当由前一个界面,push下一个界面时,如A push B,四个方法执行顺序A(ViewWillDisappear)>B(viewWillAppear)>A(viewDidDisappear)>B(viewDidAppear)
当由后一个界面pop前一个界面时,如B pop A,四个方法执行顺序B(viewWillDisappear)>A(viewWillAppear)>B(viewDidDisappear)>A(viewDidAppear)
当由前一个界面,push下一个界面时,如A push B,四个方法执行顺序A(ViewWillDisappear)>B(viewWillAppear)>A(viewDidDisappear)>B(viewDidAppear)
当由后一个界面pop前一个界面时,如B pop A,四个方法执行顺序B(viewWillDisappear)>A(viewWillAppear)>B(viewDidDisappear)>A(viewDidAppear)