Animate SKSpriteNode in circle, pause, and reverse? - ios

Question

I am trying to move a SKSpriteNode in a circular path, pause that animation, and then reverse it from where it left off.
Here is what I have so far.
CGPathRef circle = CGPathCreateWithEllipseInRect(CGRectMake(self.frame.size.width/2-75,self.frame.size.height/2-75,150,150), NULL);
self.circlePathActionClockwise = [SKAction followPath:circle asOffset:NO orientToPath:YES duration:5.0];
self.circlePathActionClockwise = [SKAction repeatActionForever:self.circlePathActionClockwise];
[self.ball runAction:self.circlePathActionClockwise];
self.ball.speed = 1.0;
This animated the ball in a circle. Then when I want to pause and reverse, I try:
[self.ball runAction:[self.circlePathActionClockwise reversedAction]];
self.ball.speed = 1.0;
This reverses the animation, but it does not start it from where it left off. Instead it treats it was a new animation and starts from the beginning.
Is there a way I can start an action from self.ball's current position?
Thanks a lot!

Answer 1

The steps to reverse the direction of a sprite that is following a circular path are
Compute the angle of the sprite at the stopping point (see point a and angle theta in the Figure 1)
Create a new circular path that starts at angle theta
Run an action in the opposite direction
Figure 1. A sprite following a counter-clockwise circular path
Here's an example of how to do that:
Step 1: compute the starting angle of the new path based on the current position of the sprite.
- (CGFloat) angleOnCircleWithPosition:(CGPoint)position andCenter:(CGPoint)center
{
CGFloat dx = position.x - center.x;
CGFloat dy = position.y - center.y;
return atan2f(dy, dx);
}
Step 2: create a circular path given a center point and radius. Create the path starting from the specified angle (in radians).
- (CGPathRef) circlePathRefWithCenter:(CGPoint)center radius:(CGFloat)radius andStartingAngle:(CGFloat)startAngle
{
CGMutablePathRef circlePath = CGPathCreateMutable();
CGPathAddRelativeArc(circlePath, NULL, center.x, center.y, radius, startAngle, M_PI*2);
CGPathCloseSubpath(circlePath);
return circlePath;
}
You will need to declare the following instance variables:
BOOL clockwise;
SKSpriteNode *sprite;
CGFloat circleRadius;
CGPoint circleCenter;
Step 3: Reverse the direction of the sprite when the user taps the screen. It assumes that you've created a sprite and added it to the scene and set the radius and center of the circle path.
- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
[sprite removeActionForKey:#"circle"];
CGFloat angle = [self angleOnCircleWithPosition:sprite.position andCenter:circleCenter];
CGPathRef path = [self circlePathRefWithCenter:circleCenter radius:circleRadius andStartingAngle:angle];
SKAction *followPath = [SKAction followPath:path asOffset:NO orientToPath:YES duration:4];
if (clockwise) {
[sprite runAction:[SKAction repeatActionForever:followPath] withKey:#"circle"];
}
else {
[sprite runAction:[[SKAction repeatActionForever:followPath] reversedAction] withKey:#"circle"];
}
// Toggle the direction
clockwise = !clockwise;
}