iOS:在完成多个后台线程之前,不要从函数返回

Question

这看起来应该很简单,但我遇到了很多麻烦.我有一个函数可以触发一些在后台运行并具有完成块的其他函数.我希望我的函数等到所有完成块都被返回之前被调用.

我无法控制我在后台执行的函数.否则我只是修改它以使用dispatch_async和我自己的队列,然后等待该队列完成.

我的情况看起来如何:

- (void)functionThatShouldBeSynchronous {
    for (int i = 0; i < 10; i++) {
        [self doSomethingInBackground:^{
            NSLog(@"completed!");
        }];
    }
    // How do I wait until all 10 threads have completed before returning?
}

- (void)doSomethingInBackground:(void(^)())completion {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        [NSThread sleepForTimeInterval:1.0f]; // Do stuff
        completion(); // Execute completion block
    });
}

提前致谢.

Answer 1

使用这样的调度组:

- (void)functionThatShouldBeSynchronous {
    dispatch_group_t taskGroup = dispatch_group_create();

    for (int i = 0; i < 10; i++) {
        dispatch_group_enter(taskGroup);
        [self doSomethingInBackground:^{
            NSLog(@"completed!");
            dispatch_group_leave(taskGroup);
        }];
    }

    // Waiting for threads
    dispatch_group_wait(taskGroup, DISPATCH_TIME_FOREVER);
    dispatch_release(taskGroup);

    // Background work complete
}

如果要等待线程超时,可以将dispatch_group_wait行更改为此

// Waiting 10 seconds before giving up
if (dispatch_group_wait(taskGroup, dispatch_time(DISPATCH_TIME_NOW, 10000000000)) != 0) {
    // Timeout
}

该参数以纳秒为单位.

正如bbum所说,你不应该阻止主线程.在这种情况下,你可以这样做:

typedef void(^MyCompletionHandler)();
-(void)functionDoingBackgroundWorkWithCompletionHandler:(MyCompletionHandler)completionHandler {
    dispatch_group_t taskGroup = dispatch_group_create();

    for (int i = 0; i < 10; i++) {
        dispatch_group_enter(taskGroup);
        [self doSomethingInBackground:^{
            NSLog(@"completed!");
            dispatch_group_leave(taskGroup);
        }];
    }

    dispatch_queue_t waitingQueue = dispatch_queue_create("com.mycompany.myapp.waitingQueue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(waitingQueue, ^{
        // Waiting for threads
        dispatch_group_wait(taskGroup, DISPATCH_TIME_FOREVER);
        dispatch_release(taskGroup);


        // Background work complete
        dispatch_async(dispatch_get_main_queue(), ^{
            // Calling the completion handler on the main thread (If you like)
            completionHandler();
        });
        dispatch_release(waitingQueue);
    });
}