the value returned by <class_doc href="LayoutView#current"/> 
  or <class_doc href="MainWindow#current_view"/> can be "nil" if no layout is shown.
  </p>

  <p>
  A layout view is the container for a variety of "visual" objects. These resources are mainly display objects like 
  annotations, markers and images. In addition, the report database system is anchored in the LayoutView object.
  The following resources are managed in the layout view:
  </p>

  <ul>
    <li><b>Annotations (rulers)</b>: an arbitrary number of annotations can be registered in the view. 
    The annotations are objects of the Annotation class (<class_doc href="Annotation"/>). Annotations
    are independent of layouts and are defined in micron units. 
    </li>
    <li><b>Images</b>: images are also objects independent of layouts. Any number of images can be 
    placed below the drawn layout.
    </li>
    <li><b>Markers</b>: markers are temporary overlay objects which can be used a highlights or to 
    add some elaborate annotation to a layout view. Markers can be layout database objects, hence
    it is possible to draw polygons or other objects over the layout. Markers can be configured
    to a large degree, so different colors can be used for example. Markers are objects of class
    Marker (<class_doc href="Marker"/>).
    </li>
    <li><b>Local configuration</b>: be default, the layout view pulls it's configuration from the global
    configuration database. It is possible however to override certain configuration parameters for a 
    particular view. This allows for example to set the background color for a particular view without
    affecting the other views.
    </li>
    <li><b>Layer properties</b>: the layer properties tree is also managed by the layout view. 
    Since there can be multiple layer properties trees in different tabs in the layer properties panel, 
    there are method to access either the current or a specific one of the layer properties trees.
    </li>
    <li><b>Custom stipples and line pattern</b>: custom stipplesu and line styles can be set in the 
    layout view and used in the layer properties. Custom stipples are bitmaps that define the fill 
    pattern used for the inside area of a polygon. Line styles are bit pattern that make lines being
    resolved into dots.
    </li>
    <li><b>Selection</b>: the layout view also manages the selection. This is a set of objects and their
    instantiation path in the layout database. It represents the set of selected objects. Each
    selected object is described by a instantiation path and the object itself. That information is
    combined in the ObjectInstPath object (<class_doc href="ObjectInstPath"/>).
    </li>
    <li><b>Transient selection</b>: this is the object that is highlighted briefly when the mouse
    hovers over it.
    </li>
    <li><b>Cell views</b>: the list of layouts and cells shown in the layout view as overlays. Cell views
    are created when layouts are loaded and deleted when layouts are closed.
    One of the cell views is the "active" one. That is the one which is selected in the drop-down box
    in the cell tree and for which the cell tree is shown.</li>
    <li><b>Cell visibility</b>: the information about what cell is visible and what cell is not. Each
    cell can be made invisible. In that case, only the cell frame is drawn and the cell is shown 
    stroked out in the cell tree.
    </li>
    <li><b>Hierarchy levels</b>: this attribute controls which hierarchy levels are shown by default.</li>
    <li><b>Viewport</b>: the geometrical dimensions of the area which is drawn in micron space.</li>
    <li><b>Report databases</b>: a layout view can have multiple report databases attached to it.
    Report databases can be shown in the marker browser and are collections of general information
    or geometrical information related to a certain position or area.</li>
    <li><b>Transactions</b>: transactions are grouped layout operations which form an atomic operation which 
    can be undone. Transactions can be created within the layout view. Transactions must be opened and closed
    before they are available as operations on the undo stack.</li>
    <li><b>Plugins</b>: plugins are a way to implement new functionality inside the layout view related
    to mouse actions. By using plugins it is possible to track the mouse and implement actions related
    to mouse activity.</li>
    <li><b>Title</b>: finally, a layout view has a title which is shown in the tab.</li>
  </ul>

  <p>
  Being the central class, the layout view naturally offers many methods and attributes. 
  Here's a brief explanation of some of these methods:
  </p>

  <ul>
    <li><class_doc href="LayoutView#active_cellview"/> and <class_doc href="LayoutView#active_cellview_index"/>: 
    gets the active CellView object or index of this object. The active cell view is the one that
    is selected in the drop-down box above the cell tree. <class_doc href="CellView#active"/> gets the active cellview
    of the current layout view.
    </li>
    <li><class_doc href="LayoutView#add_missing_layers"/> will add the layers to the layer tree for which there is no 
    layer properties entry yet. This method can be used after a layout has been created and populated to show all
    layers of the layout.
    </li>
    <li><class_doc href="LayoutView#add_stipple"/>, <class_doc href="LayoutView#clear_stipples"/> and <class_doc href="LayoutView#remove_stipple"/>: 
    manage custom stipples.
    </li>
    <li><class_doc href="LayoutView#add_line_style"/>, <class_doc href="LayoutView#clear_line_styles"/> and <class_doc href="LayoutView#remove_line_styles"/>: 
    manage custom line styles.
    </li>
    <li><class_doc href="LayoutView#ascend"/> and <class_doc href="LayoutView#descend"/>: moves the context cell up or down in the 
    hierarchy.
    </li>
    <li><class_doc href="LayoutView#begin_layers"/> and <class_doc href="LayoutView#end_layers"/>: 
    gets an start or end iterator object that allows traversing of the layer properties tree
    in a recursive or non-recursive fashion.
    See below for a description of how to deal with the layer properties.
    </li>
    <li><class_doc href="LayoutView#each_layer"/> is a convenient alternative way of iterating over the layers
    without directly using the layer tree iterator.
    </li>
    <li><class_doc href="LayoutView#box"/>: 
    gets the display area in micron units (the viewport).
    </li>
    <li><class_doc href="LayoutView#cancel"/>: 
    returns the view into idle state (nothing selected, no editing in progress, "Select" mode is active).
    </li>
    <li><class_doc href="LayoutView#cellview"/>: 
    gets the CellView object for a given index.
    </li>
    <li><class_doc href="LayoutView#cellviews"/>: 
    gets the number of cell views registered.
    </li>
    <li><class_doc href="LayoutView#clear_annotations"/>, <class_doc href="LayoutView#insert_annotation"/>, <class_doc href="LayoutView#erase_annotation"/> and <class_doc href="LayoutView#replace_annotation"/>: 
    manage annotations (rulers).
    </li>
    <li><class_doc href="LayoutView#clear_images"/>, <class_doc href="LayoutView#insert_image"/>, <class_doc href="LayoutView#erase_image"/> and <class_doc href="LayoutView#replace_image"/>: 
    manage images.
    </li>
    <li><class_doc href="LayoutView#clear_config"/>, <class_doc href="LayoutView#get_config"/>, <class_doc href="LayoutView#set_config"/> and <class_doc href="LayoutView#commit_config"/>: 
    allow manipulation of the configuration for that layout view
    only. For example it is possible to set a different background color for that specific layout view.
    </li>
    <li><class_doc href="LayoutView#clear_layers"/>, <class_doc href="LayoutView#delete_layer"/>, <class_doc href="LayoutView#delete_layer_list"/>, 
        <class_doc href="LayoutView#insert_layer"/>, <class_doc href="LayoutView#insert_layer_list"/>, <class_doc href="LayoutView#replace_layer_node"/>
        and <class_doc href="LayoutView#remove_unused_layers"/>: 
    manage the layer properties. See below for a detailed explanation.
    </li>
    <li><class_doc href="LayoutView#clear_transactions"/>: 
    clears all transactions (clears the undo stack).
    </li>
    <li><class_doc href="LayoutView#transaction"/> and <class_doc href="LayoutView#commit"/>: 
    starts or ends a transaction. All operations between the 
    start and end of a transaction can be undone in one step.
    </li>
    <li><class_doc href="LayoutView#clear_object_selection"/>: 
    clears the selection of geometrical objects (shapes or cell instances).
    </li>
    <li><class_doc href="LayoutView#create_layout"/> and <class_doc href="LayoutView#load_layout"/>: 
    creates a new layout or loads a layout. In both cases, you can either
    replace the current layouts or add the new one to the layouts present.
    </li>
    <li><class_doc href="LayoutView#current_layer"/>: 
    returns an iterator pointing to the current layer (the one that has the focus 
    frame in the layer tree.
    </li>
    <li><class_doc href="LayoutView#current_layer_list"/>: 
    returns the index of the current layer list (if multiple tabs are present
    in the layer control panel, the current layer list is the tab that is selected).</li>
    <li><class_doc href="LayoutView#each_annotation"/> and <class_doc href="LayoutView#each_annotation_selected"/>: 
    delivers all or the selected annotations.
    </li>
    <li><class_doc href="LayoutView#each_image"/> and <class_doc href="LayoutView#each_image_selected"/>: 
    delivers all or the selected images.
    </li>
    <li><class_doc href="LayoutView#each_object_selected"/> and <class_doc href="LayoutView#each_object_selected_transient"/>: 
    delivers ObjectInstPath objects (<class_doc href="ObjectInstPath"/>) 
    that point to one selected object each.
    </li>
    <li><class_doc href="LayoutView#erase_cellview"/>: 
    close a cell view, i.e. remove that specific layout from the list of loaded layouts.</li>
    <li><class_doc href="LayoutView#enable_edits"/>: Enables or disables editing. This method will enable or disable all editing
    features. This is intended for temporarily disallowing edits. This is not the same than edit and viewer mode.
    </li>
    <li><class_doc href="LayoutView#get_image"/>, <class_doc href="LayoutView#get_image_with_options"/> and <class_doc href="LayoutView#get_screenshot"/>: 
    dumps the screen content into a QImage with or without a specific resolution.</li>
    <li><class_doc href="LayoutView#init_layer_properties"/>: provides an initialization of a "LayerProperties" object for a new layer
    according to the current settings of the view.
    </li>
    <li><class_doc href="LayoutView#is_cell_hidden?"/>, <class_doc href="LayoutView#hide_cell"/>, <class_doc href="LayoutView#show_cell"/> and  <class_doc href="LayoutView#show_all_cells"/>: 
    manages cell visibility.
    </li>
    <li><class_doc href="LayoutView#load_layer_props"/> and <class_doc href="LayoutView#save_layer_props"/>: 
    loads or saves layer properties files.
    </li>
    <li><class_doc href="LayoutView#max_hier"/>, <class_doc href="LayoutView#max_hier_levels="/> and <class_doc href="LayoutView#min_hier_levels="/>: 
    manages hierarchy levels shown.
    </li>
    <li><class_doc href="LayoutView#object_selection"/>, <class_doc href="LayoutView#object_selection="/>, <class_doc href="LayoutView#select_object"/> and <class_doc href="LayoutView#clear_object_selection"/>:
    return or manipulate the selection of geometrical objects (shapes, instances). The key descriptor object for that purpose
    is <class_doc href="ObjectInstPath"/>, which refers to a geometrical object throug an instantiation path.
    </li>
    <li><class_doc href="LayoutView#pan_center"/> (and other pan... methods), <class_doc href="LayoutView#zoom_box"/> (and other zoom... methods): 
    changes the viewport.
    </li>
    <li><class_doc href="LayoutView#reload_layout"/>: 
    reloads a given layout.
    </li>
    <li><class_doc href="LayoutView#create_rdb"/>, <class_doc href="LayoutView#delete_rdb"/>, <class_doc href="LayoutView#rdb"/>
      and <class_doc href="LayoutView#num_rdbs"/>: Create, delete and get report databases stored inside the LayoutView object.
    </li>
    <li><class_doc href="LayoutView#rename_cellview"/>: 
    changes the name of a cellview.
    </li>
    <li><class_doc href="LayoutView#title"/>, <class_doc href="LayoutView#title="/> and <class_doc href="LayoutView#reset_title"/>: 
    sets or resets the layout view's title.
    </li>
    <li><class_doc href="LayoutView#save_as"/>: Saves a layout to a file (with options).</li>
    <li><class_doc href="LayoutView#show_image"/>: 
    shows or hides an image.
    </li>
    <li><class_doc href="LayoutView#viewport_width"/>, <class_doc href="LayoutView#viewport_height"/> and <class_doc href="LayoutView#viewport_trans"/>:
    gets the viewport parameters.
    </li>
  </ul>

  <h3>Implementing Undo/Redo</h3>

  <p>Undo/Redo functionality is implemented by using "transactions". Transactions are 
  groups of operations which implement one user operation. Transactions are built internally
  and automatically once a transaction is initiated. Most operations performed in the framework
  of the LayoutView and Layout objects are tracked within these transactions. When a transacting
  is finished, it needs to be committed. After that, a new operation will be available for
  "Undo" or "Redo".
  </p>

  <p>
  Transactions can be initiated with <link href="LayoutView#transaction"/> and committed with <link href="LayoutView#commit"/>.
  To ensure, every initiation of a transaction is matched by a "commit", it is recommended to employ "ensure":
  </p>

  <pre>begin

  view.transaction("Some operation")

  ... do your thing here ...
  
ensure
  view.commit
end
</pre>

  <h3>Manipulating the selection</h3>

  <p>
  The selection of geometrical objects can be manipulated by providing the necessary <class_doc href="ObjectInstPath"/> objects.
  Each such object provides a "pointer" to a shape or instance through the hierarchy. Specifically it lists all the 
  cells and their instantiation transformations down to the shape selected. By accumulating these selections, a shape can be
  addressed in a flat view, even if the shape is instantiated many levels down in the hierarchy.
  </p>

  <p>
  Generating such instantiation path objects is somewhat tedious, but usually the requirement is not 
  to generate such paths, but to take an existing selecting, manipulate it somehow and then to set the selection
  to the new one.
  This is fairly easy by taking a copy of the selection, manipulation of the shapes and setting the 
  manipulated selection as the new one.
  </p>

  <p>
  The following is a sample which replaces all shapes by their hull polygons. Note that is provides
  undo/redo support through a "transaction":
  </p>

  <pre>view = mw.current_view

begin

  view.transaction("Convert selected shapes to polygons")

  sel = view.object_selection

  sel.each do |s|
    if !s.is_cell_inst? &amp;&amp; !s.shape.is_text?
      ly = view.cellview(s.cv_index).layout
      # convert to polygon
      s.shape.polygon = s.shape.polygon
    end
  end
  
  view.object_selection = sel

ensure
  view.commit
end
</pre>

  <h3>Events</h3>

  <p>
  The LayoutView object supplies several events.  
  See <link href="/programming/events.xml"/> for details about events. 
  These are the events:
  </p>

  <ul>
    <li><b>on_active_cellview_changed</b>: 
    This event is triggered when the active cellview changes. The active cellview is the one indicated
    by the drop-down-box atop of the cell list if multiple layouts are loaded into one view.
    </li>
    <li><b>on_annotation_changed</b>: 
    This event is triggered if an annotation is changed. The ID of the annotation is sent along
    with the event.
    </li>
    <li><b>on_annotation_selection_changed</b>: 
    This event is triggered if the selection of annotations is changed.
    </li>
    <li><b>on_annotations_changed</b>: 
    This event is triggered if an annotation is added or deleted.
    </li>
    <li><b>on_cell_visibility_changed</b>: 
    This event is triggered when the visibility of a cell changes. The visibility of a cell is changed by using 
    "Hide Cell" or "Show Cell" from the cell tree's context menu.
    </li>
    <li><b>on_cellviews_changed</b>:
    This event is triggered when a new cellview is added or a cellview is removed.
    </li>
    <li><b>on_cellview_changed</b>:
    This event is triggered when a cellview changed (i.e. the current cell has been changed. The index of 
    the changed cell view is sent along with the event.
    </li>
    <li><b>on_close</b>:
    This event is triggered when a cell view is closed.
    </li>
    <li><b>on_file_open</b>:
    This event is triggered when a file is loaded.
    </li>
    <li><b>on_hide</b>:
    This event is triggered when a cell view is going to become invisible (i.e the tab changed).
    </li>
    <li><b>on_current_layer_list_changed</b>:
    This event is triggered when the current layer list was changed (i.e. the tab in the layer list
    has been changed).
    </li>
    <li><b>on_image_changed</b>:
    This event is triggered when an image was edited. The ID of the image is sent along with the event.
    </li>
    <li><b>on_image_selection_changed</b>:
    This event is triggered when an image was selected or unselected.
    </li>
    <li><b>on_images_changed</b>:
    This event is triggered when an image was added or deleted.
    </li>
    <li><b>on_layer_list_changed</b>:
    This event is triggered if a layer was changed, added or deleted.
    </li>
    <li><b>on_layer_list_deleted</b>:
    This event is triggered if a layer list was deleted (i.e. a tab was removed).
    </li>
    <li><b>on_layer_list_inserted</b>:
    This event is triggered if a layer list was inserted (i.e. a tab was added).
    </li>
    <li><b>on_rdb_list_changed</b>:
    This event is triggered when a report database is opened or removed.
    </li>
    <li><b>on_selection_changed</b>:
    This event is triggered when the selection has changed.
    </li>
    <li><b>on_show</b>:
    This event is triggered when a cell view is going to become visible (i.e the tab changed).
    </li>
    <li><b>on_transient_selection_changed</b>:
    This event is triggered when the transient selection has changed.
    </li>
    <li><b>on_viewport_changed</b>:
    This event is triggered when the viewport has changed, for example the view is zoomed in or panned.
    </li>
  </ul>

  <h3>Working with layer properties</h3>

  <p>
  The API provides methods by which the layer properties list of the layout view can be traversed and manipulated in many ways.
  In particular:
  </p>

  <ul>
    <li>Add or remove entries to or from the layer properties list: <class_doc href="LayoutView#insert_layer"/> and <class_doc href="LayoutView#delete_layer"/></li>
    <li>Clear all entries: <class_doc href="LayoutView#clear_layers"/></li>
    <li>Manage custom stipple pattern: <class_doc href="LayoutView#add_stipple"/>, <class_doc href="LayoutView#clear_stipples"/> and <class_doc href="LayoutView#remove_stipple"/></li>
    <li>Add, rename or remove tabs, get or change the current tab: <class_doc href="LayoutView#insert_layer_list"/>, <class_doc href="LayoutView#rename_layer_list"/>, <class_doc href="LayoutView#delete_layer_list"/>, <class_doc href="LayoutView#current_layer_list"/> and <class_doc href="LayoutView#set_current_layer_list"/></li>
    <li>Remove unused layers from the layer list or add entries for missing layers: <class_doc href="LayoutView#remove_unused_layers"/>, <class_doc href="LayoutView#add_missing_layers"/></li>
    <li>Load or save layer properties from or to a ".lyp" file: <class_doc href="LayoutView#load_layer_props"/> and <class_doc href="LayoutView#save_layer_props"/></li>
    <li>Obtain the selected entries from the layer properties tree: <class_doc href="LayoutView#selected_layers"/> and <class_doc href="LayoutView#current_layer"/></li>
    <li>Initialize layer properties with the default settings: <class_doc href="LayoutView#init_layer_properties"/></li>
    <li>Expand layer properties which contain wildcard entries ("stylesheet" layer properties files): <class_doc href="LayoutView#expand_layer_properties"/></li>
    <li>Manipulate layer properties by setting the properties of the <class_doc href="LayerPropertiesNodeRef"/> object returned by 
        <class_doc href="LayoutView#each_layer"/> or <class_doc href="LayerPropertiesIterator#current"/>.</li>
  </ul>

  <keyword name="LayerPropertiesIterator"/>
  <p>
  Many of these functions use <class_doc href="LayerPropertiesIterator"/> objects
  to identify entries in the layer tree. Such an object is basically a pointer into the tree. The term "iterator"
  refers means that such a pointer can be moved to neighboring entries in the layer tree.
  By default, the LayerPropertiesIterator performs a preorder, depth-first traversal of the layer properties tree (the
  virtual root object is omitted).
  This is how to work with LayerPropertiesIterator objects:
  </p>

  <pre>layout_view = Application::instance.main_window.current_view
# Get the iterator for the first entry:
lp = layout_view.begin_layers
# advance to the next entry (preorder, depth-first traversal):
lp.next
# advance to the next sibling 
lp.next_sibling(1)
# advance to the previous sibling
lp.next_sibling(-1)
# move down in the hierarchy to the first child
lp.down_first_child
# move down in the hierarchy to the last child
lp.down_last_child
# move up to the parent node
lp.up
# get the value of the current node
props = lp.current</pre>

  <p>
  The LayerPropertiesIterator has a couple of attributes:
  </p>
  
  <ul>
    <li><class_doc href="LayerPropertiesIterator#current"/>: returns the LayerPropertiesNodeRef object (<class_doc href="LayerPropertiesNodeRef"/>) which is a 
    representative for the layer the iterator points to. This object can be manipulated which has an immediate effect on the layer list of the view.</li>
    <li><class_doc href="LayerPropertiesIterator#at_end?"/>: returns true, if the iterator is at the end of the layer properties tree (if "next" is
    used to traverse) or at the end of the current child node list if other methods of traversal are used.
    It "at_end?" is true, the iterator does not point to a valid entry.
    </li>
    <li><class_doc href="LayerPropertiesIterator#is_null?"/>: returns true, if this iterator is a null iterator (i.e. default-constructed).</li>
    <li><class_doc href="LayerPropertiesIterator#at_top?"/>: returns true, if this iterator is pointing to a top-level entry.</li>
    <li><class_doc href="LayerPropertiesIterator#child_index"/>: returns the index of the current entry in the current child list.</li>
  </ul>

  <p>
  Iterators can be compared against each other. If two iterators point to the same object, the equality
  operator "==" returns true.
  </p>

  <p>
  The actual entry that the iterators "current" property is a LayerPropertiesNodeRef object (a reference to a LayerPropertiesNode object). 
  If behaves the same way than a LayerPropertiesNode object (<class_doc href="LayerPropertiesNode"/>), but modifications of the latter 
  will change the way the layer is displayed in the view.
  </p>

  <keyword name="LayerPropertiesNode"/>
  <p>
  The LayerPropertiesNode object contributes only a few methods, namely:
  </p>

  <ul>
    <li><class_doc href="LayerPropertiesNode#id"/>: an integer ID that uniquely identifies the entry in the tree.</li>
    <li><class_doc href="LayerPropertiesNode#flat"/>: computes and delivers an effective set of properties as a LayerProperties object.</li>
    <li><class_doc href="LayerPropertiesNode#bbox"/>: computes the bounding box of the drawn layer represented by this entry.</li>
    <li><class_doc href="LayerPropertiesNode#has_children?"/>: returns true, if this node is not a leaf node.</li>
    <li><class_doc href="LayerPropertiesNode#add_child"/>: adds a child node to the node. It returns a reference to the new node created inside the node's hierarchy. Is is possible to add new children to the node returned.</li>
    <li><class_doc href="LayerPropertiesNode#clear_children"/>: removes all children from the node.</li>
  </ul>

  <p>
  The actual properties of the layer are accessible through methods of the <class_doc href="LayerProperties"/> object.
  Since the parent node may override or contribute properties, a LayerProperties object has a twofold
  identity: the way it appears finally ("real") and the way it is configured ("local"). The property
  accessors have a "real" parameter and deliver the real value if this parameter is set to true and 
  the local value otherwise. There are also convenience methods which always deliver the "real" value.
  </p>

  <pre>lp = layout_view.begin_layers

# manipulate the layer 
lp.current.width = 2
lp.current.fill_color = 0x80ff40

# which is equivalent to this somewhat more efficient way:
props = lp.current.dup
props.width = 2
props.fill_color = 0x80ff40
lp.current.assign(props)</pre>

  <p>
  It is possible to directly manipulate the hierarchy this way:
  </p>

  <pre>lp = layout_view.begin_layers
# create a copy that we can manipulate
# add two child nodes
cp = RBA::LayerProperties::new
cp.source = "100/0"
lp.current.add_child(cp)
cp = RBA::LayerProperties::new
cp.source = "101/0"
lp.current.add_child(cp)
</pre>

  <p>
  New entries can be created by using LayoutView's insert_layer method and a LayerPropertiesIterator to 
  specify the location where the node shall be created. Here is an example how to create a child entry
  using that technique. Please note how "down_first_child" is used to navigate into the node's child space
  which works even if there are no children yet:
  </p>

  <pre>lp = layout_view.begin_layers
# let the iterator point to the first child, even if it does not exist
lp.down_first_child
# (lp.current may not be valid, but still lp is a valid insert position)
# prepare a new entry for insert:
props = RBA::LayerProperties.new
props.source = "100/0"
# insert the child node:
layout_view.insert_layer(lp, props)
# now, lp points to a valid object: lp.current.source == "100/0"</pre>

  <h3>LayerProperties objects</h3>
  <keyword name="LayerProperties"/>

  <p>
  The <class_doc href="LayerProperties"/> object represents one entry in the layer properties tree
  and has several basic properties. For each of these properties,
  a getter for the real and local value exists as well as a setter that installs a local value.
  For example, for the width property, the following methods are defined:
  </p>

  <ul>
    <li><b>width(real)</b>: the getter for the real ("width(true)") or local ("width(false)") value.</li>
    <li><b>width</b>: the real value.</li>
    <li><b>width=</b>: the setter for the local value.</li>
  </ul>

  <p>
  Width is a "weak" property. That means that for computing the effective width, child nodes 
  can override the settings inherited from the parent nodes. A width of 0 is considered "not set" and does not
  override parent defined widths. Other properties like visibility are "strong", i.e. 
  the parent can override the properties set for it's children. Another form of combination is "additive" where
  the effective property value is the "sum" (or in general combination) of all local properties from parent
  to child.
  </p>

  <p>
  Some properties like "fill_color" do not have a neutral value but instead they can be cleared (in that case with
  "clear_fill_color"). The LayerProperties object can be asked whether a fill color is set using the "has_fill_color?" method.
  </p>

  <p>
  This is a brief list of properties:
  </p>

  <ul>
    <li><class_doc href="LayerProperties#animation"/> (strong): specifies animation (blinking, scrolling)</li>
    <li><class_doc href="LayerProperties#dither_pattern"/> (strong): specifies the fill pattern</li>
    <li><class_doc href="LayerProperties#line_style"/> (strong): specifies the line style</li>
    <li><class_doc href="LayerProperties#fill_brightness"/> (additive): specifies the fill color's brightness</li>
    <li><class_doc href="LayerProperties#fill_color"/> (strong): specifies the fill color</li>
    <li><class_doc href="LayerProperties#frame_brightness"/> (additive): specifies the frame color's brightness</li>
    <li><class_doc href="LayerProperties#frame_color"/> (strong): specifies the frame color</li>
    <li><class_doc href="LayerProperties#lower_hier_level"/> (weak): the lower hierarchy level shown. This property has various flavors related to the definition of "lower level".</li>
    <li><class_doc href="LayerProperties#upper_hier_level"/> (weak): the upper hierarchy level shown. This property has various flavors related to the definition of "upper level".</li>
    <li><class_doc href="LayerProperties#marked?"/> (strong): specifies whether the layout is rendered with small crosses at each vertex</li>
    <li><class_doc href="LayerProperties#xfill?"/> (strong): Specifies whether a cross is drawn in rectangles</li>
    <li><class_doc href="LayerProperties#transparent?"/> (strong): specifies whether the layer is semi-transparent (color bitmap combination)</li>
    <li><class_doc href="LayerProperties#visible?"/> (strong): specifies whether the layer is visible</li>
    <li><class_doc href="LayerProperties#valid?"/> (strong): specifies whether the layer is valid</li>
    <li><class_doc href="LayerProperties#width"/> (weak): specifies the line width</li>
    <li><class_doc href="LayerProperties#source"/> (weak, additive): specifies the origin of the data. This property can be set or obtained either a string using KLayout's source notation or be accessed
      through a couple of specialized properties delivering a part of the source specification each (<class_doc href="LayerProperties#source_layer"/>, <class_doc href="LayerProperties#source_datatype"/>, <class_doc href="LayerProperties#source_name"/>, <class_doc href="LayerProperties#source_layer_index"/>, <class_doc href="LayerProperties#source_cellview"/> and <class_doc href="LayerProperties#trans"/>)</li>
  </ul>

  <p>
  In addition, a couple of getters for computed and derived values are present (i.e. "eff_frame_color"). There are
  no setters for these properties. The effective frame color for example delivers the frame color which results
  from combining the frame color and the frame brightness.
  </p>

  <h2>The CellView class</h2>
  <keyword name="CellView"/>

  <p>
  The CellView (<class_doc href="CellView"/>) identifies the cell drawn and the context the cell is drawn in. A CellView can be created as a object
  but usually it is obtained from a LayoutView object. In the following example, the active cell view is used:
  </p>

  <pre>RBA::Application::instance.main_window.current_view.active_cellview</pre>

  <p>
  Alternatively, a cell view can be addressed by index:
  </p>

  <pre>lv = RBA::Application::instance.main_window.current_view
num_cellviews = lv.cellviews # number of cell views
lv.cellview(0) # first one</pre>

  <p>
  A cellview carries the following information:
  </p>

  <ul>
    <li><class_doc href="CellView#cell"/>: a reference to the cell shown (a Cell object: <class_doc href="Cell"/>).</li>
    <li><class_doc href="CellView#layout"/>: a reference to the layout object (a Layout object: <class_doc href="Layout"/>) which contains the cell shown.</li>
    <li><class_doc href="CellView#name"/>: the unique name of the layout.</li>
    <li><class_doc href="CellView#filename"/>: the name (actually the path) of the file loaded if the layout was loaded from a file.</li>
    <li><class_doc href="CellView#path"/>: the unspecific path (see below)</li>
    <li><class_doc href="CellView#context_path"/>: the specific path (see below)</li>
    <li><class_doc href="CellView#ctx_cell"/>: the context cell (see below). Alternatively the cell index of the context cell is available by <class_doc href="CellView#ctx_cell_index"/>.</li>
    <li><class_doc href="CellView#technology"/>: the technology (name) used with this layout</li>
    <li><class_doc href="CellView#is_cell_hidden?"/>: returns a value indicating whether a given cell is hidden</li>
  </ul>

  <p>
  A cellview can be manipulated to change the cell shown in the layout view. For this purpose, assignment methods
  exist which will reconfigure the cellview:
  </p>

  <ul>
    <li><class_doc href="CellView#cell="/>: a reference to the cell shown (a Cell object: <class_doc href="Cell"/>).</li>
    <li><class_doc href="CellView#cell_index="/>: a reference to the cell shown (by cell index).</li>
    <li><class_doc href="CellView#name="/>: the unique name of the layout.</li>
    <li><class_doc href="CellView#path="/>: sets the unspecific path (see below)</li>
    <li><class_doc href="CellView#context_path="/>: sets the specific path (see below)</li>
    <li><class_doc href="CellView#technology="/>: applies the given technology to this layout</li>
    <li><class_doc href="CellView#show_cell"/>, <class_doc href="CellView#show_all_cells"/>, <class_doc href="CellView#hide_cell"/>: shows or hides cells</li>
    <li><class_doc href="CellView#close"/>: closes this cell view (removes it from the layout view)</li>
  </ul>

  <h3>Unspecific and specific path and context cell</h3>

  <p>
  In addition to the cell itself, the cell view specifies how the cell is embedded in the hierarchy.
  Embedding can happen in two ways: an unspecific and a specific way. Both ways contribute to a path which 
  leads from a top cell to the cell drawn.
  </p>

  <p>
  The first part is always the unspecific path. This path specifies, where the cell drawn is located in the cell 
  tree. That has no effect on the drawing, but is determines what entry in the cell tree is selected. 
  Giving a path for that information is required, because a cell can be child of different cells which itself can be 
  children of other cells. The unspecific path lists the top cell and further cells which are all direct or indirect 
  parents of the cell addressed.
  </p>

  <p>
  The unspecific path ends at the "context cell" which usually is identical to the cell addressed by the cell view.
  KLayout allows addressing of a specific instance of a direct or indirect child cell as the actual cell. In that
  case, the specific path comes into play. Bascially that means, that a cell is drawn within a context of embedding 
  layout. The specific path leads from the context cell to the cell view's target cell and consists of specific instances 
  (hence the name "specific path"). The "descend" and "ascend" feature
  bascially adds or removes instances from that path.
  </p>

  <p>
  The unspecific path can be obtained with the <class_doc href="CellView#path"/> method, the specific path with the <class_doc href="CellView#context_path"/> method.
  The unspecific path is just an array of cell indexes specifying the top cell and further cells down to the context cell
  and includes the context cell. The specific path is an array of InstElement objects (<class_doc href="InstElement"/>).
  Each InstElement object describes a specific instantiation (a cell instance plus information when a specific array instance
  is addressed). When there is no context, the specific path is an empty array.
  Using the setters <class_doc href="CellView#path="/> and <class_doc href="CellView#context_path="/> these paths can be changed
  to select a new cell into the layout view.
  </p>

  <h2>The Image class</h2>
  <keyword name="Image"/>

  <p>
  Images can be placed onto the drawing canvas and display colored or monochrome images below the layout.
  Images are represented by Image objects (<class_doc href="Image"/>). Basically an image is a two-dimensional
  array of pixel values with a specification how these pixels are to be displayed on the canvas. An image
  can be created an placed on the canvas like this:
  </p>

  <pre>lv = RBA::Application::instance.main_window.current_view
image = RBA::Image::new("image.png")
lv.insert_image(image)</pre>

  <p>
  An image can be configured by using different properties and attributes:
  </p>

  <ul>
    <li>The images' data can be loaded from a file by using a constructor with a file name. In addition, the image 
    can use data from an array of floating-point values using either a constructor or the <class_doc href="Image#set_data"/> method. 
    An image can be colored, in which case three channels are present or it can be monochrome. In the latter case,
    a single channel is present only. Together with the data, the dimensions of the image have to be specified (width
    and height in pixel units).
    </li>
    <li>The image's data can be manipulated per pixel using the <class_doc href="Image#get_pixel"/> or <class_doc href="Image#set_pixel"/> method.</li>
    <li>The data range for the data stored in the image can be set using the <class_doc href="Image#min_value="/> and <class_doc href="Image#max_value="/>
    attributes. The data range determines which value is considered "maximum intensity" (max_value) and "zero intensity"
    (min_value).
    </li>
    <li>For monochrome images, a data mapping can be specified. A data mapping converts a monochrome value (a scalar)
    to a color. Data mapping is specified through a ImageDataMapping object (<class_doc href="ImageDataMapping"/>)
    using the <class_doc href="Image#data_mapping"/> method. 
    </li>
    <li>The geometrical properties of an image are encapsulated in a Matrix3d object (<class_doc href="Matrix3d"/>).
    Such a matrix describes the transformation from pixel coordinates to the micron unit space of the canvas.
    A 3x3 matrix is a generic way to specify a transformation, including translation, rotation, mirror, shear or 
    perspective distortion. The matrix is obtained and set using the <class_doc href="Image#matrix"/> attribute. 
    Convenience methods like <class_doc href="Image#trans"/>, <class_doc href="Image#pixel_width"/> and <class_doc href="Image#pixel_height"/> allow accessing sub-aspects
    of the generic transformation (affine transformation, scaling).
    </li>
  </ul>

  <p>
  An image can be transformed using one of the <class_doc href="Image#transformed"/> methods. It can be hidden or shown using the 
  <class_doc href="Image#visible="/>
  method. The bounding box of the image can be obtained with the <class_doc href="Image#box"/> method.
  </p>

  <h2>The Annotation class</h2>
  <keyword name="Annotation"/>

  <p>
  Annotations (<class_doc href="Annotation"/>) are basically rulers and other "overlay objects" but can be used for other purposes as well,
  for example to simply add a text object.
  Annotations, like images, are objects stored in the LayoutView and can be selected, deleted, transformed etc.
  </p>
 
  <p>Programmatically, annotations are created this way:</p>

  <pre>lv = RBA::Application::instance.main_window.current_view
ant = RBA::Annotation::new
ant.p1 = RBA::DPoint.new(0.0, 0.0)
ant.p2 = RBA::DPoint.new(100.0, 0.0)
lv.insert_annotation(ant)</pre>

  <p>The annotation carries several attributes. Those are the same attributes that can be configured in the 
  annotation properties dialog.
  The most important properties are the two positions (start and end 
  position) accessible through the <class_doc href="Annotation#p1"/> and <class_doc href="Annotation#p2"/> properties, 
  the style (<class_doc href="Annotation#style"/> property)
  and the outline (<class_doc href="Annotation#outline"/> property).
  </p>

  <p>If properties are changed using the attribute setters, their appearance will change as well. 
  The following example demonstrates how rulers are manipulated. In this example, the style of all rulers 
  is set to "arrow on both sides". Note, how in this example transactions are used to implement
  undo/redo:
  </p>

  <pre>view = RBA::LayoutView::current

begin

  view.transaction("Restyle annotations")

  view.each_annotation do |a|
    a.style = RBA::Annotation::StyleArrowBoth
  end
  
ensure
  view.commit
end
</pre>

  <h2>The Marker class</h2>
  <keyword name="Marker"/>

  <p>
  A marker is a temporary highlight object. A marker is represented by the Marker class (<class_doc href="Marker"/>).
  Markers appear when they are created and disappear when they are destroyed. Since destruction by the garbage
  collector happens at undefined times, the destroy method can be used to destroy the marker explicitly.
  Markers accept some plain shapes (i.e. a Box) which will be displayed as the marker. Markers can be configures in 
  manifold ways, i.e. the colors, the fill pattern, line width etc. See the class documentation for details about 
  the configuration properties.
  </p>

  <p>
  This is how to create and destroy a marker:
  </p>

  <pre>lv = RBA::Application::instance.main_window.current_view
marker = RBA::Marker.new(lv)
marker.set(RBA::DBox::new(0.0, 0.0, 100.0, 200.0))
# to hide the marker:
marker.destroy</pre>

  <p>
  Markers are temporary objects intended for highlighting a certain area or shape. Markers are not 
  persisted in sessions nor can they be edited.
  </p>

  <h2>The Plugin and PluginFactory classes</h2>
  <keyword name="Plugin"/>
  <keyword name="PluginFactory"/>

  <p>
  Plugins (<class_doc href="Plugin"/>) are objects which provide modular extensions of KLayout. Plugins are 
  the only way to handle 
  mouse events in the canvas.
  The basic operation of a plugin is the following:
  </p>
  
  <ul>
    <li>For each plugin type, a PluginFactory (<class_doc href="PluginFactory"/>) object must be provided. KLayout 
    uses this object to 
    configure itself and to create a particular plugin instance for each LayoutView. The PluginFactory must provide 
    certain configuration information and can handle some events in a global manner, for example menu entries that 
    do not refer to a certain plugin instance. The PluginFactory must register itself in the KLayout framework. After doing 
    so, KLayout will provide a new button in the tool bar. If this button is selected, the plugin will be activated.
    </li>
    <li>When a LayoutView is created, it will use the PluginFactory to create a specific Plugin instance for the view.
    When the tool bar button is pressed which relates to this plugin, the plugin will be activated and mouse or other
    events will be redirected to this plugin.
    </li>
  </ul>

  <p>
  The PluginFactory itself acts as a singleton per plugin class and provides not only the ability to create
  Plugin objects but also a couple of configuration options and a global handler for configuration and menu
  events. The configuration includes: 
  </p>

  <ul>
    <li>Menu items: by configuring menu items in the PluginFactory, KLayout can create these items when 
    the plugin is initialized. Each menu entry is connected with the plugin through a symbol: this is a string
    that tells the plugin's <class_doc href="Plugin#menu_activated"/> method which menu item was selected. By configuring a menu
    rather than creating it explicitly, KLayout has a somewhat better control over what menu items belong to
    which plugin. Menu items are configured by calling <class_doc href="PluginFactory#add_menu_entry"/> in the PluginFactory's constructor.
    </li>
    <li>Configuration options: Instead of directly taking values from the configuration database, it is more
    convenient to register configuration keys in the PluginFactory's constructor using the <class_doc href="PluginFactory#add_option"/> method.
    After an option is configured, the individual Plugin objects and the PluginFactory receives "configure" calls 
    when a configuration option changes or for the initial configuration.
    </li>
  </ul>

  <p>
  A PluginFactory must be instantiated and register itself. Menu items and configuration options should be
  set before the object is registered. Upon registration, a unique name must be specified for the plugin class.
  Also, the tool button title and optionally an icon can be specified.
  </p>

  <p>
  The main objective of the PluginFactory class however is to create the actual plugin object. For this,
  the create_plugin method needs to be reimplemented. The implementation is supposed to create an object of
  the specific class.
  </p>

  <p>
  The actual implementation of the plugin is a class derived from Plugin (<class_doc href="Plugin"/>).
  The plugin comes into life, when it is activated. That is, when the tool button is pressed that is 
  associated with the plugin. When the plugin is activated, the <class_doc href="Plugin#activated"/> method is called. The method
  can be reimplemented in order to prepare the plugin for taking actions on mouse events. When the 
  plugin is not longer active, i.e. because another mode has been selected, the <class_doc href="Plugin#deactivated"/> method is called.
  </p>

  <p>
  Every plugin has the ability to receive and intercept mouse events. Various mouse events are available: mouse moved,
  mouse button clicked (button pressed and released), mouse button double clicked, mouse button pressed, mouse button
  released, entry or leave of the window and agitation of the mouse wheel. Each event follows a certain protocol depending 
  whether the plugin is
  active or not. In addition, plugins can request exclusive control over the mouse by "grabbing" the mouse. 
  Each event is associated with a certain callback. The callback has a parameter - "prio" - which determines the
  role of the event.
  The protocol is described here:
  </p>

  <ul>
    <li>First, all plugins that grabbed the mouse with grab_mouse will receive an event callback with 
    'prio' set to true in the reverse order the plugins grabbed the mouse. 
    If any one of the mouse event handlers returns true, the protocol terminates.
    </li>
    <li>
    If that is not the case or no plugin has grabbed the mouse, the active plugin receives 
    the mouse event with 'prio' set to true.
    </li>
    <li>
    If no receiver accepted the mouse event by returning true, it is sent again to all plugins with 'prio' set to false. 
    Again, the loop terminates if one of the receivers returns true. The second pass gives inactive 
    plugins a chance to monitor the mouse and implement specific actions - i.e. displaying the current position.
    </li>
  </ul>

  <p>
  In an mouse event handler, the plugin can take any action, i.e. transform objects or create/remove markers. 
  This allows implementing of interactive functionality upon KLayout's canvas object. Using "set_cursor", the 
  plugin can set the mouse cursor to a specific shape for example. A plugin should consider implementing "drag_cancel" 
  in order to terminate any pending dragging operations. <class_doc href="Plugin#drag_cancel"/> is called by KLayout to regain control 
  over the mouse in certain circumstances and is supposed to put the plugin into a "watching" instead of "dragging" state.
  </p>

</doc>

