fluidly dealing with multiple keypresses in ActionScript3

问题

I'm currently building an engine for a platformer game at the moment, but I've noticed that ActionScript3 is having difficulty in keeping fluid when multiple keypresses are in use. For example;

        function onKeyDown(event:KeyboardEvent):void {
        if (event.keyCode == Keyboard.UP || event.keyCode == Keyboard.W || event.keyCode == Keyboard.SPACE) {
            if (isTouchingGround()) {
                isJumping = true;
                yv = -100;
            }
        } else if (event.keyCode == Keyboard.DOWN || event.keyCode == Keyboard.D) {
            if (xv == 0) {
                player.gotoAndPlay(275);
            }
        } else if (event.keyCode == Keyboard.LEFT || event.keyCode == Keyboard.A) {
            if (xv == 0) {
                xv = -24;
            } else if (xv != -120) {
                xv-=2;
            }
        } else if (event.keyCode == Keyboard.RIGHT || event.keyCode == Keyboard.D) {
            if (xv == 0) {
                xv = 24;
            } else if (xv != 120) {
                xv+=2;
            }
        }
    }

So, as listed above, using the UP (or W, or Space) key triggers the player to jump (seperate onframe event handler handles gravity etc). Using the RIGHT (or D..) key triggers increases the player acceleration, which is again applied to the player in a seperate onframe event handler.

Everything works fine by itself - but the problem arises when multiple keystrokes are used. If a player starts to move to the right, and hits jump, he will cease accelerating. At the same time, he will not decelerate, as instructed in the Keyboard.UP method. Instead, he will maintain constant at his current rate, until the RIGHT key is hit again.

In short, it is as though Actionscript begins ignoring both the keyboard.down and keyboard.up methods for the RIGHT or LEFT movement keys, until they are no longer being pressed. This obviously causes for some very rigid gameplay - is there any solution anyone would be willing to share with me on this?

回答1:

Your problem lies in the fact that your if conditionals are followed by if else conditionals. Drop the else and just have the if conditionals. Basically if the user holds down space then none of the other if conditionals are going to be tested since space is being held down and it's the first if statement. So just drop the else off of the if's. Just remove the if else conditionals that are testing keystrokes, not the conditionals inside of the if statements that deal with keystrokes.

Here is what your code should look like:

function onKeyDown(event:KeyboardEvent):void {
    if (event.keyCode == Keyboard.UP || event.keyCode == Keyboard.W || event.keyCode == Keyboard.SPACE) {
        if (isTouchingGround()) {
            isJumping = true;
            yv = -100;
        }
    }
    if (event.keyCode == Keyboard.DOWN || event.keyCode == Keyboard.D) {
        if (xv == 0) {
            player.gotoAndPlay(275);
        }
    }
    if (event.keyCode == Keyboard.LEFT || event.keyCode == Keyboard.A) {
        if (xv == 0) {
            xv = -24;
        } else if (xv != -120) {
            xv-=2;
        }
    }
    if (event.keyCode == Keyboard.RIGHT || event.keyCode == Keyboard.D) {
        if (xv == 0) {
            xv = 24;
        } else if (xv != 120) {
            xv+=2;
        }
    }
}

Something else you may notice is that when the UP key and RIGHT key are both being held down, the computer seems to freeze keyboard input, however when the W key and D key are being held down you can still press other keys and the computer will register their input. The answer to that question is here.

Update:

For the fluid part, instead of triggering something when a keystroke takes place, it is better to have a boolean variable such as keyUP or UP that holds a true if the key is down or false when the key is up. Then have a function onEnterFrame(event:Event):void {} that performs an action when keyUP is true. Like so:

import flash.events.*;

public class keyEvents extends MovieClip {
    private var keyRIGHT:Boolean = false;

    public function keyEvents():void
    {
        this.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);
        this.addEventListener(KeyboardEvent.KEY_UP, onKeyUp);
        this.addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }

    function onKeyDown(event:KeyboardEvent):void
    {
        if(event.keyCode == Keyboard.RIGHT) {
            this.keyRIGHT = true;
        }
    }

    function onKeyUp(event:KeyboardEvent):void
    {
        if(event.keyCode == Keyboard.RIGHT) {
            this.keyRIGHT = false;
        }
    }

    function onEnterFrame(event:Event):void
    {
        if(this.keyRIGHT) {
            // This code is executed while the RIGHT arrow key is down.
        }
    }
}

If the above code does not work I think that your problem lies with your keyboard, not that it's broken or anything but the way it was made might be messing things up.

Let me know if this didn't help and I'll continue trying.

来源：https://stackoverflow.com/questions/14166083/fluidly-dealing-with-multiple-keypresses-in-actionscript3