无法正确设置精灵的速度

Question

我在iOS8中制作了一个类似于asteriods的应用程序。我的代码创建了一个激光镜头，并根据船舶的正面设置其速度矢量。当船在屏幕上朝上时，代码正常工作。当船在屏幕上向下定向时，精灵沿y轴没有或几乎没有速度。也就是说，physicsBody.velocity的.dy组件表现得好像接近零; .dx组件的行为正确。 LLDB会将.dy组件报告为应该的内容，但精灵会错误地移动。下面是相关代码：

func createLaser()->SKSpriteNode { 
    let laserBeam = SKSpriteNode(imageNamed: "laserBeam") 
    laserBeam.name = "laserBeam" 
    laserBeam.position.x = shipNode.position.x 
    laserBeam.position.y = shipNode.position.y 
    laserBeam.physicsBody = SKPhysicsBody(circleOfRadius: 10.0) 
    laserBeam.physicsBody?.dynamic = true 
    laserBeam.physicsBody?.affectedByGravity = false 
    laserBeam.zRotation = shipNode.zRotation 
    laserBeam.physicsBody?.mass = 1.0 
    laserBeam.physicsBody?.velocity.dx = kLaserSpeed * sin(shipNode.zRotation) * -1 //TODO apply an impulse instead? 
    laserBeam.physicsBody?.velocity.dy = kLaserSpeed * cos(shipNode.zRotation) * 1 
    println("in create Laser ship rotation = \(shipNode.zRotation), dx = \(laserBeam.physicsBody?.velocity.dx), dy = \(laserBeam.physicsBody?.velocity.dy)") 
    return laserBeam 
} 

Answer 1

试试这个：

laserBeam.physicsBody?.velocity.dx = kLaserSpeed * cos(shipNode.zRotation) 
laserBeam.physicsBody?.velocity.dy = kLaserSpeed * sin(shipNode.zRotation)