SpriteKit对具有关节的实体施加两个力

Question

问题:我想让气球在空中升起。它们可以相互碰撞，也可以旋转。垂直方向的重力效果很好。

但是现在我想把一个篮子连接到每个气球上。这样做将导致篮子像气球一样“飞”起来。但在现实生活中，篮子应该比气球“重”，所以它总是指向地面。

我该如何做到这一点？

Answer 1

使用篮子模拟气球的一种方法是将重力设置为向下方向，以将篮子拉向场景的底部，并对气球应用浮力以创建升力。下面是一个如何做到这一点的示例：

定义常量

#define kRandMax        0x7fffffff
#define kNumBalloons    5

创建气球和篮子

self.physicsWorld.gravity = CGVectorMake(0, -3);

for (int i=0;i<kNumBalloons;i++) {

    // Create a balloon near the bottom of the scene with a random x position
    SKShapeNode *balloon = [SKShapeNode shapeNodeWithCircleOfRadius:16];
    balloon.fillColor = [SKColor redColor];
            balloon.name = @"balloon";
    balloon.physicsBody.allowsRotation = NO;
    CGFloat rand1 = arc4random_uniform(kRandMax) / (double)kRandMax;
    // A value in (-view.frame.size.width/2, view.frame.size.width/2)
    CGFloat xOffset = (rand1 - 0.5) *  view.frame.size.width / 2;
    balloon.position = CGPointMake (CGRectGetMidX(view.frame)+xOffset,
                                    CGRectGetMinY(view.frame)+50);
    balloon.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:16];
    [self addChild:balloon];

    // Create the basket        
    SKSpriteNode *basket = [SKSpriteNode spriteNodeWithColor:[SKColor brownColor] size:CGSizeMake(8, 8)];
    basket.position = CGPointMake(balloon.position.x, balloon.position.y-24);
    basket.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:basket.size.width/2];
    [self addChild:basket];

    // Connect the balloon and basket with an SKPhysicsJointPin
    SKPhysicsJointPin *joint = [SKPhysicsJointPin jointWithBodyA:balloon.physicsBody
                                bodyB:basket.physicsBody anchor:balloon.position];
    [self.physicsWorld addJoint:joint];
}

在update方法中，对每个气球应用一个力以创建升力。

-(void)update:(CFTimeInterval)currentTime {
    // Apply buoyant force to all balloons in the scene
    [self enumerateChildNodesWithName:@"//balloon" usingBlock:^(SKNode *node, BOOL *stop){
        // Adjust force as needed
        [node.physicsBody applyForce:CGVectorMake(0, 17.2)];
    }];
}

Answer 2

查看SKPhysicsBody文档(可以在here中找到)。

查看Defining a Body’s Physical Properties部分。

每个物理体都有你想要的属性(比如质量)，以便获得更“真实”的行为。

如果你想要一个不太通用的答案，

发布一些代码示例。

祝你好运，伙计。

Answer 3

我现在有一个这样的解决方案：

我创建了一个向上的力场

func createForceField(strength:Float) {
    // add force field
    for i in 1...9 {
        let field = SKFieldNode.electricField()
        field.physicsBody?.categoryBitMask = PhysikcsCategory.ForceField
        field.position = CGPointMake(frame.size.width*0.1*CGFloat(i), -1000)
        field.strength = strength
        addChild(field)
    }
}

在开始游戏之前，我打开了重力和力场。

override init(size: CGSize) {

    super.init(size: size)

    // Set-Up forcefield
    createForceField(0.1)
    self.physicsWorld.gravity = CGVectorMake(0, -0.09)

    startGame()

}

在这里，我用绳子和标签创建了气球，气球被带电，所以它会对力场做出反应。

func createBalloon(vokabel:Vokabel) {

    let x = self.frame.width*0.2
    let y = self.frame.height*0.2

    // Balloon
    var balloon = BalloonNode()
    balloon.size =  CGSizeMake(balloon.size.width*0.7, balloon.size.height*0.7)
    balloon.position = CGPointMake(x-6,y)
    balloon.name = "SPRITE"
    balloon.vokabel = vokabel
    balloon.score = 50
    balloonLayer.addChild(balloon)

    // Special Start
    var specialStar = SKSpriteNode(imageNamed: "Star")
    specialStar.size =  CGSizeMake(balloon.size.width*0.7, balloon.size.height*0.7)
    specialStar.hidden = true
    specialStar.name = "STAR"  
    balloon.addChild(specialStar)


    // Rope
    let rope = SKSpriteNode(color: UIColor.blackColor(), size: CGSize(width: 2, height: 15))
    rope.position = CGPointMake(x, y-balloon.size.height/2-rope.size.height/2)
    rope.alpha = 0.2
    balloonLayer.addChild(rope)

    // Tag
    var labelText = getLabelText(vokabel)
    let tag = createTagWithText(labelText)
    tag.position = CGPointMake(x, y-balloon.size.height/2-rope.size.height)
    balloonLayer.addChild(tag)

    self.physicsWorld.gravity=CGVectorMake(0.0, -0.3)

    // physical bodies

    balloon.physicsBody = SKPhysicsBody(rectangleOfSize: balloon.size)  
    rope.physicsBody = SKPhysicsBody(rectangleOfSize: rope.size)
    tag.physicsBody = SKPhysicsBody(rectangleOfSize: tag.size)

    // physical forces
    balloon.physicsBody?.allowsRotation = false
    balloon.physicsBody?.charge = 0.5
    tag.physicsBody?.allowsRotation = false
    tag.physicsBody?.linearDamping = 1.0
    rope.physicsBody?.linearDamping = 1.0

    // anchor Points
    let anchorBalloonRope = CGPointMake(x, y-balloon.size.height/2)
    let anchorRopeTag = CGPointMake(x, y-balloon.size.height/2-rope.size.height)

    // create joints
    let joint = SKPhysicsJointPin.jointWithBodyA(balloon.physicsBody, bodyB: rope.physicsBody, anchor: anchorBalloonRope)
    let joint_rope_tag = SKPhysicsJointPin.jointWithBodyA(rope.physicsBody, bodyB: tag.physicsBody, anchor: anchorRopeTag)

    // add joints to physic
    self.physicsWorld.addJoint(joint)
    self.physicsWorld.addJoint(joint_rope_tag)

    // give it a horizontal push
    balloon.physicsBody?.applyImpulse(CGVectorMake(10.0, 0.0))



    // BalloonNode ( is just a layer, has no special position so it is the same coordinate system as the gamelayer)
    let node = BalloonNode()
    node.vokabel = vokabel
    node.score = SCORE_STANDARD
    node.name = "BALLOON"
    node.rope = rope
    node.tag = tag


}