ios - 无法正确设置 Sprite 的速度

Question

我正在 iOS8 中制作一个类似小行星的应用程序。我有创建激光射击并根据船的朝向设置其速度矢量的代码。当船在屏幕上朝上时，代码可以正常工作。当船在屏幕上朝向下方时， Sprite 没有或几乎没有沿 y 轴的速度。也就是说，physicsBody.velocity 的 .dy 组件表现得好像接近于零； .dx 组件行为正确。 LLDB 报告 .dy 组件应该是什么，但 Sprite 移动不正确。相关代码如下:

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)