如何在 Swift 4.2 SpriteKit 中让子节点跟随父节点

Question

I want to make it so that a child sprite follows its corresponding parent sprite. The child sprite has a physics body which should not mess with the parent's physics body. I have tried to in the parent sprite's subclass to override these functions like this:

public override func run(_ action: SKAction) {
    super.run(action)
    physicsSprite.run(action)
}
public override func run(_ action: SKAction, withKey key: String) {
    super.run(action, withKey: key)
    physicsSprite.run(action, withKey: key)
}
public override func removeAllActions() {
    super.removeAllActions()
    physicsSprite.removeAllActions()
}

with no success however :(
I'am exclusively moving the parent sprite with SKActions. The result is that the sprites don't stick together.

physicsSprite.zPosition = 10
physicsSprite.physicsBody = SKPhysicsBody(circleOfRadius: playerInfo.width+2)
physicsSprite.physicsBody?.affectedByGravity = false
physicsSprite.physicsBody?.isDynamic = false
physicsSprite.physicsBody?.restitution = 0
physicsSprite.physicsBody?.collisionBitMask = 0
addChild(physicsSprite)

In this case physicsSprite is the childNode in a SKSpriteNode subclass. Thanks in advance

Answer 1

Lots of ways to do this. Comparing the positions of the nodes (leader/follower) on a regular basis and updating the positions accordingly should work. Example: leader goes to some positions > positions do not match > move the follower to the location of the leader.

Code example

class GameScene: SKScene {
    private var leader: SKSpriteNode!
    private var follower: SKSpriteNode!

    override func didMove(to view: SKView) {
        self.leader = SKSpriteNode(color: UIColor.red, size: CGSize(width: 30, height: 30))
        self.follower = SKSpriteNode(color: UIColor.green, size: CGSize(width: 30, height: 30))
        self.addChild(leader)
        self.addChild(follower)
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        for t in touches {
            let target = t.location(in: self)
            // Move the leader to the touch location
            leader.run(SKAction.move(to: target, duration: 1), withKey: "moving")
        }
    }

    override func update(_ currentTime: TimeInterval) {
        // Check positions
        if follower.position != leader.position {
            // Check if the follower has a 'following' action already
            if follower.action(forKey: "following") == nil {
                // Move the follower
                follower.run(SKAction.move(to: leader.position, duration: 1), withKey: "following")
            }
        }
    }
}

Note that the update action is called once per frame, which may be too much. Depending on your situation, you could create some kind of cycle that is called every second.

Another way would be to always run the same action on the following node (possibly with a longer duration, such that you can actually see the following node).

Edit: More about collisions: Can't understand how collision bit mask works (just make sure they have different contact bit masks - ie. 0b01 and 0b10 and are not in each others collision bit mask)

Answer 2

You could try to use an SKConstraint object, that describes a mathematical constraint on a node's position or orientation.

From the docs:

Keep a node within a specified distance of another node or a point in the scene.

See https://developer.apple.com/documentation/spritekit/skconstraint

Answer 3

Try this:

parentSprite.physicsBody?.categoryBitMask = 0
parentSprite.physicsBody?.collisionBitMask = 0

physicsSprite.physicsBody?.collisionBitMask = 1
physicsSprite.physicsBody?.categoryBitMask = 1

For more information check documentation