# KeyMap

Manage maps of handlers for key events

### Table of contents

NAME
SYNOPSIS
DESCRIPTION
WRITING HANDLERS
ATTACHING TO HTML ELEMENTS
METHODS
`KeyMap`
Single-key rules
Regex rules
Antiregex rules
`eventHandler`
`observe`
`rules`
`MapAllKeys`
`Prefix`

## NAME

KeyMap - Manage maps of handlers for key events

## SYNOPSIS

      var rules = {

        // attach handlers to specific keys
        RETURN: function(event){doSomethingWith(event)},
        C_DOWN: ctrlArrowDownHandler,
        C_S_F7: ctrlShiftF7Handler,

        // special rules using regular expressions
        REGEX: [ ["",   /^[0-9]$/,      digitHandler  ],
                 ["C_", /^[aeiou]$/i, ctrlVowelHandler] ],

        // use Ctrl-X as a prefix for another set of rules
        C_X: KeyMap.Prefix({R: ctrlX_R_handler,
                            4: ctrlX_4_handler})
      };

      // create a keymap object
      var aKeyMap = new KeyMap(rules);

      // attach the corresponding handler to the keydown event (on document)
      aKeyMap.observe("keydown");

      // other way to attach : manually insert handler
      document.onkeydown = aKeyMap.eventHandler({preventDefault: true,
                                                 ignoreShift   : true});

      // dynamically change the map
      aKeyMap.rules.push(new_rules);

      // idem, temporarily ignore all keys
      aKeyMap.rules.push(KeyMap.MapAllKeys(function(){}));

      // back to previous handling state
      aKeyMap.rules.pop();

## DESCRIPTION

Provides an abstraction layer for associating handlers with HTML key events, in a browser-independent way.

A keymap is a stack of collections of rules. Each rule has a key specification or a regexp specification, and a handler to be called whenever the specification is met. The keymap object as a whole can then be registered as a usual HTML event handler associated to some DOM element (most often the document element), and will dispatch key events to appropriate handlers.

Key specifications look like A (key 'A'), C_S_A (control-shift-A), A_DELETE (alt-Delete). They are formed from :

- keynames

  For printable characters, the keyname is just that character; for special editing keys such as backspace, arrow up, etc., names are taken from the following list of builtins :

        BACKSPACE ESCAPE     TAB    RETURN LINEFEED SPACE
        PAGE_UP   PAGE_DOWN  END    HOME
        LEFT      UP         RIGHT  DOWN
        INSERT    DELETE     PAUSE  WINDOWS  PRINT_SCREEN
        CAPS_LOCK NUM_LOCK   SCROLL_LOCK
        F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12
        CTRL      SHIFT      ALT

- modifiers

  Modifiers are specified through prefixes C_, S_ and A_, corresponding to key modifiers control, shift and alt. Several prefixes may be combined, but must appear in the order just given (so for example S_C_A would be illegal).

Alternatively, key specifications may also formed from key codes instead of key names, so for example `C_13` is equivalent to `C_RETURN`. For key codes 0-9, and additional '0' is required to avoid confusion with digits: so `C_09` is equivalent to `C_TAB`, while `C_9` means "control-numeric 9".

In addition, keymap objects can also manage regex rules that cover several possible key events; details are given below.

## WRITING HANDLERS

Following the W3C event model, handlers called from the keymap object receive an event object as argument. This is the usual HTML event object, augmented with two properties `keyName` and `keyModifiers`, computed according to the specifications given above. So for example a simple handler can be

      var myHandler = function (event) {
        alert(event.keyName + " was pressed with modifiers " +
              event.keyModifiers);
      }

Further propagation of the event to other handlers is cancelled by default : W3C methods `event.stopPropagation()` and `event.preventDefault()` are called automatically by the keymap object (or, if running under Microsoft Internet Explorer, property `cancelBubble` is set to true and and property `returnValue` is set to false). This default behaviour can be disabled if necessary, as explained below.

## ATTACHING TO HTML ELEMENTS

Keymaps may be attached to HTML elements on the `keydown`, `keypress` or `keyup` event types. Choosing the proper event type is important, as it affects not only the time at which events are fired, but also the returned keycodes :

- `keydown` and `keyup`

  These are "low-level" event types that capture almost every key on the keyboard, including special keys like ESCAPE, F1, PAGE UP, etc. Returned key codes remain at a raw level, i.e. they are not translated into characters. This means that if Shift-1 is marked on your keyboard as an exclamation mark, a plus sign, or some other special character, you will not receive that keycode when capturing `keydown` events : rather, you will receive keycode 49 (ASCII character '1'). Similarly, all letters are received as uppercase.

- `keypress`

  By contrast, the `keypress` event type is higher-level in that it performs the translation from keys to characters, according to your specific keyboard. However, this event type only fires for printable characters, so you cannot observe `keypress` if you intend to capture special keys such as arrow keys, function keys, etc.

In theory, attributes such as `onkeydown` or `onkeypress` may be used with most HTML elements; but in practice, most of them will actually never fire the key events! So the most common and most sensible way for capturing key events is to attach to the `document` element.

Events `keypress` and `keydown` will repeat if the key is held down.

In order to attach the keymap to an element, you can either use the supplied /"observe" method, or call the /"eventHandler" method to get the keymap event handler, and then use your favorite technique to attach that handler to an element.

## METHODS

### `KeyMap`

      var myKeyMap = new KeyMap(rules);

Constructor for a keymap object.

#### Single-key rules

The rules argument is a map from key specifications to handlers, like for example

      { A:     function() {alert("pressed 'A'");},
        S_TAB: function() {alert("pressed 'Shift-Tab'");},
        CTRL:  function() {alert("pressed the 'Ctrl' key");},
        10:    function() {alert("pressed 'Linefeed' or maybe 'Ctrl-Return'");}
      }

Each key specification in the map corresponds to exacly one key combination, so for example `S_TAB` will not fire if the user pressed `Ctrl-Shift-Tab`.

#### Regex rules

For situations where several key combination will fire the same handler, you can insert a `REGEX` entry in the map. This should be an array of triplets, where each triplet is of shape `[modifiers, regex, handler]`, like for example

      var regexRules = [["C_",   "[0-9]",             myCtrlDigitHandler],
                        ["C_S_", /^[AEIOU]$/,         myCtrlShiftVowelHandler],
                        [null,   "RETURN|TAB|ESCAPE", someOtherHandler]   ];

Whenever a key event is received, it is converted into a keyname, and then that keynames is compared against the regex rules, in order : the first rule that matches calls the corresponding handler and terminates the event handling process.

More specifically, the members of rule triplets are :

- modifiers

  A string specifiying the key modifiers for which the rule will fire; the string a concatenation of C_, S_ and A_, as explained above. An empty string means that the rule only fires when no modifiers are pressed. By contrast, a `null` value specifies that modifiers are ignored (the rule fires in any case).

- regex

  Either a string containing a regular expression, or an already built Javascript RegExp object. Strings will be automatically converted to regular expressions, with start anchor `^` and end anchor `$` automatically added. If you supply an already built RegExp object, make sure to deal properly with the anchors; otherwise the rule might fire in unexpected cases (for example the plain regex `/[AEIOU]/` would match any builtin keyname like `RETURN` or `ESCAPE`, which is probably not the intended meaning of the rule).

- handler

  The function to be called when the rule succeeds.

#### Antiregex rules

An `ANTIREGEX` entry in the map works exactly like a `REGEX`, except that the handler is called when the regex does not match. This is useful if you want to catch most key events, except a given set.

### `eventHandler`

      document.onkeydown = aKeyMap.eventHandler(options);

Generates an event handler that can be attached to an HTML element. This method is called internally by the /"observe" method. Use `eventHandler` directly if you need fine control on how the handler is attached to the dynamic HTML model.

The `options` argument is optional. If present, it should be an inline object containing truth values for the following keys :

- `ignoreCtrl`

  ignore the `Ctrl` keyboard modifier

- `ignoreShift`

  ignore the `Shift` keyboard modifier

- `ignoreAlt`

  ignore the `Alt` keyboard modifier

- `stopPropagation`

  stop propagation of the event

- `preventDefault`

  prevent default navigator behaviour on that event

For example if `ignoreCtrl` is true, then the key specification `"C_S_TAB"` would never fire, because Ctrl-Shift-TAB key events would be encoded merely as `"S_TAB"`.

### `observe`

      aKeyMap.observe(eventType, htmlElement, options);

This is the preferred way for attaching the keymap object to an HTML element, on a given event type (`keydown`, `keypress` or `keyup`). Arguments are optional. The default event type is `"keydown"`, and the default element is `document`.

Options are passed to the /"eventHandler" method. If not explicitly given, the options default to `undefined` except for event type `keypress`, where `ignoreShift` defaults to `true`. The reason is that the Shift modifier heavily depends on which keyboard the user is using, and often the user really has no choice on pressing or not the Shift key (this would generate a different keycode). So it makes sense to just stop paying attention to the Shift key for `keypress` events.

### `rules`

      aKeyMap.rules.push(new_rules);

      aKeyMap.rules.pop();

A DHTML application may need to temporarily change the key handlers (for example when switching from navigation mode to editing mode). Therefore, a keymap object actually holds a stack of rules and publishes this stack in its `rules` property. Rules pushed on top of that stack will take precedence over pre-existing rules; conversely, popping from the stack restores the keymap to its previous state.

### `MapAllKeys`

      // grab all keys
      aKeyMap.rules.push(KeyMap.MapAllKeys(my_handler));

      // ignore all keys
      aKeyMap.rules.push(KeyMap.MapAllKeys(function (){}));

Convenience function to build a regex rule that matches all keys.

### `Prefix`

      main_rules = {C_X: KeyMap.Prefix({R: ctrlX_R_handler,
                                        4: ctrlX_4_handler})};

Specifies that a key (here `Ctrl-X`) is a prefix to another set of rules : the next key event will be passed to these rules, and after that the main rules resumes normal behaviour. Hence you can attach handlers to sequences of keys, like for example in Emacs.