Question

I’m trying to create a boundary for a player object, controlled with arrow keys, in my game using the main stage’s height and width. For example, one test point is at the top edge of the player object’s bounding box so that when the player object’s head touches the stage’s top edge, the player can’t move anymore to the north. The player object is manually instantiated to the center of the stage by using the Flash stage editor so it will start at the center before the program starts.

The problem is that right at the start of the program, I can no longer move the player object up or down with the arrow keys but I can still move it left or right. The intention is to allow the player to move north until the player object’s head touches the top edge of the main stage. Here’s the code:

package 
{
        public class Main_Playground extends MovieClip
        {
        var vx:int;
        var vy:int;

        public function Main_Playground()
        {
            init();
        }
        function init():void
        {
            //initialize variables
            vx = 0;
            vy = 0;

            //Add event listeners
            stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);
            stage.addEventListener(KeyboardEvent.KEY_UP, onKeyUp);
            addEventListener(Event.ENTER_FRAME, onEnterFrame);
        }

        function onKeyDown(event:KeyboardEvent):void
        {
            if (event.keyCode == Keyboard.LEFT)
            {
                vx = -5;
            }
            else if (event.keyCode == Keyboard.RIGHT)
            {
                vx = 5;
            }
            else if (event.keyCode == Keyboard.UP)
            {
                vy = -5;
            }
            else if (event.keyCode == Keyboard.DOWN)
            {
                vy = 5;
            }
        }
        function onKeyUp(event:KeyboardEvent):void
        {
            if (event.keyCode == Keyboard.LEFT || event.keyCode == Keyboard.RIGHT)
            {
                vx = 0;
            }
            else if (event.keyCode == Keyboard.DOWN || event.keyCode == Keyboard.UP)
            {
                vy = 0;
            }
        }
        function onEnterFrame(event:Event):void
        {
            //Move the player
            player.x += vx;
            player.y += vy;

            //determine top boundary
            if (! stage.hitTestPoint(player.x, (player.y-(player.height/2)), true ) ){
                player.y -= vy;
            }
        }
    }
}

Answer 1

Using the stage object with the shape flag set to true is will produce errors: You test, if any of the actual pixels rendered on the stage hits the point (which will probably return false, unless you have objects outside of the visible stage area, at exactly the point specified).

You can, of course, set that to false and try again (which would work better, but still leave the problem that you are testing against the bounding box around everything that is rendered on stage, rather than the actual stage area), but might I suggest a different approach?

It is more efficient, especially because your sprite is probably far smaller than the stage, to test the player’s bounding box against the stage boundaries:

function onEnterFrame (ev:Event) : void {
    player.x += vx;
    player.y += vy;

    var playerBounds:Rectangle = player.getBounds(stage);
    if (playerBounds.left < 0 || playerBounds.right > stage.stageWidth) player.x -= vx;
    if (playerBounds.top < 0 || playerBounds.bottom > stage.stageHeight) player.y -= vy;
}

The player has to be located inside the visible stage area at startup, of course, and you might have to set focus to the stage to make sure keyboard events are captured.