SpriteKit: cannot change node position in contact callback

问题

I have a node with a dynamic physics body. And I would like to make it static and change its position when it comes in contact with another body.

I managed to make the body static with the solution provided in this question: Sprite Kit failing assertion: (typeA == b2_dynamicBody || typeB == b2_dynamicBody)

However if I change the position property of the node in one of the contact callback methods (e.g didBeginContact) the new position is not taken into account.

How could I achieve that?

回答1:

I believe this is a bug in SpriteKit. (I was able to reproduce this problem with SpriteKit 7.1).

Here is a quick workaround:

- (void) didBeginContact:(SKPhysicsContact *)contact
{
     contact.bodyB.node.position = CGPointMake(newX, newY);
     contact.bodyB.node.physicsBody = contact.bodyB.node.physicsBody; // <-- Add this line
}

回答2:

JKallio's solution did not work for me (Xcode 7.2.1 on OS X 10.10.5, targeting iOS 8.1). I figured out that I was able to change the position in update: method, so I got around the bug by setting a flag in didBeginContact and changing the position in update:.

@implementation GameScene {
    SKNode *_victim;
    CGPoint _target;
}

- (void)didMoveToView:(SKView *)view {
    _victim = nil;
}

- (void)didBeginContact:(SKPhysicsContact *)contact {
    _victim = contact.bodyB.node;
    _target = CGPointMake(newX, newY);
}

- (void)update:(CFTimeInterval)currentTime {
    if (_victim != nil) {
        _victim.position = _target;
        _victim = nil;
    }
}

@end

Note that _victim serves as both the node and the flag.

My solution is more complicated that JKallio's; try that one before coming for this.

回答3:

As another option for a workaround, I instead call a method in a SKSpriteNode subclass for the object I want to move, passing in that body. The position gets set correctly.

/* In GameScene.swift */

func didBegin(_ contact: SKPhysicsContact) {
    let dotBody: SKPhysicsBody
    if contact.bodyA.categoryBitMask == 0b1 {
        dotBody = contact.bodyB
    } else {
        dotBody = contact.bodyA
    }

    if let dot = dotBody.node as? Dot {
        dot.move()
    }        
}

Here is the Dot class:

import SpriteKit
class Dot : SKSpriteNode {
    let dotTex = SKTexture(imageNamed: "dot")
    init() {
        super.init(texture: dotTex, color: .clear, size: dotTex.size())
        self.physicsBody = SKPhysicsBody(circleOfRadius: size.width / 2)
        self.physicsBody?.categoryBitMask = 0b1 << 1
        self.physicsBody?.contactTestBitMask = 0b1
    }

    func move() {
        let reset = SKAction.run {
            self.position.x = 10000
        }
        self.run(reset)
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
}

来源：https://stackoverflow.com/questions/22810237/spritekit-cannot-change-node-position-in-contact-callback

标签

sprite-kit