使用UIWindow

UIWindow并不包含任何默认的内容，但是它被当作UIView的容器，用于放置应用中所有的UIView。UIWindow更多的时候只作为UIView的顶层容器存在。
而我们最常用的方法，就是在程序刚启动时，调用UIWindow的makeAndVisible方法，代码如下：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    ViewController *vc = [[ViewController alloc] init];
    UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:vc];
    self.window.rootViewController = nav;
    [self.window makeKeyAndVisible];
    return YES;
}```
我们可以在UIWindow做一些操作：

// 取得最上面的window self代表一个view
UIWindow *window = [[UIApplication sharedApplication].windows lastObject];
[window addSubview:self];

WindowLevel属性
不是所有创建的UIWindow一定会覆盖在界面的最上面，UIWindow有一个类型为“UIWindowLevel”的属性，该属性定义了UIWindow的层级，系统定义的UIWindowLevel一共有3中取值：如下：

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

程序中默认的UIWindow的层级是UIWindowLevelNormal。

手动创建UIWindow

//创建UIWindown
_window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
_window.windowLevel = UIWindowLevelNormal;
_window.backgroundColor = [UIColor blueColor];
_window.hidden = NO;

UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(hideUIWindown)];
[_window addGestureRecognizer:tap];

/**
隐藏UIWindown
*/

• (void)hideUIWindown{
  _window.hidden = YES;
  _window = nil;
  }

在一个应用程序中，想做一个密码保护功能，在用户从应用的任何界面按Home键退出，过一段时间再从后台切换回来，显示一个密码输入界面。只有用户在输入正确的密码，才能够进入退出的界面。因为这个密码输入界面可能从任何应用界面弹出，并且需要盖在所有界面的最上层，所以它很适合用一个UIWindow来实现。

import "PasswordInputwindow.h"

@implementation PasswordInputwindow

• (instancetype)shareInstance
  {
  static id shareInstance = nil;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  shareInstance = [[self alloc] initWithFrame:[UIScreen mainScreen].bounds];
  });
  return shareInstance;
  }

• (instancetype)initWithFrame:(CGRect)frame
  {
  self = [super initWithFrame:frame];
  if (self) {
  UIViewController *vc = [[UIViewController alloc] init];
  vc.view.backgroundColor = [UIColor yellowColor];
  self.rootViewController = vc;

    UITextField *textfield = [[UITextField alloc] init];
    textfield.textColor = [UIColor blackColor];
    textfield.font = [UIFont systemFontOfSize:17];
    textfield.frame = CGRectMake(20, 120, [UIScreen mainScreen].bounds.size.width-40, 40);
    textfield.borderStyle = UITextBorderStyleRoundedRect;
    textfield.placeholder = @"在这里输入密码";
    [vc.view addSubview:textfield];
    
    UIButton *btn = [[UIButton alloc] init];
    [btn setTitle:@"隐藏界面" forState:UIControlStateNormal];
    [btn setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    btn.titleLabel.font = [UIFont systemFontOfSize:17];
    [btn sizeToFit];
    btn.center = CGPointMake([UIScreen mainScreen].bounds.size.width/2, [UIScreen mainScreen].bounds.size.height/2);
    [btn addTarget:self action:@selector(hideWindown:) forControlEvents:UIControlEventTouchUpInside];
    [vc.view addSubview:btn];
  

  }

  return self;
  }

• (void)hideWindown:(UIButton *)sender{
  [self resignKeyWindow];
  self.hidden = YES;
  }

• (void)show
  {
  [self makeKeyWindow];
  self.hidden = NO;
  }

• (void)applicationDidBecomeActive:(UIApplication *)application {

  [[PasswordInputwindow shareInstance] show];
  }

如果我们创建的UIWindow需要处理键盘事件，那么就需要合理的将其设置为keywindow。keywindow是被系统设计用来接受键盘和其他非触摸事件的UIWindow。我们可以通过makeKeyWindow和makeKeyAndVisible方法来将自己创建的UIWindow实例设置为keywindow。支付宝客户端的手势解锁功能，就是UIWindow的一个极好的应用。


/***  文中的知识摘自“iOS开发进阶”   ****/