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specification of "legacy" key events

From: Hallvord R. M. Steen <hallvord@opera.com>
Date: Sat, 26 Apr 2008 16:06:37 +0200
To: public-webapi@w3.org
Message-ID: <op.t971hb14a3v5gv@hr-opera.oslo.opera.com>
Below are some attempts at specifying the "DOM0" legacy key events (mainly  
keypress events, also how to determine keyCode/charCode for keydown and  
keyup). I believe it's very important to specify this in DOM3 Events  
because I've seen several problems caused by new web applications trying  
to define keyboard shortcuts and running into incompatibilities in key  
event handling.

I've never really written English specification prose before (though I've  
been an avid reader of W3C TRs) so please give this stuff a critical but  
patient review ;-)

The text below refers to a table of virtual key codes. This is attached in  
OpenOffice spreadsheet format, and can also be viewed here:
http://spreadsheets.google.com/pub?key=p32PJW4aXyuBDbcEUj4dmaw

Rough table of contents is introduction blurb, spec text, notes, todo, and  
references. I hope this is useful.


INTRODUCTION
The specification of key events describes processing of input from  
keyboard devices. Key events rely on hardware support and DOM applications  
should not assume that any specific keys will be available, meaning they  
should be designed so that no information or command can be accessed only  
through generating a key event.

On many operating system, the system will handle character mappings but on  
devices where such mappings are not available an implementation may  
include its own conversion tables and for example decide to send ENTER key  
events with key code 13 if a joystick-type control in a mobile phone UI is  
pressed down.

Key input normally has a default action, e.g. inserting text. The default  
action of a key depends on what sort of element has focus, for example if  
focus is in a TEXTAREA the default action of the ENTER key is to insert a  
line break. If focus is on a link, the default action for ENTER usually is  
to follow the link. What the default action for a certain key is can not  
be determined from an event listener. The default action can be prevented  
by calling the event.preventDefault() method or returning false from an  
event handler, but implementations MAY make this configurable and let the  
user specify either globally or on a per-key basis that event listeners  
are not allowed to override the defaults.

INPUT METHOD EDITORS
Input method editors or IMEs are frequently used to type for example  
Chinese, Japanese and Korean characters. An active IME captures all  
keypresses and processes them to determine what character the user wants  
to insert.

Depending on the IME and its settings, most keys will either initiate,  
contribute to or finish the conversion process. For example, with the  
standard Microsoft Japanese IME active in Hiragana mode, pressing the "K"  
key will start a conversion process. Pressing "U" will contribute to it  
and the IME will show the Hiragana "ku" character. Pressing Enter will  
complete the conversion and insert the character into the document.

In this specification, the expression "input used for IME character  
creation" means input the IME is processing to transform into the actual  
output character(s). The expression "IME character insertion" means the  
insertion that occurs when the IME is done and the user confirms the  
generated characters to insert them into the text.

Note that IMEs can also insert text without any key events occurring at  
all, e.g. with handwriting recognition or on-screen keyboards.

KEY TYPES
Some key types defined by ranges of Microsoft Windows virtual key codes:
	* Alphanumerical keys: all keys with key codes in the ranges 48-57 and  
65-90.
	* function keys: keys with key code in range 112 - 135
	* dead keys: keys that are used to add accents to the next key that is  
typed. One example is an U+00A8 DIARESIS key. On many keyboard layouts,  
dead keys are in the OEM range and their placement and key codes vary by  
keyboard layout. It is not possible to specify virtual key codes for them  
and the implementation must query the system whether the key that was  
pressed was a dead key or not.
	* navigation keys: keys with key code in range 33 - 40 (arrows, Home/End,  
page navigation)

RELATIONSHIP BETWEEN EVENT TYPES

There are two types of key events:
	* Hardware reference events. The keydown and keyup events report that a  
key was pressed down and released. These events include keyboard reference  
information but do not confirm what character(s) if any will be inserted.  
Keyboard reference codes are hardware-, software-, locale- and  
system-dependent, but implementations should map as many keys as possible  
to virtual key codes as defined below.
	* Text insertion events. The keypress and textInput events include  
complete information about the character the input is generating, if any,  
taking into account shift states, previous dead keys and other contextual  
information required to decide what character will be inserted by a key  
press. The textInput event also shows what character(s) were generated by  
an active input method editor. The keypress event MUST NOT fire when an  
IME is processing the input for conversion.

All keys except dead keys fire the hardware reference events. What other  
events are fired depends on the type of key that is pressed and response  
 from the event listeners. Typically, alphanumerical keys and punctuation  
keys fire text insertion events while control keys and navigation keys do  
not. Dead keys may fire text insertion events if pressed twice or followed  
by a space. The first time a dead key is pressed fires no events at all.

KEY CODE CALCULCATION FOR HARDWARE REFERENCE EVENTS
The key code for keydown / keyup events is calculated as follows:
	1. If input is a numerical character (0-9), return the ASCII code of the  
character.
	2. If input is a lower case character (not limited to English a-z),  
return the ASCII code of the upper case equivalent. [TODO#1]
	3. Look up key code in table of Microsoft Windows virtual key codes,  
return it if found. [NOTE#2]
	4. Read virtual key code from system if possible. [TODO#2]
	5. If no key code was found, return 0.

PROCESSING OF KEYPRESS EVENTS

When a key is pressed,	and the keydown event has been processed an  
implementation must behave as if it implemented this algorithm when firing  
keypress events:

	* If the keypress follows a keydown event on the same key and the earlier  
keydown event had its default action cancelled, terminate this algorithm.   
[NOTE#1]

	* If the input is key input used for IME character creation, terminate  
this algorithm. [TODO#3]

	* If the key does not cause text input and is not the Escape key (i.e. if  
the key is not is an alphanumerical key, a punctuation key, a repeated  
press of a dead key or the Escape key), terminate this algorithm.

	* Set event meta key properties (shiftKey, ctrlKey, altKey, metaKey)  
depending on what meta keys are pressed, if any

	* For backwards compatibility reasons the character code property has two  
different names. Define charCode and keyCode, set both to the decimal  
value of the unicode reference number of the corresponding character.

	* Fire the event. If it was not cancelled with event.preventDefault()  
proceed to fire a textInput event.

	* If the same keystroke inserts several characters, fire one keypress  
event for each

	* If the key is held down, repeatedly fire one keydown event and one or  
more keypress events at a rate determined by the system's repeat key  
interval setting


NOTE#1: follow Firefox or IE on what to do if keydown's preventDefault()  
was called? Test results:
  * IE, Safari: fire keydown, keyup. No text insertion. (Above text  
standardises this)
  * Firefox: fires keydown, keypress (does not insert text), keyup (and  
apparently web content relies on this)
  * Opera: ignores preventDefault() on keydown, does not insert text if  
preventDefault() is called on keypress

NOTE#2: Microsoft virtual key codes attached as spreadsheet (Open Office  
format). Since step 1 and 2 of this algorithm handle alphanumerical keys,  
this step will mainly deal with punctuation characters. Their placements  
and virtual key codes vary greatly between keyboard layouts on Windows.  
The reason key codes for punctuation characters are so weird in a typical  
MSIE implementation are that they are usually placed on the keys Microsoft  
defines as "OEM keys". These are numbered OEM 1 - OEM 7 (with another  
OEM_102 on some layouts) plus OEM_COMMA, OEM_PERIOD, OEM_PLUS and  
OEM_MINUS.

However, different keyboard layouts change what OEM reference a certain  
key has! So the letter "" on a Norwegian keyboard is considered an OEM_3  
key (code 192) but on EN-US layout the very same key is called OEM_1 and  
has code 186. Thus, even striking the *very same* key gives different  
virtual key codes on different keyboard layouts. Neither are those virtual  
key codes mapped to actual input: typing the *very same* character on  
different keyboard layouts can produce different key codes because these  
keys are associated with different "OEM" keys under the hood.

OEM_COMMA, OEM_PERIOD, OEM_PLUS, OEM_MINUS are obviously meant to map to  
the corresponding punctuation on most keyboards, but even this is not  
consistently applied. For example, on a Norwegian keyboard layout the +  
character is on the OEM_PLUS key and returns the virtual key code 187 but  
on the Icelandic layout + is on the OEM_2 key and returns its virtual key  
code 191.

There is no really consistent way to figure out how this should work for  
any OS and device. The assignment of keys to virtual key codes in  
Microsoft's OEM range is too chaotic to emulate, and not useful to  
authors. The way forward would be to specify explicit keyCode/charCode  
values for the following keys that never change with keyboard layouts:
	Backspace
	Tab
	Caps Lock
	Shift
	Control
	Alt
	Enter
And those punctuation characters that are *meant* to not change virtual  
codes between keyboard layouts:
	Period (.)
	Comma (,)
	Plus (+)
	Minus (-)
	Decimal key (on numpad)
	Space
Then leave other punctuation characters implementation dependent and  
inform script authors that they should listen to keypress or textInput  
events to reliably detect those.

TODO#1: note that IE does NOT take the upper-case value of certain  
non-English character (for example / on Norwegian keyboards). I believe  
doing so makes the model cleaner and is unlikely to cause compatibility  
problems - this needs investigation though. Here we also probably need to  
specify some specific algorithm for upper/lower-casing characters?

TODO#2: Step 4 of this algorithm is incomplete, probably needs to specify  
how to get a virtual key from system? The issue step 4 is trying to solve  
is: If a given key, say the I key is mapped to something else, say  
"Hiragana I" keydown/keyup will still have the key code of an upper-case I  
in reference implementations but not according to this algorithm without  
some magic in step 4. Hence we need to fall back to reading virtual key  
codes from the system in step 4, but how to do this exactly is  
underspecified and will probably vary between operating systems.

TODO#3: If the IME is NOT in the middle of a conversion, keys that do not  
initiate a conversion in that IME (such as ESC and Enter) *do* fire  
keypress events. Is the text clear enough?

TODO: dead keys pressed twice fire two keypress events. Dead keys followed  
by space fire keydown space, keypress for the dead key's accent, keyup  
space (!). Dead keys are currently a bit underspecified in the above text.


USEFUL LINKS ON WINDOWS KEY HANDLING
http://www.microsoft.com/globaldev/handson/dev/inputinwin.mspx
http://www.woodmann.com/fravia/sources/WINUSER.H
http://tonesnotes.com/blog/2003/03/keycodes_defined_in_winuserh.html
http://www.microsoft.com/globaldev/tools/msklc.mspx

-- 
Hallvord R. M. Steen
Core QA JavaScript tester, Opera Software
http://www.opera.com/
Opera - simply the best Internet experience


Received on Saturday, 26 April 2008 14:07:07 GMT

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