Spritekit physics: One ball hit another ball

Question

I'm all new to SpriteKit.

I'm trying to solve this problem: I have two balls in my scene and when I drag one ball an "hit" the other ball, this ball should roll away using the correct physics.

In my test code I only can "move" the second ball, he is not using the "force" of the hit...

Here is my scene code:

#import "HittingScene.h"

@interface HittingScene()

@property (nonatomic, strong) SKShapeNode *targetBall;
@property (nonatomic, strong) SKShapeNode *mainBall;
@property (nonatomic, weak) SKShapeNode *draggedNode;

@end

@implementation HittingScene

- (id)initWithSize:(CGSize)size
{
    if (self = [super initWithSize:size]) {
        [self addChild:[self createMainBall]];
        [self addChild:[self createTargetBall]];

        [self setPhysicsBody:[SKPhysicsBody bodyWithEdgeLoopFromRect:self.frame]];
    }
    return self;
}

- (SKShapeNode *)createTargetBall
{
    self.targetBall = [SKShapeNode node];

    CGPathRef path = CGPathCreateWithEllipseInRect((CGRect){{-20, -20}, {40, 40}}, NULL);
    [self.targetBall setPath:path];
    CGPathRelease(path);

    [self.targetBall setPosition:CGPointMake(200, 200)];
    [self.targetBall setName:@"targetBall"];

    [self.targetBall setPhysicsBody:[SKPhysicsBody bodyWithCircleOfRadius:20.0]];
    self.targetBall.physicsBody.dynamic = YES;
    self.targetBall.physicsBody.affectedByGravity = NO;
    self.targetBall.physicsBody.restitution = 0.7;

    return self.targetBall;
}

- (SKShapeNode *)createMainBall
{
    self.mainBall = [SKShapeNode node];

    CGPathRef path = CGPathCreateWithEllipseInRect((CGRect){{-20, -20}, {40, 40}}, NULL);
    [self.mainBall setPath:path];
    CGPathRelease(path);

    [self.mainBall setPosition:CGPointMake(100, 100)];
    [self.mainBall setName:@"mainBall"];

    [self.mainBall setPhysicsBody:[SKPhysicsBody bodyWithCircleOfRadius:20.0]];
    self.mainBall.physicsBody.dynamic = YES;
    self.mainBall.physicsBody.affectedByGravity = NO;
    self.mainBall.physicsBody.restitution = 0.7;

    return self.mainBall;
}

- (void)touchesBegan:(NSSet*) touches withEvent:(UIEvent*) event
{
    self.draggedNode = (SKShapeNode *)[self nodeAtPoint:[[touches anyObject]     locationInNode:self]];
}

- (void)touchesMoved:(NSSet*) touches withEvent:(UIEvent*) event
{
    self.draggedNode.position = [[touches anyObject] locationInNode:self];
}

- (void)touchesEnded:(NSSet*) touches withEvent:(UIEvent*) event
{
    self.draggedNode = nil;
}
@end

Anyone with an idea how to solve this?

Thanks, Urkman

Answer 1

And here is a possible solution:

#import "HittingScene.h"

@interface HittingScene() <SKPhysicsContactDelegate>

@property (nonatomic, strong) SKShapeNode *targetBall;
@property (nonatomic, strong) SKShapeNode *mainBall;
@property (nonatomic) CGPoint lastTouch;
@property (nonatomic) bool isDragging;

@end

@implementation HittingScene

static const uint32_t mainBallCategory = 0x1 << 0;
static const uint32_t targetBallCategory = 0x1 << 1;

- (id)initWithSize:(CGSize)size
{
    if (self = [super initWithSize:size]) {

        self.physicsWorld.gravity = CGVectorMake(0, 0);

        [self addChild:[self createMainBall]];
        [self addChild:[self createTargetBall]];

        [self setPhysicsBody:[SKPhysicsBody bodyWithEdgeLoopFromRect:self.frame]];
        self.physicsWorld.contactDelegate = self;
        [self.targetBall.physicsBody applyForce:CGVectorMake(30, 30)];
    }
    return self;
}

- (SKShapeNode *)createTargetBall
{
    self.targetBall = [SKShapeNode node];

    CGPathRef path = CGPathCreateWithEllipseInRect((CGRect){{-20, -20}, {40, 40}}, NULL);
    [self.targetBall setPath:path];
    CGPathRelease(path);

    [self.targetBall setPosition:CGPointMake(200, 200)];
    [self.targetBall setName:@"targetBall"];

    [self.targetBall setPhysicsBody:[SKPhysicsBody bodyWithCircleOfRadius:20.0]];
    self.targetBall.physicsBody.dynamic = YES;
    self.targetBall.physicsBody.affectedByGravity = YES;
    self.targetBall.physicsBody.restitution = 0.7;
    self.targetBall.physicsBody.categoryBitMask = targetBallCategory;
    self.targetBall.physicsBody.contactTestBitMask = mainBallCategory;

    return self.targetBall;
}

- (SKShapeNode *)createMainBall
{
    self.mainBall = [SKShapeNode node];

    CGPathRef path = CGPathCreateWithEllipseInRect((CGRect){{-20, -20}, {40, 40}}, NULL);
    [self.mainBall setPath:path];
    CGPathRelease(path);

    [self.mainBall setPosition:CGPointMake(100, 100)];
    [self.mainBall setName:@"mainBall"];

    [self.mainBall setPhysicsBody:[SKPhysicsBody bodyWithCircleOfRadius:20.0]];
    self.mainBall.physicsBody.dynamic = NO;
    self.mainBall.physicsBody.affectedByGravity = NO;
    self.mainBall.physicsBody.restitution = 0.7;
    self.mainBall.physicsBody.categoryBitMask = mainBallCategory;

    return self.mainBall;
}    

- (void)touchesBegan:(NSSet*) touches withEvent:(UIEvent*) event
{
    if ((SKShapeNode *)[self nodeAtPoint:[[touches anyObject] locationInNode:self]] == self.mainBall) {
        self.isDragging = YES;
    }
}

- (void) didBeginContact:(SKPhysicsContact *)contact
{
    if (self.isDragging) {
        float angle = atan2f (self.lastTouch.y - self.mainBall.position.y, self.lastTouch.x - self.mainBall.position.x) ;
        float distance = hypotf(self.lastTouch.x - self.mainBall.position.x, self.lastTouch.y - self.mainBall.position.y);

        // calculate vector
        CGFloat thrust = distance;
        CGVector thrustVector = CGVectorMake(thrust*cosf(angle), thrust*sinf(angle));

        // apply force or impluse to target node

        // force is not working
        // [self.targetBall.physicsBody applyForce:thrustVector];

        // impulse is working
        [self.targetBall.physicsBody applyImpulse:thrustVector];
    }
}

- (void)touchesMoved:(NSSet*) touches withEvent:(UIEvent*) event
{
    self.lastTouch = [[touches anyObject] locationInNode:self];
    if (self.isDragging) {
        [self.mainBall runAction:[SKAction moveTo:[[touches anyObject] locationInNode:self] duration:0.01]];
    }
}

- (void)touchesEnded:(NSSet*) touches withEvent:(UIEvent*) event
{
    self.isDragging = NO;
}

@end

Answer 2

I think the problem is that you're just changing the position of the ball on touch. Doing so just places the node at a new point and doesn't actually involve any force. You will need to set up a collision system where a certain amount of force is applied to the target ball when the main ball collides with it.

applyForce:atPoint: in SKPhysicsBody looks like a good place to start.

https://developer.apple.com/library/ios/documentation/SpriteKit/Reference/SKPhysicsBody_Ref/Reference/Reference.html