无法分配给'let'值'action'

我正在尝试在switch语句之外定义一个常量,以便我可以在switch语句执行完毕后使用它并在switch语句中分配它:

let action: SKAction!

switch (whatever) {
case 0:
    sprite.position = CGPointMake(0, self.scene.size.height * lengthDiceroll)
    action = SKAction.moveTo(CGPointMake(self.scene.size.width, self.scene.size.height * (1 - lengthDiceroll)), duration: 1) // error here
    // other actions
default:
    println("meh")       

// add the sprite to the scene
let sequence = SKAction.sequence([action, action2])
sprite.runAction(SKAction.repeatActionForever(sequence))
self.addChild(sprite)

但是我收到了错误Cannot assign to 'let' value 'action'.如何分配操作以便我可以在交换机外部使用它？

如果我尝试:

action! = SKAction.moveTo(CGPointMake(self.scene.size.width, self.scene.size.height * (1.0 - lengthDiceroll)), duration: 1)

我收到了错误"Could not find an overload for '*' that accepts the supplied arguments".