手动创建UIWindow及UIWindowLevel的说明

定义了UIWindow的层级，系统有3个取值

UIKIT_EXTERN const UIWindowLevel UIWindowLevelNormal;
UIKIT_EXTERN const UIWindowLevel UIWindowLevelAlert;
UIKIT_EXTERN const UIWindowLevel UIWindowLevelStatusBar;

NSLog(@"UIWindowLevelNormal:%f,UIWindowLevelAlert:%f,UIWindowLevelStatusBar:%f",UIWindowLevelNormal,UIWindowLevelAlert,UIWindowLevelStatusBar);

output:

UIWindowLevelNormal:0.000000,UIWindowLevelAlert:2000.000000,UIWindowLevelStatusBar:1000.000000;

手动创建UIWindow，区别于UIView，UIWindow一旦被创建，将自动地添加到整个界面上~

#import "ViewController.h"

@interface ViewController ()
{
    UIWindow *_myWindow;
}
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.view.backgroundColor = [UIColor redColor];

    //创建Button，点击创建UIWindow
    UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 100, 50)];
    [button setTitle:@"创建window" forState:UIControlStateNormal];
    [button addTarget:self action:@selector(_creatAction:) forControlEvents:UIControlEventTouchUpInside];
    button.center = self.view.center;
    [self.view addSubview:button];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)_creatAction:(UIButton *)btn {
    //创建一个UIWindow
    _myWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    _myWindow.windowLevel = UIWindowLevelNormal;
    _myWindow.backgroundColor = [UIColor blackColor];
    _myWindow.alpha = 0.7;
    _myWindow.hidden = NO;

    //添加手势
    UIGestureRecognizer *gesture = [[UITapGestureRecognizer alloc] init];
    [gesture addTarget:self action:@selector(hideWindow:)];
    [_myWindow addGestureRecognizer:gesture];

//    在window上添加一个UILabel
    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(100, 100, 200, 20)];
    label.text = @"LABEL";
    [label setTextColor:[UIColor whiteColor]];
    [_myWindow addSubview:label];
}

- (void)hideWindow:(UIGestureRecognizer *)gesture {
    _myWindow.hidden = YES;
    _myWindow = nil;
}

1、UIWindow不可滥用理应由ViewController实现的效果，就应该用ViewController addSubview来实现；

2、不可将需要弹出的界面都设置为单例，需要时调用显示，这会使得新创建的UIWindow一直得不到释放，出现多个UIWindow需要下过户有层级覆盖关系式，实现起来会比较复杂

（单例模式的意思就是只有一个实例。单例模式确保某一个类只有一个实例，而且自行实例化并向整个系统提供这个实例。这个类称为单例类。）

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