ios生命周期

1.view即将显示

- (void)  viewWillAppear:(Bool)animated {}

2.view完成显示

- (void)  viewDidAppear:(Bool)animated {}

3.view即将消失

- (void)  viewWillDisappear:(Bool)animated {}

4.view真的消失

- (void)  viewDidDisappear:(Bool)animated {}

5.view即将卸载

- (void)  viewWillUnload {} 

6.view卸载完成

- (void)  viewDidUnload {}