Reproduce the catapult with elastic Angry bird SpriteKit - ios7

Question

I want create a slingshot like Angry Bird and my question is how can I create a elastic which follow my sprite (a bird for example) when I load the launched and which hang to my slingshot ?
How can I do that with SpriteKit ?
Thank you for your help.

Answer 1

This kind of logic coding you really need to figure out on your own. I will give you something to work with. This is from my personal prototype:
-(void)fireRocket
{
if (timesRan <= 0) {
timesRan++;
}
else
{
[self enumerateChildNodesWithName:#"rocket" usingBlock: ^(SKNode *node, BOOL *stop) {
SKSpriteNode *rocket = (SKSpriteNode *) node;
[rocket removeFromParent];
[rocket removeAllActions];
}];
}
SKSpriteNode * rocket = [SKSpriteNode spriteNodeWithImageNamed:#"rocket-310663_150"];
rocket.size = CGSizeMake(30, 32);
rocket.name = #"rocket";
rocket.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:16];
rocket.position = CGPointMake(50, 185);
//CGFloat angle = M_PI_4;
CGFloat magnitude = 26;
CGFloat angle = [self angleOfRocketFire];
//CGFloat angle = M_PI_4;
[self addChild:rocket];
[rocket.physicsBody applyImpulse:CGVectorMake(magnitude*cos(angle),
magnitude*sin(angle))];
}
-(CGFloat)angleOfRocketFire
{
__block CGFloat angle;
__block CGPoint handlePosition;
[self enumerateChildNodesWithName:#"handle" usingBlock: ^(SKNode *node, BOOL *stop) {
SKSpriteNode *handle = (SKSpriteNode *) node;
CGFloat xCoord = handle.position.x + (handle.size.width / 2);
CGFloat yCoord = handle.position.y + (handle.size.height / 2);
handlePosition = CGPointMake(xCoord, yCoord);
}];
[self enumerateChildNodesWithName:#"rocketLauncher" usingBlock: ^(SKNode *node, BOOL *stop) {
SKSpriteNode *rocketLauncher = (SKSpriteNode *) node;
CGFloat xCoord = rocketLauncher.position.x + (rocketLauncher.size.width / 2);
CGFloat yCoord = rocketLauncher.position.y + (rocketLauncher.size.height / 2);
CGPoint launcherPosition = CGPointMake(xCoord, yCoord);
CGFloat slope = (handlePosition.y - launcherPosition.y) / (handlePosition.x - launcherPosition.x);
//angle = atan(slope);
angle = atan(slope);
NSLog(#"%f", slope);
}];
return angle;
}