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關于UIWindow的一點兒思考

關于UIWindow的windowLevel以及UIApplication的keyWindow的一些思考

　　每一個IOS程序都有一個UIWindow，在我們通過模板簡歷工程的時候，xcode會自動幫我們生成一個window，然后讓它變成keyWindow并顯示出來。這一切都來的那么自然，以至于我們大部分時候都忽略了自己也是可以創建UIWindow對象。

　　通常在我們需要自定義UIAlertView的時候（IOS 5.0以前AlertView的背景樣式等都不能換）我們可以使用UIWindow來實現（設置windowLevel為Alert級別），網上有很多例子，這里就不詳細說了。

　　我的上一篇文章UIWindowLevel詳解，中講到了關于Windowlevel的東西，當時還只是看了看文檔，知道有這么一回兒事。今天剛好遇到這塊兒的問題，就順便仔細看了一下UIWindow方面的東西，主要包括：WindowLevel以及keyWindow兩個方面。

一、UIWindowLevel

　　我們都知道UIWindow有三個層級，分別是Normal，StatusBar，Alert。打印輸出他們三個這三個層級的值我們發現從左到右依次是0，1000，2000，也就是說Normal級別是最低的，StatusBar處于中等水平，Alert級別最高。而通常我們的程序的界面都是處于Normal這個級別上的，系統頂部的狀態欄應該是處于StatusBar級別，UIActionSheet和UIAlertView這些通常都是用來中斷正常流程，提醒用戶等操作，因此位于Alert級別。

　　上一篇文章中我也提到了一個猜想，既然三個級別的值之間相差1000，而且我們細心的話查看UIWindow的頭文件就會發現有一個實例變量_windowSublevel，那我們就可以定義很多中間級別的Window。例如可以自定義比系統UIAlertView級別低一點兒的window。于是寫了一個小demo，通過打印發現系統的UIAlertView的級別是1996，而與此同時UIActionSheet的級別是2001，這樣也驗證了subLevel的確存在。

    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];

　　下面是程序運行截圖：

　　根據window顯示級別優先的原則，級別高的會顯示在上面，級別低的在下面，我們程序正常顯示的view位于最底層，至于具體怎樣獲取UIAlertView和UIActionSheet的level，我會在下面第二部分keyWindow中介紹并給出相應的代碼。

二、KeyWindow

　　什么是keyWindow，官方文檔中是這樣解釋的"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." 翻譯過來就是說，keyWindow是指定的用來接收鍵盤以及非觸摸類的消息，而且程序中每一個時刻只能有一個window是keyWindow。

　　下面我們寫個簡單的例子看看非keyWindow能不能接受鍵盤消息和觸摸消息，程序中我們在view中添加一個UITextField，然后新建一個alert級別的window，然后通過makeKeyAndVisible讓它變成keyWindow并顯示出來。代碼如下：

- (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];
}

　　運行截圖如下：

　　從圖中可以看出，雖然我們自己新建了一個然后設置為keyWindow并顯示，但是點擊程序中默認window上添加的textField還是可以喚出鍵盤，而且還可以正常接受鍵盤輸入，只是鍵盤被擋住了，說明非keyWindow也是可以接受鍵盤消息，這一點和文檔上說的不太一樣。

　　觀察UIWindow的文檔，我們可以發現里面有四個關于window變化的通知：

　　UIWindowDidBecomeVisibleNotification

　　UIWindowDidBecomeHiddenNotification

　　UIWindowDidBecomeKeyNotification

　　UIWindowDidResignKeyNotification

　　這四個通知對象中的object都代表當前已顯示（隱藏），已變成keyWindow（非keyWindow）的window對象，其中的userInfo則是空的。于是我們可以注冊這個四個消息，再打印信息來觀察keyWindow的變化以及window的顯示，隱藏的變動。

　　代碼如下：

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