每一个IOS程序都有一个UIWindow，在我们通过模板简历工程的时候，xcode会自动帮我们生成一个window，然后让它变成keyWindow并显示出来。这一切都来的那么自然，以至于我们大部分时候都忽略了自己也是可以创建UIWindow对象。
通常在我们需要自定义UIAlertView的时候（IOS 5.0以前AlertView的背景样式等都不能换）我们可以使用UIWindow来实现（设置windowLevel为Alert级别），网上有很多例子，这里就不详细说了。
我的上一篇文章UIWindowLevel详解，中讲到了关于Windowlevel的东西，当时还只是看了看文档，知道有这么一回儿事。今天刚好遇到这块儿的问题，就顺便仔细看了一下UIWindow方面的东西，主要包括：WindowLevel以及keyWindow两个方面。
一、UIWindowLevel
我们都知道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。而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。
上一篇文章中我也提到了一个猜想，既然三个级别的值之间相差1000，而且我们细心的话查看UIWindow的头文件就会发现有一个实例变量_windowSublevel，那我们就可以定义很多中间级别的Window。例如可以自定义比系统UIAlertView级别低一点儿的window。于是写了一个小demo，通过打印发现系统的UIAlertView的级别是1996，而与此同时UIActionSheet的级别是2001，这样也验证了subLevel的确存在。
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Alert View" message:@"Hello Wolrd, i'm AlertView!!!" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:@"Cancel", nil]; [alertView show]; [alertView release]; UIActionSheet *actionSheet = [[UIActionSheet alloc] initWithTitle:@"ActionSheet" delegate:nil cancelButtonTitle:@"Cancel" destructiveButtonTitle:@"Don't do that!" otherButtonTitles:@"Hello Wolrd", nil]; [actionSheet showInView:self.view]; [actionSheet release];
下面是程序运行截图：
根据window显示级别优先的原则，级别高的会显示在上面，级别低的在下面，我们程序正常显示的view位于最底层，至于具体怎样获取UIAlertView和UIActionSheet的level，我会在下面第二部分keyWindow中介绍并给出相应的代码。
二、KeyWindow
什么是keyWindow，官方文档中是这样解释的"The key window is the one that is designated to receive keyboard and other non-touch related events. Only one window at a time may be the key window." 翻译过来就是说，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。
下面我们写个简单的例子看看非keyWindow能不能接受键盘消息和触摸消息，程序中我们在view中添加一个UITextField，然后新建一个alert级别的window，然后通过makeKeyAndVisible让它变成keyWindow并显示出来。代码如下：
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease]; // Override point for customization after application launch. self.viewController = [[[SvUIWindowViewController alloc] initWithNibName:@"SvUIWindowViewController" bundle:nil] autorelease]; self.window.rootViewController = self.viewController; [self.window makeKeyAndVisible]; UIWindow *window1 = [[UIWindow alloc] initWithFrame:CGRectMake(0, 80, 320, 320)]; window1.backgroundColor = [UIColor redColor]; window1.windowLevel = UIWindowLevelAlert; [window1 makeKeyAndVisible]; return YES; }
- (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. [self registerObserver]; // add a textfield UITextField *filed = [[UITextField alloc] initWithFrame:CGRectMake(0, 0, 320, 60)]; filed.placeholder = @"Input something here"; filed.clearsOnBeginEditing = YES; filed.borderStyle = UITextBorderStyleRoundedRect; [self.view addSubview:filed]; [filed release]; }
运行截图如下：
从图中可以看出，虽然我们自己新建了一个然后设置为keyWindow并显示，但是点击程序中默认window上添加的textField还是可以唤出键盘，而且还可以正常接受键盘输入，只是键盘被挡住了，说明非keyWindow也是可以接受键盘消息，这一点和文档上说的不太一样。
观察UIWindow的文档，我们可以发现里面有四个关于window变化的通知：
UIWindowDidBecomeVisibleNotification
UIWindowDidBecomeHiddenNotification
UIWindowDidBecomeKeyNotification
UIWindowDidResignKeyNotification
这四个通知对象中的object都代表当前已显示（隐藏），已变成keyWindow（非keyWindow）的window对象，其中的userInfo则是空的。于是我们可以注册这个四个消息，再打印信息来观察keyWindow的变化以及window的显示，隐藏的变动。
代码如下：
SvUIWindowViewController.m
// // SvUIWindowViewController.m // SvUIWindowSample // // Created by maple on 11/14/12. // Copyright (c) 2012 maple. All rights reserved. // #import "SvUIWindowViewController.h" #import "SvCustomWindow.h" @interface SvUIWindowViewController () - (void)presentAlertView; - (void)presentActionSheet; // observer - (void)registerObserver; - (void)unregisterObserver; // observer handler - (void)windowBecomeKey:(NSNotification*)noti; - (void)windowResignKey:(NSNotification*)noti; - (void)windowBecomeVisible:(NSNotification*)noti; - (void)windowBecomeHidden:(NSNotification*)noti; @end @implementation SvUIWindowViewController - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. [self registerObserver]; // add a textfield UITextField *filed = [[UITextField alloc] initWithFrame:CGRectMake(0, 0, 320, 60)]; filed.placeholder = @"Input something here"; filed.clearsOnBeginEditing = YES; filed.borderStyle = UITextBorderStyleRoundedRect; [self.view addSubview:filed]; [filed release]; } - (void)viewDidUnload { [super viewDidUnload]; // Release any retained subviews of the main view. [self unregisterObserver]; } - (void)viewDidAppear:(BOOL)animated { [super viewDidAppear:animated]; // [self presentAlertView]; // [self presentActionSheet]; } - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation { return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown); } #pragma mark - #pragma mark observer - (void)registerObserver { [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(windowBecomeKey:) name:UIWindowDidBecomeKeyNotification object:nil]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(windowResignKey:) name:UIWindowDidResignKeyNotification object:nil]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(windowBecomeVisible:) name:UIWindowDidBecomeVisibleNotification object:nil]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(windowBecomeHidden:) name:UIWindowDidBecomeHiddenNotification object:nil]; } - (void)unregisterObserver { [[NSNotificationCenter defaultCenter] removeObserver:self]; } #pragma mark - #pragma mark Test AlertView & ActionSheet - (void)presentAlertView { UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Alert View" message:@"Hello Wolrd, i'm AlertView!!!" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:@"Cancel", nil]; [alertView show]; [alertView release]; } - (void)presentActionSheet { UIActionSheet *actionSheet = [[UIActionSheet alloc] initWithTitle:@"ActionSheet" delegate:nil cancelButtonTitle:@"Cancel" destructiveButtonTitle:@"Don't do that!" otherButtonTitles:@"Hello Wolrd", nil]; [actionSheet showInView:self.view]; [actionSheet release]; } #pragma mark - #pragma mark Notification handler - (void)windowBecomeKey:(NSNotification*)noti { UIWindow *window = noti.object; NSArray *windows = [UIApplication sharedApplication].windows; NSLog(@"current window count %d", windows.count); NSLog(@"Window has become keyWindow: %@, window level: %f, index of windows: %d", window, window.windowLevel, [windows indexOfObject:window]); } - (void)windowResignKey:(NSNotification*)noti { UIWindow *window = noti.object; NSArray *windows = [UIApplication sharedApplication].windows; NSLog(@"current window count %d", windows.count); NSLog(@"Window has Resign keyWindow: %@, window level: %f, index of windows: %d", window, window.windowLevel, [windows indexOfObject:window]); } - (void)windowBecomeVisible:(NSNotification*)noti { UIWindow *window = noti.object; NSArray *windows = [UIApplication sharedApplication].windows; NSLog(@"current window count %d", windows.count); NSLog(@"Window has become visible: %@, window level: %f, index of windows: %d", window, window.windowLevel, [windows indexOfObject:window]); } - (void)windowBecomeHidden:(NSNotification*)noti { UIWindow *window = noti.object; NSArray *windows = [UIApplication sharedApplication].windows; NSLog(@"current window count %d", windows.count); NSLog(@"Window has become hidden: %@, window level: %f, index of windows: %d", window, window.windowLevel, [windows indexOfObject:window]); } @end
a、当我们打开viewDidAppear中“[self presentAlertView];”的时候，控制台输出如下：
根据打印的信息我们可以看出流程如下：
1、程序默认的window先显示出来
2、默认的window再变成keyWindow
3、AlertView的window显示出来
4、默认的window变成非keyWindow
5、最终AlertView的window变成keyWindow
总体来说就是“要想当老大（keyWindow），先从小弟（非keyWindow）开始混起” 而且根据打印的信息我们同事可以知道默认的window的level是0，即normal级别；AlertView的window的level是1996，比Alert级别稍微低了一点儿。
b、当我们打开viewDidAppear中“[self presentActionSheet];”的时候，控制台输出如下：
keyWindow的变化和window的显示和上面的流程一样，同时我们可以看出ActionSheet的window的level是2001。
c、接着上一步，我们点击弹出ActionSheet的cancel的时候，控制台输出如下：
我们看出流程如下：
1、首先ActionSheet的window变成非keyWindow
2、程序默认的window变成keyWindow
3、ActionSheet的window在隐藏掉
总体就是“想隐居幕后可以，但得先交出权利”。
以上是这两天遇到的，总结一下，欢迎补充，如果有不对的地方也请多多指正。
注：本文欢迎转载，转载请注明出处。同时欢迎加我qq，期待与你一起探讨更多问题。
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