A Completed Game: Belt Commander - Beginning iOS6 Games Development - page 290

if (point.y < self.center.y){
[self setState:VPR_STATE_UP];
} else if (point.y > self.center.y){
[self setState:VPR_STATE_DOWN];
}
motion = [LinearMotion linearMotionFromPoint:[self center] toPoint:point AtSpeed:1.2f];
[motion setDelegate:self];
[motion setStopAtPoint:YES];
[motion setPointToStopAt:point];
[motion setStayInRect:YES];
CGSize gameSize = [gameController gameAreaSize];
[motion setRectToStayIn:CGRectMake(63, 31, 2, gameSize.height - 63)];
[motion setWrap:NO];
[[self behaviors] addObject:motion];
setMoveToPoint:within: . In this task, the goal is to set up a
object that will move the Viper to the specified point. The first step is to see if we have
LinearMotion object stored at the variable motion. If so, we want to remove it from our
CGPoint called point, based on the CGPoint
find a point that keeps the Viper horizontally fixed. Once we know where our point is, we can set our
state to either VPR_STATE_UP or VPR_STATE_DOWN .
Creating the new LinearMotion is as simple as passing the Viper's current center and point to the
constructor linearMotionFromPoint:toPoint:AtSpeed: . Setting self as the delegate , and telling it to
stop when it reaches the specified point, further modifies the LinearMotion . The last modification is
to set LinearMotion so that it stays within the specified rectangle. This prevents the Viper from going
outside the bounds of the screen.
We have looked at how the Viper moves about the screen. The rest of the interesting features are
implemented in the task step: , as shown in Listing 12-20.
Listing 12-20. Viper.m (step:)
-(void)step:(GameController*)gameController{
long stepNumber = [gameController stepNumber];
if (stepNumber - lastShot > stepsPerShot - damage){
[gameController playAudio:AUDIO_BLIP];
CGPoint center = [self center];
CGSize gameSize = [gameController gameAreaSize];