GCD 隊列組：dispatch_group

有時候我們會有這樣的需求：分別異步執(zhí)行2個耗時任務(wù)，然后當(dāng)2個耗時任務(wù)都執(zhí)行完畢后再回到主線程執(zhí)行任務(wù)。這時候我們可以用到 GCD的隊列組。

• 調(diào)用隊列組的 dispatch_group_async 先把任務(wù)放到隊列中，然后將隊列放入隊列組中。或者使用隊列組的 dispatch_group_enter、dispatch_group_leave 組合 來實現(xiàn)dispatch_group_async。
• 調(diào)用隊列組的 dispatch_group_notify 回到指定線程執(zhí)行任務(wù)?；蛘呤褂?dispatch_group_wait回到當(dāng)前線程繼續(xù)向下執(zhí)行（會阻塞當(dāng)前線程）。

dispatch_group_notify

監(jiān)聽 group中任務(wù)的完成狀態(tài)，當(dāng)所有的任務(wù)都執(zhí)行完成后，追加任務(wù)到 group中，并執(zhí)行任務(wù)。

/**
 * 隊列組 dispatch_group_notify
 */
- (void)groupNotify {
    
    // 打印當(dāng)前線程
    NSLog(@"currentThread-->%@",[NSThread currentThread]);
    NSLog(@"group-->begin");
    dispatch_group_t group =  dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    dispatch_group_async(group, queue, ^{
        // 追加任務(wù)1
        for (int i = 0; i < 2; ++i) {
            // 模擬耗時操作
            [NSThread sleepForTimeInterval:2];
            // 打印當(dāng)前線程
            NSLog(@"1-->%@",[NSThread currentThread]);
        }
    });
    dispatch_group_async(group, queue, ^{
        // 追加任務(wù)2
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"2-->%@",[NSThread currentThread]);
        }
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // 等前面的異步任務(wù)1、任務(wù)2都執(zhí)行完畢后，回到主線程執(zhí)行下邊任務(wù)
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"3-->%@",[NSThread currentThread]);
        }
        NSLog(@"group-->end");
    });
}

輸出結(jié)果

currentThread--><NSThread: 0x2818cae80>{number = 1, name = main}
group-->begin
1--><NSThread: 0x281844340>{number = 5, name = (null)}
2--><NSThread: 0x281840740>{number = 6, name = (null)}
2--><NSThread: 0x281840740>{number = 6, name = (null)}
1--><NSThread: 0x281844340>{number = 5, name = (null)}
3--><NSThread: 0x2818cae80>{number = 1, name = main}
3--><NSThread: 0x2818cae80>{number = 1, name = main}
group-->end

從dispatch_group_notify相關(guān)代碼運行輸出結(jié)果可以看出：當(dāng)所有任務(wù)都執(zhí)行完成之后，才執(zhí)行dispatch_group_notify block中的任務(wù)。

dispatch_group_wait

暫停當(dāng)前線程（阻塞當(dāng)前線程），等待指定的 group中的任務(wù)執(zhí)行完成后，才會往下繼續(xù)執(zhí)行。

/**
 * 隊列組 dispatch_group_wait
 */
- (void)groupWait {
    
    NSLog(@"currentThread-->%@",[NSThread currentThread]);
    NSLog(@"group-->begin");
    dispatch_group_t group =  dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_group_async(group, queue, ^{
        // 追加任務(wù)1
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"1-->%@",[NSThread currentThread]);
        }
    });
    dispatch_group_async(group, queue, ^{
        // 追加任務(wù)2
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"2-->%@",[NSThread currentThread]);
        }
    });
    // 等待上面的任務(wù)全部完成后，會往下繼續(xù)執(zhí)行（會阻塞當(dāng)前線程）
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"group-->end");
}

輸出結(jié)果

currentThread--><NSThread: 0x281246e40>{number = 1, name = main}
group-->begin
2--><NSThread: 0x2812cde00>{number = 5, name = (null)}
1--><NSThread: 0x2812c8640>{number = 4, name = (null)}
2--><NSThread: 0x2812cde00>{number = 5, name = (null)}
1--><NSThread: 0x2812c8640>{number = 4, name = (null)}
group-->end

從dispatch_group_wait相關(guān)代碼運行輸出結(jié)果可以看出：當(dāng)所有任務(wù)執(zhí)行完成之后，才執(zhí)行 dispatch_group_wait之后的操作。但是，使用dispatch_group_wait 會阻塞當(dāng)前線程。

dispatch_group_enter、dispatch_group_leave

• dispatch_group_enter標(biāo)志著一個任務(wù)追加到 group，執(zhí)行一次，相當(dāng)于group 中未執(zhí)行完畢任務(wù)數(shù)+1
• dispatch_group_leave標(biāo)志著一個任務(wù)離開了 group，執(zhí)行一次，相當(dāng)于group 中未執(zhí)行完畢任務(wù)數(shù)-1。
• 當(dāng) group中未執(zhí)行完畢任務(wù)數(shù)為0的時候，才會使dispatch_group_wait解除阻塞，以及執(zhí)行追加到dispatch_group_notify中的任務(wù)。

/**
 * 隊列組 dispatch_group_enter、dispatch_group_leave
 */
- (void)groupEnterAndLeave {
    
    NSLog(@"currentThread-->%@",[NSThread currentThread]);
    NSLog(@"group-->begin");
    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, ^{
        // 追加任務(wù)1
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"1-->%@",[NSThread currentThread]);
        }
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        // 追加任務(wù)2
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"2-->%@",[NSThread currentThread]);
        }
        dispatch_group_leave(group);
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // 等前面的異步操作都執(zhí)行完畢后，回到主線程.
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];
            NSLog(@"3-->%@",[NSThread currentThread]);
        }
        NSLog(@"group_notify-->end");
    });
    // 等待上面的任務(wù)全部完成后，會往下繼續(xù)執(zhí)行（會阻塞當(dāng)前線程）
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"group---end");
}

輸出

currentThread--><NSThread: 0x282901a80>{number = 1, name = main}
group-->begin
1--><NSThread: 0x28294b7c0>{number = 4, name = (null)}
2--><NSThread: 0x282948cc0>{number = 3, name = (null)}
2--><NSThread: 0x282948cc0>{number = 3, name = (null)}
1--><NSThread: 0x28294b7c0>{number = 4, name = (null)}
group---end
3--><NSThread: 0x282901a80>{number = 1, name = main}
3--><NSThread: 0x282901a80>{number = 1, name = main}
group_notify-->end

從dispatch_group_enter、dispatch_group_leave相關(guān)代碼運行結(jié)果中可以看出：當(dāng)所有任務(wù)執(zhí)行完成之后，才執(zhí)行dispatch_group_notify 中的任務(wù)。這里的dispatch_group_enter、dispatch_group_leave組合，其實等同于dispatch_group_async