ActionScript3: What code should I use to stop the player controlled sprite from moving? - actionscript-3

Question

I'm very new to ActionScript3 and am making an asteroids-type game. Right now, the ship continues floating in a straight line when you let go of the movement buttons, and I want to be able to stop that from happening. I'm thinking either a dedicated button for braking, like the b key, or if the keys are not pressed to stop movement, whichever would be easier. Like I said I'm really new to AS3 so not even sure what part of my code is making them keep flying in a straight line. Here is the code to control movement for reference:
// register key presses
public function keyDownFunction(event:KeyboardEvent) {
if (event.keyCode == 37) {
leftArrow = true;
} else if (event.keyCode == 39) {
rightArrow = true;
} else if (event.keyCode == 38) {
upArrow = true;
//Add event listener for down arrow
} else if (event.keyCode == 40) {
downArrow = true;
// show thruster
if (gameMode == "play") ship.gotoAndStop(2);
} else if (event.keyCode == 32) { // space
var channel:SoundChannel = shootSound.play();
newMissile();
} else if (event.keyCode == 90) { // z
startShield(false);
var channel:SoundChannel = shieldSound.play();
}
}
// register key ups
public function keyUpFunction(event:KeyboardEvent) {
if (event.keyCode == 37) {
leftArrow = false;
} else if (event.keyCode == 39) {
rightArrow = false;
} else if (event.keyCode == 38) {
upArrow = false;
//Add listener for down arrow
} else if (event.keyCode == 40) {
downArrow = false;
// remove thruster
if (gameMode == "play") ship.gotoAndStop(1);
}
}
// animate ship
public function moveShip(timeDiff:uint) {
// rotate and thrust
if (leftArrow) {
ship.rotation -= shipRotationSpeed*timeDiff;
} else if (rightArrow) {
ship.rotation += shipRotationSpeed*timeDiff;
} else if (upArrow) {
shipMoveX += Math.cos(Math.PI*ship.rotation/180)*thrustPower;
shipMoveY += Math.sin(Math.PI*ship.rotation/180)*thrustPower;
//Added down arrow movement to allow player to move backwards
} else if (downArrow) {
shipMoveX -= Math.cos(Math.PI*ship.rotation/180)*thrustPower;
shipMoveY -= Math.sin(Math.PI*ship.rotation/180)*thrustPower;
}
// move
ship.x += shipMoveX;
ship.y += shipMoveY;

Answer 1

I agree that there is a little logic problem in your code. Your ship will move on 'til
judgement day because the two variables responsible for the speed of it's motion - shipMoveX
and shipMoveY - aren't automatically degraded over time.
Now there are literally thousands of ways of how this could be achieved but let's keep things
simple.
You are using a class variable called thrustPower - make sure to set it to 0.1 and that the value of both shipMoveX & shipMoveY is 0.
Additionally add these class variables:
private var thrustHorizontal:Number = 0;
private var thrustVertical:Number = 0;
private var speed:Number = 0;
private var maxSpeed:Number = 5;
private var decay:Number = 0.97;
Replace the moveShip function with this:
public function moveShip(timeDiff:uint):void
{
thrustHorizontal = Math.sin(Math.PI * ship.rotation / 180);
thrustVertical = Math.cos(Math.PI * ship.rotation / 180);
// rotate and thrust
if (leftArrow)
{
ship.rotation -= shipRotationSpeed * timeDiff;
}
else if (rightArrow)
{
ship.rotation += shipRotationSpeed * timeDiff;
}
else if (upArrow)
{
shipMoveX += thrustPower * thrustHorizontal;
shipMoveY += thrustPower * thrustVertical;
}
else if (downArrow)
{
shipMoveX -= thrustPower * thrustHorizontal;
shipMoveY -= thrustPower * thrustVertical;
}
if (!upArrow && !downArrow)
{
shipMoveX *= decay;
shipMoveY *= decay;
}
speed = Math.sqrt((shipMoveX * shipMoveX) + (shipMoveY * shipMoveY));
if (speed > maxSpeed)
{
shipMoveX *= maxSpeed / speed;
shipMoveY *= maxSpeed / speed;
}
ship.x += shipMoveX;
ship.y -= shipMoveY;
}
As you can see, there is this new if-block:
if (!upArrow && !downArrow)
{
shipMoveX *= decay;
shipMoveY *= decay;
}
Here we're handling the case, if the player neither pressed up nor down and multiply
the spaceships horizontal/vertical speed by decay. If you scroll back a bit, you notice
it's value is 0.97.
You can generally say, if you multiply a positive number x by 0 < y < 1, x will become smaller.
So if you're spaship is currently moving horizontally at 3 pixels per frame (shipMoveX=3 ; shipMoveY=0)
it will become 2.91 the next frame. The next frame it will be 2.8227, then 2.738019 ... and so on before it will finally reach zero in infinity.