flash本身失去焦点的事件：Event.DEACTIVATE

// initialize arrow variables
var leftArrow:Boolean = false;
var rightArrow:Boolean = false;
var upArrow:Boolean = false;
var downArrow:Boolean = false;

var mascot:Mascot = new Mascot;
mascot.x = 200;
mascot.y = 200;
addChild(mascot);

stage.addEventListener(KeyboardEvent.KEY_DOWN, keyPressedDown);
stage.addEventListener(KeyboardEvent.KEY_UP, keyPressedUp);
stage.addEventListener(Event.ENTER_FRAME, moveMascot);
stage.addEventListener(Event.DEACTIVATE, missingStage);

// set arrow variables to true
function keyPressedDown(event:KeyboardEvent)
{
    if (event.keyCode == 37)
    {
        leftArrow = true;
    }
    else if (event.keyCode == 39)
    {
        rightArrow = true;
    }
    else if (event.keyCode == 38)
    {
        upArrow = true;
    }
    else if (event.keyCode == 40)
    {
        downArrow = true;
    }
}

// set arrow variables to false
function keyPressedUp(event:KeyboardEvent)
{
    if (event.keyCode == 37)
    {
        leftArrow = false;
    }
    else if (event.keyCode == 39)
    {
        rightArrow = false;
    }
    else if (event.keyCode == 38)
    {
        upArrow = false;
    }
    else if (event.keyCode == 40)
    {
        downArrow = false;
    }
}

// set arrow variables to false
function missingStage(e:Event)
{

    leftArrow = false;

    rightArrow = false;

    upArrow = false;

    downArrow = false;

}

// move every frame
function moveMascot(event:Event)
{
    var speed:Number = 5;
    if (leftArrow)
    {
        mascot.x -= speed;
    }
    if (rightArrow)
    {
        mascot.x += speed;
    }
    if (upArrow)
    {
        mascot.y -= speed;
    }
    if (downArrow)
    {
        mascot.y += speed;
    }
}