一、UIWindow簡(jiǎn)介

UIWindow是最頂級(jí)的界面容器。
UIWindow繼承自UIView。

UIWindow的主要功能：

1. 作為UIView的最頂級(jí)容器，包含應(yīng)用顯示所需要的所有的UIView。
2. 傳遞觸摸消息和鍵盤事件給UIView。

UIWindow添加UIView有兩種方式：

1. 通過(guò)調(diào)用addSubView方法。
2. 通過(guò)設(shè)置其特有的rootViewController屬性。通過(guò)設(shè)置該屬性為要添加view對(duì)應(yīng)的UIViewController，UIWindow將自動(dòng)將其view添加到當(dāng)前window中，同時(shí)負(fù)責(zé)ViewController和view的生命周期。

二、UIWindow的使用

UIWindow通過(guò)UIWindowLevel設(shè)置window的層級(jí)。
例創(chuàng)建UIWindow的子類，實(shí)現(xiàn)調(diào)出輸入密碼界面。

#import <UIKit/UIKit.h>

@interface PasswordInputWindow : UIWindow
+ (PasswordInputWindow *)sharedInstance;
- (void)show;
@end

#import "PasswordInputWindow.h"

@implementation PasswordInputWindow{
    UITextField *_textField;
}

+(PasswordInputWindow *)sharedInstance{
    static id sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc]initWithFrame:[UIScreen mainScreen].bounds];
    });
    return sharedInstance;
}
- (instancetype)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
        UILabel *label = [[UILabel alloc]initWithFrame:CGRectMake(10, 50, 200, 20)];
        label.text = @"請(qǐng)輸入密碼";
        [self addSubview:label];
        UITextField *textField = [[UITextField alloc]initWithFrame:CGRectMake(10, 80, 200, 20)];
        textField.backgroundColor = [UIColor yellowColor];
        textField.secureTextEntry = YES;
        [self addSubview:textField];
        
        UIButton *button = [[UIButton alloc]initWithFrame:CGRectMake(10, 110, 200, 44)];
        [button setBackgroundColor:[UIColor redColor]];
        button.titleLabel.textColor = [UIColor whiteColor];
        [button setTitle:@"登錄" forState:UIControlStateNormal];
        [button addTarget:self action:@selector(completeButtonPressed:) forControlEvents:UIControlEventTouchUpInside];
        [self addSubview:button];
        
        self.backgroundColor = [UIColor yellowColor];
        _textField = textField;
    }
    return self;
}
- (void)show{
    [self makeKeyWindow];
    self.hidden = NO;
}
- (void)completeButtonPressed:(id)sender{
    if ([_textField.text isEqualToString:@"1234"]) {
        //正確
        [_textField resignFirstResponder];
        [self resignKeyWindow];
        self.hidden = YES;
    }else{
        UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"提示" message:@"密碼輸入錯(cuò)誤" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
        [alert show];
    

    }
}

//后臺(tái)進(jìn)入時(shí)調(diào)用

    [[PasswordInputWindow sharedInstance] show];

一般，手勢(shì)解鎖頁(yè)、啟動(dòng)介紹頁(yè)、應(yīng)用內(nèi)的通知提醒、彈窗廣告等適合用UIWindow來(lái)實(shí)現(xiàn)。