iOS UIKit ：UIWindow

 

UIQWindow定义了一个window对象来管理views。一个软件只能有一个window。window的主要职能使为view提供显示取和向view传递事件。想要改变软件显示的内容，你可以改变window的root view。

 

UIWindow的screen属性指定了window的显示属性包括：bounds, mode, and brightness.

window notifications用来监听window 和 screen的改变，包括：

UIWindowDidBecomeVisibleNotificationUIWindowDidBecomeHiddenNotificationUIWindowDidBecomeKeyNotificationUIWindowDidResignKeyNotification 

UIWindow继承自UIView，关于这一点可能有点逻辑障碍，画框怎么继承自画布呢？不要过于去专牛角尖，画框的形状不就是跟画布一样吗？拿一块画布然后用一些方法把它加强，是不是可以当一个画框用呢？这也是为什么 一个view可以直接加到另一个view上去的原因了。一个应用程序只能有一个画框。

看一下系统的初始化过程（在application didFinishLauchingWithOptions里面）：

 

- (BOOL)application:(UIApplication *)application willFinishLaunchingWithOptions:(NSDictionary *)launchOptions {   UIWindow *window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];   myViewController = [[MyViewController alloc] init];   window.rootViewController = myViewController;   [window makeKeyAndVisible];   return YES;}

 

 

创建一个an external display：

- (void)checkForExistingScreenAndInitializeIfPresent{    if ([[UIScreen screens] count] > 1)    {        // Get the screen object that represents the external display.        UIScreen *secondScreen = [[UIScreen screens] objectAtIndex:1];        // Get the screen's bounds so that you can create a window of the correct size.        CGRect screenBounds = secondScreen.bounds;         self.secondWindow = [[UIWindow alloc] initWithFrame:screenBounds];        self.secondWindow.screen = secondScreen;         // Set up initial content to display...        // Show the window.        self.secondWindow.hidden = NO;    }}- (void)setUpScreenConnectionNotificationHandlers{    NSNotificationCenter *center = [NSNotificationCenter defaultCenter];     [center addObserver:self selector:@selector(handleScreenDidConnectNotification:)            name:UIScreenDidConnectNotification object:nil];    [center addObserver:self selector:@selector(handleScreenDidDisconnectNotification:)            name:UIScreenDidDisconnectNotification object:nil];}- (void)handleScreenDidConnectNotification:(NSNotification*)aNotification{    UIScreen *newScreen = [aNotification object];    CGRect screenBounds = newScreen.bounds;     if (!self.secondWindow)    {        self.secondWindow = [[UIWindow alloc] initWithFrame:screenBounds];        self.secondWindow.screen = newScreen;         // Set the initial UI for the window.    }} - (void)handleScreenDidDisconnectNotification:(NSNotification*)aNotification{    if (self.secondWindow)    {        // Hide and then delete the window.        self.secondWindow.hidden = YES;        self.secondWindow = nil;     } }