iOS开发中在指定的某些线程执行完之后去执行其他线程

2.1 利用在组中所有的线程执行完之后再去执行其他的线程

2.2 利用wait

 dispatch\_group\_t group = dispatch\_group\_create(); 

 // 并行队列 

 dispatch\_queue\_t queue = dispatch\_get\_global\_queue(DISPATCH\_QUEUE\_PRIORITY\_DEFAULT, 0); 

 // 进入组（进入组和离开组必须成对出现, 否则会造成死锁） 

 dispatch\_group\_enter(group); 

 dispatch\_group\_async(group, queue, ^{ 

     // 执行异步任务1 

     [NSThread sleepForTimeInterval:2]; 

     for (int i = 0; i < 3; i ++) { 

         NSLog(@”1—%@”,[NSThread currentThread ]);    // 子线程 

     } 

     \_str1 = @”str1”; 

     dispatch\_group\_leave(group); 

 });

// 进入组
dispatch_group_enter(group);
dispatch_group_async(group, queue, ^{
    // 执行异步任务2
    [NSThread sleepForTimeInterval:2];
    for (int i = 3; i < 6; i ++) {
        NSLog(@"2---%@",[NSThread currentThread ]);
    }
    _str2 = @"str2";
    dispatch_group_leave(group);

});
// wait
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

dispatch_async(dispatch_get_global_queue(0, 0), ^{
    NSLog(@"%@", [NSThread currentThread]);
    _str1 = @"str..";
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"%@", _str1);
        NSLog(@"%@", _str2);
        NSLog(@"%@", [NSThread currentThread]); // 主线程
    });
});

return;
dispatch_group_notify(group, queue, ^{  // 监听组里所有线程完成的情况

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
       NSLog(@"%@", [NSThread currentThread]);
        _str1 = @"str..";

        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"%@", _str1);
            NSLog(@"%@", _str2);
            NSLog(@"%@", [NSThread currentThread]); // 主线程
            NSLog(@"完成...");
        });

    });

});