Simple SpriteKit Slingshot

SlingScene.m

//
//  SlingScene.m
//
//  Demonstrates a simple slingshot mechanism.
//
//  Created by Andrew Clissold on 5/3/14.
//  Copyright (c) 2014 Andrew Clissold. All rights reserved.
//

/* Contents of SlingScene.h:
#import <SpriteKit/SpriteKit.h>
@interface SlingScene : SKScene
@end */

#import "SlingScene.h"

@interface SlingScene ()

@property (nonatomic, strong) SKAction *slingAction;

@end

@implementation SlingScene

- (instancetype)initWithSize:(CGSize)size {
    self = [super initWithSize:size];
    if (self) {
        CGPoint center = CGPointMake(self.size.width/2.0, self.size.height/2.0);

        self.slingAction = [SKAction sequence:
                            @[[SKAction waitForDuration:0.1],
                              [SKAction runBlock:
                               ^{
                                   [self.physicsWorld removeAllJoints];
                               }
                               ]]];

        // Create a square, which will be slung by the spring
        SKSpriteNode *square =
            [SKSpriteNode spriteNodeWithColor:[SKColor whiteColor]
                                         size:CGSizeMake(60.0, 60.0)];
        square.position = CGPointMake(center.x, center.y - 2*square.size.height);
        square.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:square.size];
        square.physicsBody.collisionBitMask = 0;
        square.name = @"square";

        // Create a post to anchor the square to
        SKShapeNode *post = [SKShapeNode node];
        post.path = CGPathCreateWithEllipseInRect(
                CGRectMake(0.0, 0.0, 60.0, 60.0), NULL);
        post.fillColor = [SKColor brownColor];
        post.strokeColor = [SKColor brownColor];
        post.position = CGPointMake(center.x-30.0, center.y-30.0);
        post.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:60.0 center:center];

        // Give the post a near-infinite mass so the square won't tug at it
        // and move it around
        post.physicsBody.mass = 1000000;
        post.physicsBody.affectedByGravity = NO;

        // Set their collision bit masks to the same value to allow them to pass
        // through each other
        post.physicsBody.collisionBitMask = 0;
        square.physicsBody.collisionBitMask = 0;

        // Add them to the scene
        [self addChild:post];
        [self addChild:square];

        // Connect them via a spring
        SKPhysicsJointSpring *spring =
            [SKPhysicsJointSpring jointWithBodyA:post.physicsBody
                                           bodyB:square.physicsBody
                                         anchorA:center
                                         anchorB:square.position];
        spring.damping = 0.4;
        spring.frequency = 1.0;
        [self.physicsWorld addJoint:spring];

        // Lower gravity from default {0.0, -9.8} to allow the
        // square to be slung farther
        self.physicsWorld.gravity = CGVectorMake(0.0, -4.0);
    }
    return self;
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    // Move the square to the touch position
    SKSpriteNode *square = (SKSpriteNode *)[self childNodeWithName:@"square"];
    CGPoint location = [[touches anyObject] locationInNode:self];
    [square runAction:[SKAction moveTo:location duration:0.1]];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    // Sling the square by
    //   1. allowing the spring to accelerate it, and
    //   2. removing the spring altogether
    [self runAction:self.slingAction];
}

@end

This can easily be played around with in an existing SpriteKit app:

SlingScene *slingScene = [[SlingScene alloc] initWithSize:self.view.bounds.size];
SKView *spriteView = (SKView *)self.view;
[spriteView presentScene:slingScene];