Only visible ViewControllers can be seen.
只能看到可见的ViewController。
Using ValidationRule keeps validation processing separate from the data model and ViewControllers.
使用ValidationRule使验证处理同数据模型和ViewController分隔开。
ViewControllers can be implemented in two ways: through inheritance or through delegation. Both are shown here.
有两种方法可以实现ViewController：通过继承或通过委托。
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