前言：

常常遇到一个页面有多个请求，这些请求当中缺存在某些依赖关系的情况，鉴于网络请求当中，大多属于异步请求，无法及时得到我们想要的数据。于是需要用到一些方法，使异步请求执行同步操作。

方法一：使用GCD串行队列
首先要搞清楚在队列当中，有串行和并行

// DISPATCH_QUEUE_CONCURRENT + async 开启多条线程
// DISPATCH_QUEUE_SERIAL + async 不开启多条线程

dispatch_queue_t queue = dispatch_queue_create("my queue", DISPATCH_QUEUE_SERIAL);
    
    dispatch_async(queue, ^{
        for (int i=0; i<5; i++) {
            NSLog(@"task-1---%d---%@",i,[NSThread currentThread]);
            sleep(1.0);
        }
    });
    
    dispatch_async(queue, ^{
        for (int i=0; i<5; i++) {
            NSLog(@"task-2---%d---%@",i,[NSThread currentThread]);
            sleep(1.0);
        }
    });
    
    dispatch_async(queue, ^{
        for (int i=0; i<5; i++) {
            NSLog(@"task-3---%d---%@",i,[NSThread currentThread]);
            sleep(1.0);
        }
    });

看看打印结果，执行完task-1后，依次执行task-2，再执行了task-3

1556785739282.jpg

方法二：barrier GCD

    dispatch_queue_t queue = dispatch_queue_create("my concurrent queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_suspend(queue);// 挂起
    
    dispatch_async(queue, ^{
        [self loadDataA:^(BOOL success) {
            NSLog(@" A 加载完成");
        }];
    });


    dispatch_barrier_async(queue, ^{
        NSLog(@"--this is a barrier--");
    });

    dispatch_async(queue, ^{
        [self loadDataB:^(BOOL success) {
            NSLog(@" B 加载完成");
        }];
    });

    dispatch_resume(queue);// 恢复

image.png

方法三：dispatch_group_enter + dispatch_group_leave
注意：dispatch_group_enter() 和 dispatch_group_leave() 一定是配套使用的

dispatch_group_t group = dispatch_group_create();
   dispatch_group_enter(group);
   [self loadDataA:^(BOOL success) {
       if (success) {
           NSLog(@"A load finished...");
           dispatch_group_leave(group);
       }else {
           finished(NO);
       }
   }];
   
   dispatch_group_enter(group);
   [self loadDataB:^(BOOL success) {
       if (success) {
           NSLog(@"B load finished...");
           dispatch_group_leave(group);
       }else {
           finished(NO);
       }
   }];
   
   dispatch_group_notify(group, dispatch_get_main_queue(), ^{
       finished(YES);
       NSLog(@"全部加载完成。。。%@",[NSThread currentThread]);
   });

image.png

方法四：NSOperation 设置依赖关系

   NSOperationQueue *operarionQueue = [[NSOperationQueue alloc]init];
   
   NSBlockOperation *lastOperation = [NSBlockOperation blockOperationWithBlock:^{
       sleep(1.0);
       NSLog(@"最后的任务！");
   }];
   [operarionQueue addOperation:lastOperation];
   for (int i=0; i<4; i++) {
       NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
           sleep(i);
           NSLog(@"任务%d",i+1);
       }];
       if (i==2) {
           [operation addDependency:lastOperation];
       }
       [operarionQueue addOperation:operation];
   }

image.png

if (i==2) {
         [operation addDependency:lastOperation];
}

将lastOperation依赖于operation2执行，所以只有任务2执行完成，才会执行lastOperation

本文仅个人理解，欢迎指正和补充。。。觉得有用的可以点个赞哈！