ViewController的生命周期

—>init
—>loadView
—>viewDidLoad
—>viewWillAppear
—>viewWillLayoutSubViews
—>viewDidLayoutSubViews
—>viewDidAppear

—>viewWillDisappear
—>viewDidDisappear
—>viewWillUnload[已废弃]
—>viewDidUnload[已废弃]
—>dealloc