How to detect a held key in a flash game?

Question

What is the correct way to detect held keys in a flash game? For example, I want to know that the right arrow is held to move the player.

Naive code:

function handleKeyDown(event:KeyboardEvent) {
    held[event.keyCode] = true;
}

function handleKeyUp(event:KeyboardEvent) {
    held[event.keyCode] = false;
}

stage.addEventListener(KeyboardEvent.KEY_DOWN, handleKeyDown);
stage.addEventListener(KeyboardEvent.KEY_UP, handleKeyUp);

The naive code has problems on some computers. The KEY_DOWN event is alternating with KEY_UP many times for a held key there. That makes the key appear to be released in some frames.

An example of the seen events:

[Just holding a single key.]
KEY_DOWN,KEY_UP,KEY_DOWN,KEY_UP,KEY_DOWN,KEY_UP,...

Answer 1

here's a quick fix , with the limitation that it can only work one key at a time

var currentKey:uint;

function handleKeyDown(event:KeyboardEvent) {
    held[event.keyCode] = true;

    //make sure the currentKey value only changes when the current key 
    //has been released. The value is set to 0 , 
    //but it should be any value outside the keyboard range
    if( currentKey == 0 )
    {
        currentKey = event.keyCode;

       //limitation: this can only work for one key at a time
       addEventListener(Event.ENTER_FRAME , action );
    }
}

function handleKeyUp(event:KeyboardEvent) {
    held[event.keyCode] = false;

    if( currentKey != 0 )
    {
        //reset
        removeEventListener(Event.ENTER_FRAME , action );
        currentKey = 0;
    }
}

function action(event:Event):void
{
   //your code here
}

stage.addEventListener(KeyboardEvent.KEY_DOWN, handleKeyDown);
stage.addEventListener(KeyboardEvent.KEY_UP, handleKeyUp);

Answer 2

My workaround is to remember the keys that were seen down at least once in this frame.

function handleKeyDown(event:KeyboardEvent) {
    held[event.keyCode] = true;
    justPressed[event.keyCode] = true;
}

function handleKeyUp(event:KeyboardEvent) {
    held[event.keyCode] = false;
}

// You should clear the array of just pressed keys at the end
// of your ENTER_FRAME listener.
function clearJustPressed() {
    justPressed.length = 0;
}

And I use a function to check if a key was down in this frame:

function pressed(keyCode:int) {
    return held[keyCode] || justPressed[keyCode];
}