CODEWITHSUNDEEP
iosobjective-csprite-kitskphysicsjoint
user1272965
user1272965

Reputation: 3064

Simple pin joint in SpriteKit doesn't work as expected

I'd like to make a pendulum. Starting with an SKScene and everything defaulted, I do the following...

- (void)createSceneContents {
    self.physicsBody = [SKPhysicsBody bodyWithEdgeLoopFromRect:self.view.bounds];

    // object 1 is the fulcrum
    SKSpriteNode *object1 = [SKSpriteNode spriteNodeWithColor:[UIColor redColor] size:CGSizeMake(5, 5)];
    [self addChild:object1];
    object1.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:object1.frame.size];
    object1.physicsBody.dynamic = NO;
    object1.position = self.view.center;

    // object2 is like a broomstick, which I will pin to the fulcrum
    SKSpriteNode *object2 = [SKSpriteNode spriteNodeWithColor:[UIColor greenColor] size:CGSizeMake(90, 2)];
    [self addChild:object2];
    object2.anchorPoint = CGPointMake(0.0, 0.5);
    object2.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:object2.frame.size];
    object2.position = self.view.center;

    // pin the physics bodies
    SKPhysicsJointPin *pinJoint = [SKPhysicsJointPin jointWithBodyA:object1.physicsBody bodyB:object2.physicsBody anchor:object1.position];
    [self.physicsWorld addJoint:pinJoint];
}

If I don't add the pin logic, as expected the fulcrum stays in the middle of the scene and the broomstick falls to the floor, so I know the broomstick is subject to gravity. After adding the pin, I get this:

enter image description here

No motion. Why? This causes no motion either... 

[object2.physicsBody applyForce:CGVectorMake(0, -5)];

I expect the broomstick to swing and oscillate because it's pinned to the fulcrum. I've seen articles about positioning the nodes first, before joints, but I've done that. Am I wrong to expect the broom to swing? How can I get it to do so?

Upvotes: 3

Views: 745

Answers (2)

Maetschl
Maetschl

Reputation: 1339

Node 2 have bad defined anchorPoint, here an upload an example:

Full code of example

Image:

3 Seconds running code

Swift 3 code:

    let nodeSize = CGSize(width: 10, height: 10)
    let node = SKSpriteNode(color: .red, size: nodeSize)
    node.physicsBody = SKPhysicsBody(rectangleOf: nodeSize)
    node.physicsBody?.isDynamic = false
    self.addChild(node)

    let node2Size = CGSize(width: 60, height: 8)
    let node2 = SKSpriteNode(color: .green, size: node2Size)
    node2.position = CGPoint(x: 30, y: 0)
    node2.physicsBody = SKPhysicsBody(rectangleOf: node2Size)
    node2.physicsBody?.mass = 1.0
    self.addChild(node2)


    let a = SKPhysicsJointPin.joint(withBodyA: node.physicsBody! , bodyB: node2.physicsBody!, anchor: CGPoint(x: 0.0, y: 0.0))
    self.physicsWorld.add(a)

Objective-C:

SKSpriteNode *object1 = [SKSpriteNode spriteNodeWithColor:[UIColor redColor] size:CGSizeMake(5, 5)];
[self addChild:object1];
object1.position = self.view.center;
object1.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:object1.frame.size];
object1.physicsBody.dynamic = NO;

SKSpriteNode *object2 = [SKSpriteNode spriteNodeWithColor:[UIColor greenColor] size:CGSizeMake(90, 2)];
[self addChild:object2];
object2.position = CGPointMake(self.view.center.x+45, self.view.center.y);

object2.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:object2.frame.size];

SKPhysicsJointPin *pinJoint = [SKPhysicsJointPin jointWithBodyA:object1.physicsBody bodyB:object2.physicsBody anchor:self.view.center];
[self.physicsWorld addJoint:pinJoint];

Upvotes: 4

user1272965
user1272965

Reputation: 3064

Thanks to @Maetschl, I was able to make it work in Objective-c as follows (removing the anchor point and just positioning the swinging node to one edge)...

SKSpriteNode *object1 = [SKSpriteNode spriteNodeWithColor:[UIColor redColor] size:CGSizeMake(5, 5)];
[self addChild:object1];
object1.position = self.view.center;
object1.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:object1.frame.size];
object1.physicsBody.dynamic = NO;

SKSpriteNode *object2 = [SKSpriteNode spriteNodeWithColor:[UIColor greenColor] size:CGSizeMake(90, 2)];
[self addChild:object2];
object2.position = CGPointMake(self.view.center.x+45, self.view.center.y);

object2.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:object2.frame.size];

SKPhysicsJointPin *pinJoint = [SKPhysicsJointPin jointWithBodyA:object1.physicsBody bodyB:object2.physicsBody anchor:self.view.center];
[self.physicsWorld addJoint:pinJoint];

Upvotes: 0

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