【iOS】玩转-GCD

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GCD介绍


Grand Central Dispatch (GCD)是Apple开发的一个多核编程的解决方法
基于C语言,提供了非常多强大的函数

术语


同步 (Synchronous)

在当前线程中执行任务,不具备开启新线程的能力
提交的任务在执行完成后才会返回
同步函数: dispatch_sync()

异步 (Asynchronous)

在新线程中执行任务,具备开启新线程的能力
提交的任务立刻返回,在后台队列中执行
异步函数: dispatch_async()

串行 (Serial)

一个任务执行完毕后,再执行下一个任务

并发 (Concurrent)

多个任务同时执行(自动开启多个线程),只有在异步函数下才有效

描述 说明
queue 队列
main 主队列
global 全局队列
dispatch_queue_t 描述队列
dispatch_block_t 描述任务
dispatch_once_t 描述一次性
dispatch_time_t 描述时间
dispatch_group_t 描述队列组
dispatch_semaphore_t 描述信号量
函数 说明
dispatch_sync() 同步执行
dispatch_async() 异步执行
dispatch_after() 延时执行
dispatch_once() 一次性执行
dispatch_apply() 提交队列
dispatch_queue_create() 创建队列
dispatch_group_create() 创建队列组
dispatch_group_async() 提交任务到队列组
dispatch_group_enter() / dispatch_group_leave() 将队列组中的任务未执行完毕的任务数目加减1(两个函数要配合使用)
dispatch_group_notify() 监听队列组执行完毕
dispatch_group_wait() 设置等待时间(返回 0成功,1失败)
注意:

1.所有的执行都放到队列中(queue),队列的特点是FIFO(先提交的先执行)
2.必须在主线程访问 UIKit 的类
3.并发队列只在异步函数下才有效

基本使用


 NSLog(@"当前线程: %@", [NSThread currentThread]);
 //获取主队列
 dispatch_queue_t mainQueue = dispatch_get_main_queue();
 
 //获取全局并发队列
 dispatch_queue_t otherQueue =   dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
 //同步函数(在当前线程中执行,不具备开启新线程的能力)
 dispatch_sync(otherQueue, ^{
      NSLog(@"同步 %@", [NSThread currentThread]);
 });
    
 //异步函数(在另一条线程中执行,具备开启新线程的能力)
 dispatch_async(otherQueue, ^{
      NSLog(@"异步 %@", [NSThread currentThread]);
 });
    
 //输出:   当前线程: <NSThread: 0x7f81bac06670>{number = 1, name = main}
 //输出:   同步 <NSThread: 0x7f81bac06670>{number = 1, name = main}
 //输出:   异步 <NSThread: 0x7f81bae35be0>{number = 3, name = (null)}
延时执行 dispatch_after()

dispatch_after()延迟一段时间把一项任务提交到队列中执行,返回之后就不能取消
常用来在在主队列上延迟执行一项任务

    NSLog(@"当前线程 %@", [NSThread currentThread]);

    //GCD延时调用(主线程)(主队列)
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"GCD延时(主线程) %@", [NSThread currentThread]);
    });
    
    //GCD延时调用(其他线程)(全局并发队列)
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"GCD延时(其他线程) %@", [NSThread currentThread]);
    });

    //输出:   当前线程 <NSThread: 0x7f8cb9701990>{number = 1, name = main}
    //输出:   GCD延时(主线程) <NSThread: 0x7f8cb9701990>{number = 1, name = main}
    //输出:   GCD延时(其他线程) <NSThread: 0x7f8cb9513ee0>{number = 3, name = (null)}
一次性执行 dispatch_once()

整个程序运行中,只会执行一次 (默认线程是安全的)
dispatch_once() 以线程安全的方式执行且仅执行其代码块一次

    for (NSInteger i = 0; i < 10; i++) {
      
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            NSLog(@"GCD一次性执行(默认线程是安全的)");
        });
        
    }

 //输出:   GCD一次性执行(默认线程是安全的)
//使用GCD初始化单例
+ (instancetype)sharedManager { 
  
    static PhotoManager *sharedPhotoManager = nil; 
    static dispatch_once_t onceToken; 
    dispatch_once(&onceToken, ^{ 
        sharedPhotoManager = [[PhotoManager alloc] init]; 
    }); 

    return sharedPhotoManager; 
} 
提交 dispatch_apply()

把一项任务提交到队列中多次执行,具体是并行执行还是串行执行由队列本身决定
dispatch_apply不会立刻返回,在执行完毕后才会返回,是同步的调用。

队列

任务1,任务2依次执行,所有任务都执行成功后回到主线程
(效率不高)

 NSLog(@"当前线程 %@", [NSThread currentThread]);
   
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //全局并发队列
        for (NSInteger i = 0; i < 5; i++) {
            NSLog(@"任务1 %@", [NSThread currentThread]);
        }
        
        for (NSInteger i = 0; i < 5; i++) {
            NSLog(@"任务2 %@", [NSThread currentThread]);
        }
        
        dispatch_async(dispatch_get_main_queue(), ^{
            //主队列
            NSLog(@"主线程执行(刷新UI) %@", [NSThread currentThread]);
        });
        
    });
    
    //输出:   当前线程 <NSThread: 0x7fa78040b8b0>{number = 1, name = main}
    //输出:   任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
    //输出:   主线程(刷新UI) <NSThread: 0x7f9da1705940>{number = 1, name = main}
队列组

任务1,任务2同时执行,所有任务都执行成功后回到主线程
(效率高)

    NSLog(@"当前线程 %@", [NSThread currentThread]);
    
    //(1)创建一个队列组
    dispatch_group_t group= dispatch_group_create();
    
    //(2)开启任务1
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
       
        for (NSInteger i = 0; i < 5; i++) {
            NSLog(@"任务1 %@", [NSThread currentThread]);
        }
        
    });
    
    //(3)开启任务2
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
       
        for (NSInteger i = 0; i < 5; i++) {
            NSLog(@"任务2 %@", [NSThread currentThread]);
        }
        
    });
    
    //(4)所有任务执行完毕,回到主线程
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        
        NSLog(@"主线程(刷新UI) %@", [NSThread currentThread]);
        
    });
    
    //输出:   当前线程 <NSThread: 0x7fd951704e70>{number = 1, name = main}
    //输出:   任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
    //输出:   任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
    //输出:   任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
    //输出:   任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
    //输出:   任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
    //输出:   任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
    //输出:   任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
    //输出:   主线程(刷新UI) <NSThread: 0x7ff65a406fb0>{number = 1, name = main}

串行与并发


各个队列的执行效果
串行队列
串行队列

一个任务执行完毕后,再执行下一个任务
主队列是GCD自带的一种特殊的串行队列,放在主队列中的任务,都会放到主线程中执行

    //(1)使用dispatch_queue_create函数创建串行队列
    //参数1: 队列名称
 //参数2: 队列属性 (一般用NULL)
    dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", NULL);
    
    //(2)使用主队列(跟主线程相关联的队列)
    dispatch_queue_t serialMainQueue = dispatch_get_main_queue();
并发队列
并发队列

多个任务并发执行(自动开启多个线程同时执行任务)

<u>并发功能只有在异步(dispatch_async)函数下才有效!!!</u>

GCD默认已经提供了全局的并发队列,供整个应用使用,不需要手动创建

并发队列优先级 快捷值 优先级
DISPATCH_QUEUE_PRIORITY_HIGH 2
DISPATCH_QUEUE_PRIORITY_DEFAULT 0 中(默认)
DISPATCH_QUEUE_PRIORITY_LOW (-2)
DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN 后台
 //(1)使用dispatch_get_global_queue函数获得全局的并发队列
    //参数1: 优先级
 //参数2: 暂时无用参数 (传0)
    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
异步函数_并发队列

(开启新线程,并发执行任务)

    NSLog(@"当前线程 %@", [NSThread currentThread]);
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"任务1 %@", [NSThread currentThread]);
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"任务2 %@", [NSThread currentThread]);
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"任务3 %@", [NSThread currentThread]);
    });
    
    //输出:   当前线程 <NSThread: 0x7fbe83c067c0>{number = 1, name = main}
    //输出:   任务1 <NSThread: 0x7fb84160ed90>{number = 3, name = (null)}
    //输出:   任务3 <NSThread: 0x7fb841752940>{number = 4, name = (null)}
    //输出:   任务2 <NSThread: 0x7fb8414167b0>{number = 5, name = (null)}
异步函数_串行队列

(开启新线程,串行执行任务)

    NSLog(@"当前线程 %@", [NSThread currentThread]);
    //创建串行队列
    dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", NULL);
    
    dispatch_async(serialQueue, ^{
        NSLog(@"任务1 %@", [NSThread currentThread]);
    });
    
    dispatch_async(serialQueue, ^{
        NSLog(@"任务2 %@", [NSThread currentThread]);
    });
    
    dispatch_async(serialQueue, ^{
        NSLog(@"任务3 %@", [NSThread currentThread]);
    });
    
    //输出:   当前线程 <NSThread: 0x7fc192e05380>{number = 1, name = main}
    //输出:   任务1 <NSThread: 0x7fc192c16670>{number = 3, name = (null)}
    //输出:   任务2 <NSThread: 0x7fc192c16670>{number = 3, name = (null)}
    //输出:   任务3 <NSThread: 0x7fc192c16670>{number = 3, name = (null)}
同步函数_并发队列

(不会开启新线程,并发执行任务失效!)

    NSLog(@"当前线程 %@", [NSThread currentThread]);
    dispatch_sync(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务1 %@", [NSThread currentThread]);
    });
    
    dispatch_sync(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务2 %@", [NSThread currentThread]);
    });
    
    dispatch_sync(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务3 %@", [NSThread currentThread]);
    });
    
    //输出:   当前线程 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
    //输出:   任务1 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
    //输出:   任务2 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
    //输出:   任务3 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
同步函数_串行队列

(不会开启新线程,串行执行任务)

 NSLog(@"当前线程 %@", [NSThread currentThread]);
 //创建串行队列
 dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", NULL);
 dispatch_sync(serialQueue, ^{
     NSLog(@"任务1 %@", [NSThread currentThread]);
 });

 dispatch_sync(serialQueue, ^{
      NSLog(@"任务2 %@", [NSThread currentThread]);
 });

 dispatch_sync(serialQueue, ^{
     NSLog(@"任务3 %@", [NSThread currentThread]);
 });

 //输出:   当前线程 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}
 //输出:   任务1 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}
 //输出:   任务2 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}
 //输出:   任务3 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}

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