GCD的group方法只有async（異步方法）即

void dispatch_group_async(dispatch_group_t group,
    dispatch_queue_t queue,
    dispatch_block_t block);

使用group要傳2個(gè)參數(shù) group、queue，其中g(shù)roup使用

//創(chuàng)建group
dispatch_group_t group = dispatch_group_create();

queue我們可以使用以下幾種方式

dispatch_queue_t queue = dispatch_get_main_queue()
//或者 
queue = dispatch_get_global_queue(0, 0);
//或者 并行隊(duì)列
queue = dispatch_queue_create("com.sccc", DISPATCH_QUEUE_CONCURRENT);
//或者串行隊(duì)列
queue = dispatch_queue_create("com.sccc", DISPATCH_QUEUE_SERIAL);

這里主要區(qū)分串行和并行的方式

- (void)doSomething:(void (^)())handler {
    if (handler) {
        sleep(2);
        handler();
    }
}

//異步串行g(shù)roup
- (void)groupAsyncSerialTest {
    //創(chuàng)建串行隊(duì)列
    dispatch_queue_t queue = dispatch_queue_create("com.sccc", DISPATCH_QUEUE_SERIAL);
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_async(group, queue, ^{
        [self doSomething:^() {
            NSLog(@"任務(wù)一");
        }];
    });
    dispatch_group_async(group, queue, ^{
        [self doSomething:^() {
            NSLog(@"任務(wù)二");
        }];
    });
    dispatch_group_async(group, queue, ^{
        [self doSomething:^() {
            NSLog(@"任務(wù)三");
        }];
    });
    dispatch_group_notify(group, queue, ^{
        NSLog(@"前面的任務(wù)已完成");
    });
}

結(jié)果

串行隊(duì)列打印結(jié)果

值得注意的是按照順序每2s打印一次

串行隊(duì)列的情況如下：

//異步并行
- (void)groupAsyncConcurrentTest {
    //創(chuàng)建并行隊(duì)列
    dispatch_queue_t queue = dispatch_queue_create("com.sccc", DISPATCH_QUEUE_CONCURRENT);
    queue = dispatch_get_global_queue(0, 0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue, ^{
        [self doSomething:^() {
            NSLog(@"任務(wù)一");
        }];
    });
    dispatch_group_async(group, queue, ^{
        [self doSomething:^() {
            NSLog(@"任務(wù)二");
        }];
    });
    dispatch_group_async(group, queue, ^{
        [self doSomething:^() {
            NSLog(@"任務(wù)三");
        }];
    });
    dispatch_group_notify(group, queue, ^{
        NSLog(@"前面的任務(wù)已完成");
    });
}

結(jié)果如下

并行打印結(jié)果

多次試驗(yàn)總結(jié)的結(jié)果是任務(wù)完成先后順序不定，前面的任務(wù)都完成后才會(huì)執(zhí)行notify中的任務(wù)

當(dāng)然以上的情況不適用于網(wǎng)絡(luò)請求
網(wǎng)絡(luò)請求使用group時(shí)需要注意

dispatch_group_enter(group);
dispatch_group_leave(group);

搭配使用
下面是沒有搭配使用的情況

//異步并行
- (void)groupAsyncConcurrentTest {
    //創(chuàng)建并行隊(duì)列
    dispatch_queue_t queue = dispatch_queue_create("com.sccc", DISPATCH_QUEUE_CONCURRENT);
    queue = dispatch_get_global_queue(0, 0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue, ^{
        [[CMAppRequest sharedInstance] guideMessage:^() {
           NSLog(@"任務(wù)一");
        }];
    });
    dispatch_group_async(group, queue, ^{
        [[CMAppRequest sharedInstance] guideMessage:^() {
            NSLog(@"任務(wù)二");
        }];
    });
    dispatch_group_async(group, queue, ^{
        [[CMAppRequest sharedInstance] guideMessage:^() {
            NSLog(@"任務(wù)三");
        }];
    });
    dispatch_group_notify(group, queue, ^{
        NSLog(@"前面的任務(wù)已完成");
    });
}

結(jié)果如下

網(wǎng)絡(luò)請求打印結(jié)果

你會(huì)發(fā)現(xiàn)notify不起作用了，這時(shí)候就要用到enter、leave了
修改代碼如下：

//異步并行
- (void)groupAsyncConcurrentTest {
    //創(chuàng)建并行隊(duì)列
    dispatch_queue_t queue = dispatch_queue_create("com.sccc", DISPATCH_QUEUE_CONCURRENT);
    queue = dispatch_get_global_queue(0, 0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_enter(group);
    [[CMAppRequest sharedInstance] guideMessage:^() {
           dispatch_group_leave(group);
           NSLog(@"任務(wù)一");
    }];
    dispatch_group_enter(group);
    [[CMAppRequest sharedInstance] guideMessage:^() {
            dispatch_group_leave(group);
            NSLog(@"任務(wù)二");
    }];
    dispatch_group_enter(group);
    [[CMAppRequest sharedInstance] guideMessage:^() {
            dispatch_group_leave(group);
            NSLog(@"任務(wù)三");
    }];
    dispatch_group_notify(group, queue, ^{
        NSLog(@"前面的任務(wù)已完成");
    });
}

//考慮到請求是異步隊(duì)列，不在需要使用dispatch_group_async

結(jié)果如下：

這就正常了，網(wǎng)絡(luò)請求的就不在做串行和并行的測試了，以上網(wǎng)絡(luò)請求部分使用的是串行隊(duì)列，串行的打印結(jié)果只和網(wǎng)絡(luò)請求的返回順序有關(guān)。

• 最后特別需要注意，enter和leave一定要成對出現(xiàn)；否則如實(shí)enter多了，notify里的任務(wù)就會(huì)永遠(yuǎn)執(zhí)行不了，若是levae多了則會(huì)照成crash

當(dāng)然這只是group的一部分還有dispatch_group_wait等功能沒有一一實(shí)現(xiàn)，以上內(nèi)容只代表個(gè)人觀點(diǎn)，如有問題歡迎指正，謝謝~~