UIQWindow定義了一個window對象來管理views。一個軟件只能有一個window。window的主要職能使為view提供顯示取和向view傳遞事件。想要改變軟件顯示的內容，你可以改變window的root view。
UIWindow的screen屬性指定了window的顯示屬性包括：bounds, mode, and brightness.
window notifications用來監聽window 和 screen的改變，包括：
UIWindowDidBecomeVisibleNotification
UIWindowDidBecomeHiddenNotification
UIWindowDidBecomeKeyNotification
UIWindowDidResignKeyNotification

UIWindow繼承自UIView，關於這一點可能有點邏輯障礙，畫框怎麼繼承自畫布呢？不要過於去專牛角尖，畫框的形狀不就是跟畫布一樣嗎？拿一塊畫布然後用一些方法把它加強，是不是可以當一個畫框用呢？這也是為什麼 一個view可以直接加到另一個view上去的原因了。一個應用程序只能有一個畫框。

看一下系統的初始化過程（在application didFinishLauchingWithOptions裡面）：

- (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;
}



創建一個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;
 
    }
 
}