队列组 iOS之多线程GCD（二）

逃离我推掉我的手 2022-01-19 03:17 249阅读 0赞

## dispatch\_group\_async、dispatch\_group\_notify ##

**特点**：当任务管理中的任务执行完会通知函数 dispatch\_group\_notify

我们经常遇到这样的面试题：异步下载几张图片、等待所有图片下载完成、合并一张大图、更新UI等等之类的需求。今天我们就用队列组解决这个问题。这里要用到dispatch\_group\_notify函数。

效果如下图所示：（为了便于理解、三张异步下载的小图也加载出来）

**代码部分如下**

//三张小图数组
        NSMutableArray *array1;
        //一张大图数组
        NSMutableArray *array2;
        
        //图片一二进制数据
        NSData *data1;
        //图片二二进制数据
        NSData *data2;
        //图片三二进制数据
        NSData *data3;
        
    - (void)groupNotify
    {
        NSLog(@"当前的线程为：%@",[NSThread currentThread]);
        NSLog(@"开始");
        dispatch_group_t group = dispatch_group_create();
        
        
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // 追加任务1
            for (int i = 0; i < 1; ++i) {
                [NSThread sleepForTimeInterval:1];
                
                NSLog(@"我就是任务一、来自线程：%@",[NSThread currentThread]);
                
                NSURL *url = [NSURL URLWithString:@"http://imgsrc.baidu.com/imgad/pic/item/34fae6cd7b899e51fab3e9c048a7d933c8950d21.jpg"];
                self->data1 = [NSData dataWithContentsOfURL:url];
                UIImageView *imageView = self->array1[0];
                dispatch_async(dispatch_get_main_queue(), ^{
                    
                    imageView.image = [UIImage imageWithData:self->data1];
                    
                });
                
            }
        });
     
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // 追加任务2
            for (int i = 0; i < 1; ++i) {
                [NSThread sleepForTimeInterval:1];
                
                NSLog(@"我就是任务二、来自线程：%@",[NSThread currentThread]);
                
                NSURL *url = [NSURL URLWithString:@"http://img17.3lian.com/d/file/201702/18/2b5f1b6298411b0045c5562da02fc6ac.jpg"];
                self->data2 = [NSData dataWithContentsOfURL:url];
                UIImageView *imageView = self->array1[1];
                dispatch_async(dispatch_get_main_queue(), ^{
                    
                    imageView.image = [UIImage imageWithData:self->data2];
                
                });
                
            }
        });
        
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // 追加任务3
            for (int i = 0; i < 1; ++i) {
                [NSThread sleepForTimeInterval:1];
                
                NSLog(@"我就是任务三、来自线程：%@",[NSThread currentThread]);
                
                NSURL *url = [NSURL URLWithString:@"http://img17.3lian.com/d/file/201702/23/dc458f76475470db9a5791848cb67801.jpg"];
                self->data3 = [NSData dataWithContentsOfURL:url];
                UIImageView *imageView = self->array1[2];
                dispatch_async(dispatch_get_main_queue(), ^{
                   
                    imageView.image = [UIImage imageWithData:self->data3];
                
                });
            }
            
        });
        
        dispatch_group_notify(group, dispatch_get_main_queue(), ^{
            // 所有图片下载完毕、合成一张图
            for (int i = 0; i < 1; ++i) {
                [NSThread sleepForTimeInterval:1];
                
                UIImageView *imageView1 = self->array2[0];
                UIImageView *imageView2 = self->array2[1];
                UIImageView *imageView3 = self->array2[2];
                imageView1.image = [UIImage imageWithData:self->data1];
                imageView2.image = [UIImage imageWithData:self->data2];
                imageView3.image = [UIImage imageWithData:self->data3];
                NSLog(@"我是要等任务-、任务二、任务三完成以后的最终任务、来自线程：%@",[NSThread currentThread]);
            }
            
            NSLog(@"结束");
        });
    }
    复制代码

**打印结果：**

2019-01-24 15:54:22.755688+0800 多线程demo[8498:280372] 当前的线程为：<NSThread: 0x60400006b9c0>{number = 1, name = main}
    2019-01-24 15:54:22.755843+0800 多线程demo[8498:280372] 开始
    2019-01-24 15:54:23.760460+0800 多线程demo[8498:280446] 我就是任务三、来自线程：<NSThread: 0x604000477640>{number = 5, name = (null)}
    2019-01-24 15:54:23.760459+0800 多线程demo[8498:280447] 我就是任务二、来自线程：<NSThread: 0x604000477680>{number = 4, name = (null)}
    2019-01-24 15:54:23.760459+0800 多线程demo[8498:280444] 我就是任务一、来自线程：<NSThread: 0x600000264b80>{number = 3, name = (null)}
    2019-01-24 15:54:31.972052+0800 多线程demo[8498:280372] 我是要等任务-、任务二、任务三完成以后的最终任务、来自线程：<NSThread: 0x60400006b9c0>{number = 1, name = main}
    2019-01-24 15:54:31.972156+0800 多线程demo[8498:280372] 结束
    复制代码

分析：任务一、任务二、任务三采用异步下载（有各自的线程）、所以下载过程中是交错的而不是顺序执行，这也使得UI流畅，体验感好。所有任务完成时候、会走进dispatch\_group\_notify函数、在主线程更新UI，从而完成需求。

## dispatch\_group\_enter、dispatch\_group\_leave ##

同样是类似上面的需求、当不用dispatch\_group\_async函数时、结合以上三个函数也能实现。

**dispatch\_group\_enter、dispatch\_group\_leave成对出现、前者任务+1后者任务—1。当为0时才会追加到dispatch\_group\_notify函数中。**

代码如下：(第一种注释dispatch\_group\_enter、dispatch\_group\_leave第二种打开注释)

NSLog(@"当前线程为：%@",[NSThread currentThread]);  // 打印当前线程
        NSLog(@"开始");
        
        dispatch_group_t group = dispatch_group_create();
        dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    //    dispatch_group_enter(group);
        dispatch_async(queue, ^{
            // 追加任务1
            for (int i = 0; i < 2; ++i) {
                 sleep(1);
                NSLog(@"我是任务一、来自线程%@",[NSThread currentThread]);
            }
    //        dispatch_group_leave(group);
        });
        
    //    dispatch_group_enter(group);
        dispatch_async(queue, ^{
            // 追加任务2
            for (int i = 0; i < 2; ++i) {
                sleep(1);
                NSLog(@"我是任务二、来自线程%@",[NSThread currentThread]);
            }
    //        dispatch_group_leave(group);
        });
        
        dispatch_group_notify(group, dispatch_get_main_queue(), ^{
            
            for (int i = 0; i < 2; ++i) {
                 sleep(1);
                NSLog(@"我是任务三、来自线程%@",[NSThread currentThread]);
            }
            NSLog(@"结束");
        });
    复制代码

**打印结果一**：（注释dispatch\_group\_enter、dispatch\_group\_leave）

2019-01-24 16:55:42.769970+0800 多线程demo[9519:323831] 当前线程为：<NSThread: 0x604000071100>{number = 1, name = main}
    2019-01-24 16:55:42.770108+0800 多线程demo[9519:323831] 开始
    2019-01-24 16:55:43.773884+0800 多线程demo[9519:323889] 我是任务一、来自线程<NSThread: 0x60400047a000>{number = 3, name = (null)}
    2019-01-24 16:55:43.773902+0800 多线程demo[9519:323890] 我是任务二、来自线程<NSThread: 0x600000276d40>{number = 4, name = (null)}
    2019-01-24 16:55:43.800804+0800 多线程demo[9519:323831] 我是任务三、来自线程<NSThread: 0x604000071100>{number = 1, name = main}
    2019-01-24 16:55:44.776018+0800 多线程demo[9519:323889] 我是任务一、来自线程<NSThread: 0x60400047a000>{number = 3, name = (null)}
    2019-01-24 16:55:44.776010+0800 多线程demo[9519:323890] 我是任务二、来自线程<NSThread: 0x600000276d40>{number = 4, name = (null)}
    2019-01-24 16:55:44.802165+0800 多线程demo[9519:323831] 我是任务三、来自线程<NSThread: 0x604000071100>{number = 1, name = main}
    2019-01-24 16:55:44.802362+0800 多线程demo[9519:323831] 结束
    复制代码

分析结果：没有dispatch\_group\_enter、dispatch\_group\_leave函数、三个任务交错执行、互不影响。

**打印结果二**：（打开注释）

2019-01-24 17:00:05.609413+0800 多线程demo[9608:327439] 当前线程为：<NSThread: 0x604000068ac0>{number = 1, name = main}
    2019-01-24 17:00:05.609562+0800 多线程demo[9608:327439] 开始
    2019-01-24 17:00:06.614555+0800 多线程demo[9608:327506] 我是任务二、来自线程<NSThread: 0x60400046de00>{number = 3, name = (null)}
    2019-01-24 17:00:06.614622+0800 多线程demo[9608:327507] 我是任务一、来自线程<NSThread: 0x60400027f4c0>{number = 4, name = (null)}
    2019-01-24 17:00:07.620524+0800 多线程demo[9608:327506] 我是任务二、来自线程<NSThread: 0x60400046de00>{number = 3, name = (null)}
    2019-01-24 17:00:07.620543+0800 多线程demo[9608:327507] 我是任务一、来自线程<NSThread: 0x60400027f4c0>{number = 4, name = (null)}
    2019-01-24 17:00:08.620973+0800 多线程demo[9608:327439] 我是任务三、来自线程<NSThread: 0x604000068ac0>{number = 1, name = main}
    2019-01-24 17:00:09.621951+0800 多线程demo[9608:327439] 我是任务三、来自线程<NSThread: 0x604000068ac0>{number = 1, name = main}
    2019-01-24 17:00:09.622129+0800 多线程demo[9608:327439] 结束
    复制代码

**分析结果**：任务三始终在任务一、二结束完毕执行。任务一、任务二交错执行。

## dispatch\_group\_wait ##

**特点**：group中的任务完成之后 才能执行下面的函数。 代码如下：

NSLog(@"当前线程为：%@",[NSThread currentThread]);  // 打印当前线程
        NSLog(@"开始");
        
        dispatch_group_t group =  dispatch_group_create();
        
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // 任务1
            for (int i = 0; i < 2; i ++) {
                sleep(1);
                NSLog(@"我是任务一、来自线程：%@",[NSThread currentThread]);
            }
        });
        
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // 任务2
            for (int i = 0; i < 2; i ++) {
                sleep(1);
                NSLog(@"我是任务二、来自线程：%@",[NSThread currentThread]);
            }
        });
        
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // 任务3
            for (int i = 0; i < 2; i ++) {
                sleep(1);
                NSLog(@"我是任务三、来自线程：%@",[NSThread currentThread]);
            }
        });
        
        dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
        
        NSLog(@"我一定要最后离开、因为我爱你");
    复制代码

**打印结果**：

1.没dispatch\_group\_wait函数

2019-01-24 17:08:22.912309+0800 多线程demo[9777:334058] 当前线程为：<NSThread: 0x600000261500>{number = 1, name = main}
    2019-01-24 17:08:22.912460+0800 多线程demo[9777:334058] 开始
    2019-01-24 17:08:22.912600+0800 多线程demo[9777:334058] 我一定要最后离开、因为我爱你
    2019-01-24 17:08:23.918052+0800 多线程demo[9777:334133] 我是任务三、来自线程：<NSThread: 0x6000004697c0>{number = 4, name = (null)}
    2019-01-24 17:08:23.918048+0800 多线程demo[9777:334131] 我是任务一、来自线程：<NSThread: 0x60000046efc0>{number = 3, name = (null)}
    2019-01-24 17:08:23.918050+0800 多线程demo[9777:334132] 我是任务二、来自线程：<NSThread: 0x604000660800>{number = 5, name = (null)}
    2019-01-24 17:08:24.923557+0800 多线程demo[9777:334132] 我是任务二、来自线程：<NSThread: 0x604000660800>{number = 5, name = (null)}
    2019-01-24 17:08:24.923557+0800 多线程demo[9777:334133] 我是任务三、来自线程：<NSThread: 0x6000004697c0>{number = 4, name = (null)}
    2019-01-24 17:08:24.923557+0800 多线程demo[9777:334131] 我是任务一、来自线程：<NSThread: 0x60000046efc0>{number = 3, name = (null)}
    
    复制代码

**分析**： NSLog(@"我一定要最后离开、因为我爱你");并没有最后离开。所有任务交错执行。

2.有dispatch\_group\_wait函数

2019-01-24 17:14:23.603205+0800 多线程demo[9928:339432] 当前线程为：<NSThread: 0x60400006c280>{number = 1, name = main}
    2019-01-24 17:14:23.603338+0800 多线程demo[9928:339432] 开始
    2019-01-24 17:14:24.605474+0800 多线程demo[9928:339503] 我是任务三、来自线程：<NSThread: 0x600000467bc0>{number = 3, name = (null)}
    2019-01-24 17:14:24.605474+0800 多线程demo[9928:339495] 我是任务二、来自线程：<NSThread: 0x6040002781c0>{number = 4, name = (null)}
    2019-01-24 17:14:24.605474+0800 多线程demo[9928:339493] 我是任务一、来自线程：<NSThread: 0x600000467b00>{number = 5, name = (null)}
    2019-01-24 17:14:25.606206+0800 多线程demo[9928:339495] 我是任务二、来自线程：<NSThread: 0x6040002781c0>{number = 4, name = (null)}
    2019-01-24 17:14:25.606220+0800 多线程demo[9928:339503] 我是任务三、来自线程：<NSThread: 0x600000467bc0>{number = 3, name = (null)}
    2019-01-24 17:14:25.606261+0800 多线程demo[9928:339493] 我是任务一、来自线程：<NSThread: 0x600000467b00>{number = 5, name = (null)}
    2019-01-24 17:14:25.606992+0800 多线程demo[9928:339432] 我一定要最后离开、因为我爱你
    复制代码

**分析**：确实最后离开了、dispatch\_group\_wait阻塞了当前线程、dispatch\_group\_wait后面的方法不能执行。

转载于:https://juejin.im/post/5c490d1b6fb9a04a0b2285f5