关于UIWindow的一点儿思考-阿里云开发者社区

UIWindow的一点儿思考

　　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