var window: UIWindow?
func application(application: UIApplication!, didFinishLaunchingWithOptions launchOptions: NSDictionary!) -> Bool {
// Override point for customization after application launch.
self.window = UIWindow(frame: UIScreen.mainScreen().bounds)
self.window!.backgroundColor = UIColor.whiteColor()
self.window!.makeKeyAndVisible()
在这段代码中，我们初始化了一个可选的UIWindow?。要为它赋值，我假设我会将值赋给 self.window!，因为这是可选值所在的位置。但是如您所见，我们需要使用 self.window。为什么会这样。为什么注释下第一行不像self.window! = UIWindow(frame: ....
请您参考如下方法:
感叹号用于强制展开可选值。在您的代码中，window 尚未定义，尝试强制展开它会引发错误。
来自 The Swift Programming Language Guide
Trying to use ! to access a non-existent optional value triggers a runtime error. Always make sure that an optional contains a non-nil value before using ! to force-unwrap its value.
