ios - dispatch_queue 和返回数据

Question

我正在尝试编写返回 NSArray 的方法。我的 NSMutableArray (friendUsers) 添加了正确的对象，但在 dispatch_async 之外，数组是空的。我尝试在主队列中添加用户(如图所示)，但数组为空。有任何想法吗 ？感谢您的帮助。

- (NSArray *)checkUsersInGroup {

    NSMutableArray *friendUsers = [[NSMutableArray alloc] init];

    dispatch_queue_t checkUSers = dispatch_queue_create("CheckUsers", NULL);
    dispatch_async(checkUSers, ^{

        NSArray *totalUsers = [VVDataRead lecturaDades];
        NSArray *usersToSearch = [_grup objectForKey:@"groupFriends"];

        for (NSString *tempUserId in usersToSearch){
            for (NSDictionary *user in totalUsers){
                NSString *id = [user objectForKey:@"id"];
                    if ([tempUserId isEqualToString:id])
                        dispatch_async(dispatch_get_main_queue(), ^{
                            [friendUsers addObject:user];
                        });
            }
        }

    });
    NSLog(@"people:%@",friendUsers);
    return [friendUsers copy];
}

Answer 1

你可以使用积木，在这种情况下它可以让你的生活更轻松。

- (void)checkUsersInGroupWithCompleteBlock:(void(^)(NSMutableArray * resultArray))completeBlock {

    NSMutableArray *friendUsers = [[NSMutableArray alloc] init];

    dispatch_queue_t checkUSers = dispatch_queue_create("CheckUsers", NULL);
    dispatch_async(checkUSers, ^{

        NSArray *totalUsers = [VVDataRead lecturaDades];
        NSArray *usersToSearch = [_grup objectForKey:@"groupFriends"];

        for (NSString *tempUserId in usersToSearch){
            for (NSDictionary *user in totalUsers){
                NSString *id = [user objectForKey:@"id"];
                if ([tempUserId isEqualToString:id])
                    dispatch_async(dispatch_get_main_queue(), ^{
                        [friendUsers addObject:user];
                    });
            }
        }

        // call the complete block with the result when you finished
        if (completeBlock) completeBlock(friendUsers);
    });
}

...下面是调用方法的方法:

- (void)anyMethod {

    // ... do whetever you want here before

    [self checkUsersInGroupWithCompleteBlock:^(NSMutableArray *resultArray) {
        NSLog(@"%@", resultArray);
    }];

    // ... or after

}

---

已编辑:

注意:这是另一种可能的解决方案，但在您的情况下，它只是暂停主线程(这绝对不好)，因此您使用此解决方案除了痛苦之外不会有任何收获在主线程上，但如果您在两个后台线程上，此解决方案可以提供一个很好的线程间同步示例。

- (NSArray *)checkUsersInGroup {

    NSMutableArray *friendUsers = [[NSMutableArray alloc] init];

    // our semaphore is here
    dispatch_semaphore_t _semaphore = dispatch_semaphore_create(0);

    dispatch_queue_t checkUSers = dispatch_queue_create("CheckUsers", NULL);
    dispatch_async(checkUSers, ^{

        NSArray *totalUsers = [VVDataRead lecturaDades];
        NSArray *usersToSearch = [_grup objectForKey:@"groupFriends"];

        for (NSString *tempUserId in usersToSearch){
            for (NSDictionary *user in totalUsers){
                NSString *id = [user objectForKey:@"id"];
                if ([tempUserId isEqualToString:id])
                    dispatch_async(dispatch_get_main_queue(), ^{
                        [friendUsers addObject:user];
                    });
            }
        }
        // the process finished
        dispatch_semaphore_signal(_semaphore);

    });

    // ... we are wainitng for the semaphore's signal
    dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
    dispatch_release(_semaphore);

    NSLog(@"people:%@",friendUsers);
    return [friendUsers copy];

}