• 對(duì)同步、異步、串行、并行的執(zhí)行順序，之前總是模棱兩可，今天寫個(gè)Demo測(cè)試一下，廢話不多說（其實(shí)也不會(huì)說），直接上測(cè)試代碼

一、首先定義兩個(gè)方法,一個(gè)同步執(zhí)行相應(yīng)代碼，一個(gè)異步執(zhí)行

//同步執(zhí)行
- (void)syncTestWithQueue:(dispatch_queue_t)queue withBarrier:(BOOL)barrer{
    NSLog(@"1");
    dispatch_sync(queue, ^{
        NSLog(@"start 1");
        sleep(3);
        NSLog(@"end 1");
    });
    NSLog(@"2");
    dispatch_sync(queue, ^{
        NSLog(@"start 2");
        sleep(3);
        NSLog(@"end 2");
    });
    NSLog(@"3");
    dispatch_sync(queue, ^{
        NSLog(@"start 3");
        sleep(3);
        NSLog(@"end 3");
    });
    NSLog(@"4");
    dispatch_sync(queue, ^{
        NSLog(@"start 4");
        sleep(3);
        NSLog(@"end 4");
    });
    NSLog(@"5");
    
    if (barrer) {
        dispatch_barrier_sync(queue, ^{
            NSLog(@"barrier start 1");
            sleep(3);
            NSLog(@"barrier end 1");
        });
        NSLog(@"6");
        dispatch_sync(queue, ^{
            NSLog(@"start 5");
            sleep(3);
            NSLog(@"end 5");
        });
        NSLog(@"7");
    }
}

//異步執(zhí)行
- (void)asyncTestWithQueue:(dispatch_queue_t)queue withBarrier:(BOOL)barrer{
    NSLog(@"1");
    dispatch_async(queue, ^{
        NSLog(@"start 1");
        sleep(3);
        NSLog(@"end 1");
    });
    NSLog(@"2");
    dispatch_async(queue, ^{
        NSLog(@"start 2");
        sleep(3);
        NSLog(@"end 2");
    });
    NSLog(@"3");
    dispatch_async(queue, ^{
        NSLog(@"start 3");
        sleep(3);
        NSLog(@"end 3");
    });
    NSLog(@"4");
    
    if (barrer) {
        dispatch_barrier_async(queue, ^{
            NSLog(@"barrier start 1");
            sleep(3);
            NSLog(@"barrier end 1");
        });
        NSLog(@"5");
        dispatch_async(queue, ^{
            NSLog(@"start 4");
            sleep(3);
            NSLog(@"end 4");
        });
        NSLog(@"6");
    }
}

二、在自行創(chuàng)建串行隊(duì)列執(zhí)行這兩個(gè)方法

測(cè)試代碼

    //自行創(chuàng)建串行隊(duì)列
    dispatch_queue_t serialQueue=dispatch_queue_create("串行隊(duì)列", DISPATCH_QUEUE_SERIAL);
    //1.同步
    [self syncTestWithQueue:serialQueue withBarrier:YES];
    //2.異步
    [self asyncTestWithQueue:serialQueue withBarrier:YES];

輸出結(jié)果

syncTestWithQueue: withBarrier:

asyncTestWithQueue: withBarrier:

結(jié)論：

1.同步與異步block的執(zhí)行順序都是按順序執(zhí)行的，不同的是同步會(huì)阻塞線程，異步不會(huì)
2.dispatch_barrier,執(zhí)行順序 和 dispatch_sync與dispatch_async加入的block是一樣的

三、主線程串行隊(duì)列

測(cè)試代碼

    dispatch_queue_t mainSerialQueue=dispatch_get_main_queue();
    //1.同步
    //會(huì)阻塞，不用想了
    [self syncTestWithQueue:mainSerialQueue withBarrier:YES];
    //2.異步
    [self asyncTestWithQueue:mainSerialQueue withBarrier:YES];

輸出結(jié)果

asyncTestWithQueue: withBarrier:

結(jié)論

同步會(huì)阻塞，異步和自行創(chuàng)建串行隊(duì)列是一樣的

四、自行創(chuàng)建并行隊(duì)列

測(cè)試code

    dispatch_queue_t concurrentQueue=dispatch_queue_create("并行隊(duì)列", DISPATCH_QUEUE_CONCURRENT);
    //1.同步
    [self syncTestWithQueue:concurrentQueue withBarrier:YES];
    //2.異步
    [self asyncTestWithQueue:concurrentQueue withBarrier:YES];

輸出

syncTestWithQueue: withBarrier:

asyncTestWithQueue: withBarrier:

結(jié)論

1.同步：由于會(huì)阻塞線程，所以并行與串行執(zhí)行順序是一樣的
2.異步：所有block都會(huì)一起執(zhí)行，dispatch_barrier執(zhí)行時(shí)之后加入的block，會(huì)等待barrier執(zhí)行完再執(zhí)行

五、全局并行隊(duì)列

測(cè)試code

    dispatch_queue_t globalConcurrentQueue=dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    //1.同步
    [self syncTestWithQueue:globalConcurrentQueue withBarrier:YES];
    //2.異步
    [self asyncTestWithQueue:globalConcurrentQueue withBarrier:YES];

輸出

syncTestWithQueue: withBarrier:

asyncTestWithQueue: withBarrier:

結(jié)論

1.同步：與自行創(chuàng)建并發(fā)隊(duì)列是一樣的
2.異步：所有block都會(huì)一起執(zhí)行，但是，dispatch_barrier執(zhí)行時(shí)之后加入的block，不會(huì)等待barrier執(zhí)行完再執(zhí)行(dispatch_barrier不要用在全局并發(fā)隊(duì)列里)