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：

UIWindowDidBecomeVisibleNotification
UIWindowDidBecomeHiddenNotification
UIWindowDidBecomeKeyNotification
UIWindowDidResignKeyNotification 

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;
 
    }
 
}

 

 

UIWindowLevel

UIWindow有三个层级，分别是Normal，StatusBar，Alert。打印输出他们三个这三个层级的值我们发现从左到右依次是0，1000，2000，也就是说Normal级别是最低的，StatusBar处于中等水平，Alert级别最高。

通常我们的程序的界面都是处于Normal这个级别上的，系统顶部的状态栏应该是处于StatusBar级别，UIActionSheet和UIAlertView这些通常都是用来中断正常流程，提醒用户等操作，因此位于Alert级别。

KeyWindow

keyWindow是指定的用来接收键盘以及非触摸类的消息，而且程序中每一个时刻只能有一个window是keyWindow。