ios - dispatch_group_notify 不等待一个 dispatch_group_async

Question

两个并发的后台任务需要修补两个单独的数组，需要合并到一个dispatch_group_notify block 中。问题是，第一个 block 超出了，但是dispatch_group_notify超出了，没有等待第二个后台任务的执行。它们之间的唯一区别是第一个进行本地搜索，第二个对 Web 服务进行远程调用。知道为什么跳过第二个吗？

编辑:我也尝试了 https://stackoverflow.com/a/19580584/859742 中提到的方法使用 dispatch_barrier_async 但仍然相同。

 dispatch_group_t taskGroup = dispatch_group_create(); 
 dispatch_queue_t mainQueue = dispatch_get_main_queue();

__block NSArray *localAddresses;
__block NSArray *remoteAddresses;

//Get local array in the background 
dispatch_group_async(taskGroup, mainQueue, ^{
    //localAddresses  is set
});

//get remote array from server
dispatch_group_async(taskGroup, mainQueue, ^{
 [[MDAddressManager instance] searchForPlacesContainingText:query
                                                      location:alocation
                                                    completion:^(NSArray* addresses, MDError *error){
       //remoteAddresses is set
});


//Merge two arrays
dispatch_group_notify(taskGroup, mainQueue, ^{
   //remoteAddresses and local addresses are merged
});

远程搜索方法是这样的

- (void)searchForPlacesContainingText:(NSString *)searchText
                         location:(CLLocation *)alocation
                       completion:(MDAddressManagerBlock)completionBlock
{
NSDictionary *parameters = [[NSMutableDictionary alloc] init];

[parameters setValue:searchText forKey:@"input"];

[[MDHTTPClient sharedHTTPClient] getPath:@"v1/remotePlaces.json"
                              parameters:parameters
                                 success:^(AFHTTPRequestOperation *operation, id dict) {

    if ([MDHTTPClient isResponseValid:dict])
    {
            completionBlock(returnArray, nil);
    }
    else
    {
            completionBlock(nil, nil);
    }

} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    EDLog(@"%@", error);
    completionBlock(nil, [MDError errorAFNetworking:error]);
}];
}

Answer 1

这是因为您的 getPath 方法是异步运行的。您需要它在完成 block 运行之前不要离开组。因此，与其执行 dispatch_group_async，不如手动执行 dispatch_group_enter 和 dispatch_group_leave。

您可以更改您的代码:

dispatch_group_async(taskGroup, mainQueue, ^{
    [[MDAddressManager instance] searchForPlacesContainingText:query
                                                      location:alocation
                                                    completion:^(NSArray* addresses, MDError *error){
        //remoteAddresses is set
    }];
});

收件人:

dispatch_group_enter(taskGroup);

[[MDAddressManager instance] searchForPlacesContainingText:query
                                                  location:alocation
                                                completion:^(NSArray* addresses, MDError *error){
    //remoteAddresses is set
    dispatch_group_leave(taskGroup);
});

这将确保您在调用完成 block 之前不会离开组。

---

或者，您可以更改 searchForPlacesContainingText 以使用 dispatch_group_t 参数:

- (void)searchForPlacesContainingText:(NSString *)searchText
                             location:(CLLocation *)alocation
                                group:(dispatch_group_t)group
                           completion:(MDAddressManagerBlock)completionBlock
{
    dispatch_group_enter(group);

    NSDictionary *parameters = [[NSMutableDictionary alloc] init];

    [parameters setValue:searchText forKey:@"input"];

    [[MDHTTPClient sharedHTTPClient] getPath:@"v1/remotePlaces.json"
                                  parameters:parameters
                                     success:^(AFHTTPRequestOperation *operation, id dict) {

        if ([MDHTTPClient isResponseValid:dict])
        {
                completionBlock(returnArray, nil);
        }
        else
        {
                completionBlock(nil, nil);
        }

        dispatch_group_leave(group);

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        EDLog(@"%@", error);
        completionBlock(nil, [MDError errorAFNetworking:error]);

        dispatch_group_leave(group);
    }];

}

并更改您的调用，使其不执行 dispatch_group_async，而只是传递 taskGroup 参数:

[[MDAddressManager instance] searchForPlacesContainingText:query
                                                  location:alocation
                                                     group:taskGroup
                                                completion:^(NSArray* addresses, MDError *error) {
    //remoteAddresses is set
});