Starling handbook Preview

Don’t forget to assign birdVelocity a value (zero, in this case). AS3 has a very
peculiar default value for variables of type Number: NaN (not a number). So much
for language designers not having a sense of humor!
The next step is to update birdVelocity each frame, and to move the bird by that velocity.
public function advanceTime(passedTime:Number):void
{
/* ... */
}
advancePhysics(passedTime); 1
private function advancePhysics(passedTime:Number):void
{
_bird.y += _birdVelocity * passedTime; 2
_birdVelocity += GRAVITY * passedTime; 3
}
public function flapBird():void 4
{
_birdVelocity = FLAP_VELOCITY;
}
1 The new method advancePhysics will contain our tiny physics engine.
2 Move the bird along the y-axis, according to its current velocity.
3 Add the GRAVITY to the bird’s current velocity.
4 This method will be called when the player touches the screen. It instantly sets the bird’s
velocity to a fixed value.
That’s actually all our physics code. You probably agree that adding a full-fledged physics library
would have been an overkill in this situation!
There’s one more step to do: allowing the player to control the bird, i.e. to call that new flapBird
method. Please open up the Game class and make the following modifications:
20