Swift - 延迟后改变常量属性

Question

当我执行下面的片段时，我最终会发出警告文字'mama'而不是警告，文字'hey'会在3秒内变为'mama'。

我无法弄清楚为什么会这样。

  override func viewDidAppear(animated: Bool) {
    super.viewDidAppear(animated)
    let alert = UIAlertView(title: "hey", message: "hey", delegate: nil, cancelButtonTitle: "cheese")
    alert.show()

    dispatch_after(dispatch_time_t(3000000000), dispatch_get_main_queue()){
        alert.message = "mama"
    }
  }

更新如下面的回答和评论中所述，问题在于我如何使用dispatch_after()。

这也有效：

 dispatch_after(dispatch_time(
        DISPATCH_TIME_NOW,
        Int64(3 * Double(NSEC_PER_SEC))
        ), dispatch_get_main_queue()){
        alert.message = "mama"
    }

Answer 1

试试这个：

override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)
        let alert = UIAlertView(title: "hey", message: "hey", delegate: nil, cancelButtonTitle: "cheese")
        alert.show()
        delay(3) {
            alert.message = "mama"
        }
    }

    func delay(delay:Double, closure:()->()) {
        dispatch_after(
            dispatch_time(
                DISPATCH_TIME_NOW,
                Int64(delay * Double(NSEC_PER_SEC))
            ),
            dispatch_get_main_queue(), closure)
    }