UIViewController加载过程
UIViewController是视图和数据的桥梁，UIViewController是所有controller的基类，ios内置了很多试图控制器，如导航控制器，tableViewController等。
视图控制本身包含一个视图，是view。UIScreen是连接物理屏幕的标示，UIWindow是为UIScreen提供了一个画布。
加载过程分为以下几步：
-(void)loadView{ NSLog(@"loadView");// UIView *view=[[UIView alloc] initWithFrame:[UIScreen mainScreen].applicationFrame];// view.backgroundColor=[UIColor greenColor];// self.view=view;// [view release]; //添加子视图}- (void)viewDidLoad{ [super viewDidLoad]; UIView *view=[[UIView alloc] initWithFrame:[UIScreen mainScreen].applicationFrame]; view.backgroundColor=[UIColor greenColor]; [self.view addSubview:view]; [view release]; NSLog(@"view Did Load"); //注意上面的操作会判断view是否为空，如果view为空，就会调用loadview方法// Do any additional setup after loading the view.}
第四、- (void)viewWillAppear:(BOOL)animated; // Called when the view is about to made visible. Default does nothing
第五、- (void)viewDidAppear:(BOOL)animated; // Called when the view has been fully transitioned onto the screen. Default does nothing
