前言：大家都知道在安卓上的window 有相关的level，那么iOS上是不是也有相关的level呢？其实是有的只不过iOS的开发不常用而已
先简单说一下UIWindow的几个方法

1. 新建window 方法

   self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
   self.window = [UIWindow new]; // 如果支持iOS9之前不能使用这个
   

这样建立的window 的windowLevel 是 0 hidden 是 YES

2. makeKeyAndVisible:

显示一个UIWindow，同时设置为keyWindow，并将其显示在同一windowLevel的其它任何UIWindow之上,此时其Hidden会自动变成NO
官方解释 // convenience. most apps call this to show the main window and also make it key. otherwise use view hidden property

 [self.window makeKeyAndVisible];
 (lldb) po self.window.windowLevel
    0
 (lldb) po self.window.isKeyWindow
    YES
 (lldb) po self.window.isHidden
    NO

3. keyWindow:

设置keyWindow与否并不当前影响视图层级显示，仅来接收键盘及其它非触摸事件。如果没有专门设置过keyWindow的hiden为NO，而且也没有其它非隐藏的UIWindow，那么APP会黑屏
如果想解除keywindow
resignKeyWindow
[[UIApplication sharedApplication] keyWindow]获取正在显示的UIWindow是极其不准确的，下面会有例子解释这个概念

4. UIWindowLevel:

   UIWindowLevel（float） 苹果的几个level 注意 在TVOS 上 UIWindowLevelStatusBar 是禁止的
   UIKIT_EXTERN const UIWindowLevel UIWindowLevelNormal;
   UIKIT_EXTERN const UIWindowLevel UIWindowLevelAlert;
   UIKIT_EXTERN const UIWindowLevel UIWindowLevelStatusBar __TVOS_PROHIBITED;
   新的window默认的level是UIWindowLevelNormal 0.0

5. 销毁:

self.window.hidden = YES;
self.window = nil;

6. 注意启动app时不要更换window:
   -------注意不要在 - (BOOL)application: didFinishLaunchingWithOptions: 更换window，否则会出现以下错误
   *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Application windows are expected to have a root view controller at the end of application launch'

实战

1. 在view 上建立按钮

   - (void)viewDidLoad {
      [super viewDidLoad];
      self.view.backgroundColor = [UIColor orangeColor];
      
      NSLog(@"windows%@",[UIApplication sharedApplication].windows);
      for (UIWindow * wind in [UIApplication sharedApplication].windows) {
          NSLog(@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,wind,wind.windowLevel);
      }
      // 创建测试按钮
      UIButton *tempBtn = [UIButton buttonWithType:UIButtonTypeSystem];
      tempBtn.frame = CGRectMake(44,44,200,100);
      [tempBtn setTitle:@"创建一个NormalWindow" forState:UIControlStateNormal];
      [tempBtn addTarget:self action:@selector(NormalWindow) forControlEvents:UIControlEventTouchUpInside];
      
      [self.view addSubview:tempBtn];
      // 显示window 的信息
      
      UILabel * label = [[UILabel alloc]init];
      label.text = [NSString stringWithFormat:@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,[UIApplication sharedApplication].keyWindow,[UIApplication sharedApplication].keyWindow.windowLevel];
      label.numberOfLines = 0;//表示label可以多行显示
      label.textColor = [UIColor blackColor];
      NSDictionary *attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:14]};
      CGSize labelSize = [label.text boundingRectWithSize:CGSizeMake(200, 5000) options: NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:attribute context:nil].size;
      label.font = [UIFont systemFontOfSize:14];
      label.frame = (CGRect){{50,200},labelSize};
      [self.view addSubview:label]; 
}

log1.png
png1.PNG

2. 创建一个window 默认使用UIWindowLevelNormal 0.0 系统默认的keyWindow 的windowLevel 是 Normal 那么 要优先显示创建的windowLevel 必须大于等于Normal 才会展示在上层。
   创建 window 不用添加到任何的控件上面，直接创建完毕 即自动添加到window 上

     - (void)NormalWindow
    {
        UIWindow * testWindows  =  [UIWindow new]; // 以后 默认了 window的大小
        _myWindow1 = testWindows;
        testWindows.hidden = NO;
        self.myWindow1.backgroundColor = [UIColor yellowColor];
        UIButton *windowBtn = [UIButton buttonWithType:UIButtonTypeCustom];
        [windowBtn setTitle:@"创建一个StatusBarwindow" forState:UIControlStateNormal];
        windowBtn.backgroundColor = [UIColor redColor];
        windowBtn.frame = CGRectMake(44,44,200,100);;
        [windowBtn addTarget:self action:@selector(StatusBarwindow) forControlEvents:UIControlEventTouchUpInside];
        [self.myWindow1 addSubview:windowBtn];
        
        
        
        UILabel * label = [[UILabel alloc]init];
        label.text = [NSString stringWithFormat:@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,testWindows,testWindows.windowLevel];
        label.numberOfLines = 0;//表示label可以多行显示
        label.textColor = [UIColor blackColor];
        NSDictionary *attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:14]};
        CGSize labelSize = [label.text boundingRectWithSize:CGSizeMake(200, 5000) options: NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:attribute context:nil].size;
        label.font = [UIFont systemFontOfSize:14];
        label.frame = (CGRect){{50,200},labelSize};
        [testWindows addSubview:label];
        
        for (UIWindow * wind in [UIApplication sharedApplication].windows) {
            NSLog(@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,wind,wind.windowLevel);
        }
    }

log2.png

通过log 可以发现新的window 追加在了数组后面 0x10060be90 level 0.0 (新建的normal window)

pn2.PNG

在normal上新建一个statusBar 的window

- (void)StatusBarwindow
{
    //window 销毁
    //    self.myWindow1.hidden = YES; //可有可无 看 UI效果
    //    self.myWindow1 = nil; // 这个方法是真正移除 UIWindow
    UIWindow * testWindows = [UIWindow new];
    //    [testWindows makeKeyAndVisible];
    testWindows.backgroundColor = [UIColor blueColor];
    // 设置 window 的 windowLevel
    testWindows.windowLevel = UIWindowLevelStatusBar; //TODO: Normal，StatusBar，Alert 分别 为 0，1000，2000 可以修改这里体验 层级变化 对 展示 window的影响
    testWindows.hidden = NO;
    self.myWindow2 = testWindows;
    UIButton *windowBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    [windowBtn setTitle:@"StatusBarNewNorWindow" forState:UIControlStateNormal];
    windowBtn.backgroundColor = [UIColor redColor];
    windowBtn.frame = CGRectMake(15, 90, self.view.frame.size.width - 15 * 2, 64);
    [windowBtn addTarget:self action:@selector(StatusBarNewNorWindow) forControlEvents:UIControlEventTouchUpInside];
    [testWindows addSubview:windowBtn];
    // 显示window 的信息
    
    UILabel * label = [[UILabel alloc]init];
    label.text = [NSString stringWithFormat:@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,testWindows,testWindows.windowLevel];
    label.numberOfLines = 0;//表示label可以多行显示
    label.textColor = [UIColor blackColor];
    NSDictionary *attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:14]};
    CGSize labelSize = [label.text boundingRectWithSize:CGSizeMake(200, 5000) options: NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:attribute context:nil].size;
    label.font = [UIFont systemFontOfSize:14];
    label.frame = (CGRect){{50,200},labelSize};
    [testWindows addSubview:label];
    
    
    for (UIWindow * wind in [UIApplication sharedApplication].windows) {
        NSLog(@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,wind,wind.windowLevel);
    }
    
}

log3.png

通过log 可以发现新的window 追加在了数组后面 0x100320be0（level 1000.0） 现在数组中是4个数据

pn3.PNG

那么如果接下来增加一个 level 是normal的window 如何

- (void)StatusBarNewNorWindow
{
    //window 销毁
    //    self.myWindow1.hidden = YES; //可有可无 看 UI效果
    //    self.myWindow1 = nil; // 这个方法是真正移除 UIWindow
    UIWindow * testWindows = [UIWindow new];
    //    [testWindows makeKeyAndVisible];
    testWindows.backgroundColor = [UIColor blueColor];
    // 设置 window 的 windowLevel
    testWindows.windowLevel = UIWindowLevelNormal; //TODO: Normal，StatusBar，Alert 分别 为 0，1000，2000 可以修改这里体验 层级变化 对 展示 window的影响
    testWindows.hidden = NO;
    self.myWindow3 = testWindows;
    UIButton *windowBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    [windowBtn setTitle:@"UIWindowLevelAlert" forState:UIControlStateNormal];
    windowBtn.backgroundColor = [UIColor redColor];
    windowBtn.frame = CGRectMake(15, 90, self.view.frame.size.width - 15 * 2, 64);
    [windowBtn addTarget:self action:@selector(UIWindowLevelAlert) forControlEvents:UIControlEventTouchUpInside];
    [testWindows addSubview:windowBtn];
    // 显示window 的信息
    
    UILabel * label = [[UILabel alloc]init];
    label.text = [NSString stringWithFormat:@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,testWindows,testWindows.windowLevel];
    label.numberOfLines = 0;//表示label可以多行显示
    label.textColor = [UIColor blackColor];
    NSDictionary *attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:14]};
    CGSize labelSize = [label.text boundingRectWithSize:CGSizeMake(200, 5000) options: NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:attribute context:nil].size;
    label.font = [UIFont systemFontOfSize:14];
    label.frame = (CGRect){{50,200},labelSize};
    [testWindows addSubview:label];
    
    
    for (UIWindow * wind in [UIApplication sharedApplication].windows) {
        NSLog(@"%s windows =  %@ ---windowLevel= %f",__FUNCTION__,wind,wind.windowLevel);
    }
    
}

log4.png

通过log 可以发现新的window 追加在了数组多了一个0x10031fd10（level 0.0） 并且是追加在了0x10060be90 level 0.0 后面 但是最后打印的依旧是 0x100320be0 ，由此可见，要优先显示创建的windowLevel 必须大于等于当前的level才会展示在上层。

注意
以上的keyWindow都是 **0x1003093f0 **如下图

keyWindow.png

结论

1. 创建 window 不用添加到任何的控件上面，直接创建完毕 即自动添加到window 上

2.创建一个window 默认使用UIWindowLevelNormal 0.0 系统默认的keyWindow 的windowLevel 是 Normal 那么 要优先显示创建的windowLevel 必须大于等于当前的window level 才会展示在上层。