1.viewDidLoad 里调用presentViewController(presentModalViewController)无效果的问题
场景：在viewDidLoad里面调用了presentViewController 发现没有预期效果，最终放弃在该方法里面操作视图控制器（用push 可以有预期的效果，why ?）,放在了viewWillAppear里面去present
参考：When viewDidLoad is called, there is no guarantee that the view controller hierarchy is loaded in the navigation tree. Moving the logic to a later stage (for example: viewWillAppear) should resolve that issue as presentingController should be loaded by then.
