有個 API 如下：

+ (void)updateByUser:(ALUser *)user completion:(void (^)(void))completion
{
    AVQuery *query = [AVQuery queryWithClassName:@"Feed"];
    query.limit = 15;
    [query orderByDescending:@"createdAt"];
    [query whereKey:@"user" equalTo:[AVObject objectWithoutDataWithClassName:@"User" objectId
 
:user.objectId]];
    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        if (!error) {
            for (AVObject *feed in objects) {
                [self saveFeed:feed];
            }

            if (completion) {
                dispatch_async(dispatch_get_main_queue
 
(), ^{
                    completion();
                });
            }
        } else {
            NSLog(@"Error: %@ %@", error, [error userInfo]);
        }
    }];
}

一次呼叫如下：

[ALFeed updateByUser:user completion:^{
    [self.tableView reloadData];
}];

但如果需要多次呼叫後再執行回撥：

for (ALUser *user
 
 in users) {
    [ALFeed updateByUser:user completion:^{

    }];
}

如何在所有 updateByUser:completion: 執行完後，再執行 [self.tableView reloadData]; ？

JavaScript 可以用 promise 庫（比如 bluebird ）解決這樣的問題：

var files = [];
for (var i = 0; i < 100; ++i) {
    files.push(fs.writeFileAsync("file-" + i + ".txt", "", "utf-8"));
}
Promise.all(files).then(function() {
    console.log("all the files were created");
});

但在 Objective-C 中，如何解決？ 

這就要用到GCD 中dispatch_group 了：

示例：

dispatch_group_t group = dispatch_group_create(); 

for (ALUser *user in self.users) { 
dispatch_group_enter(group); 
[ALFeed updateByUser:user completion:^{ 
dispatch_group_leave(group); 
}]; 
} 

dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ 
[self.tableView reloadData]; 
}); 

=================================================================

另外的解決思路，沒有實踐，供大家參考：

1，ReactiveCocoa

2，combineLatest 

3，promisekit  庫

4，NSOperation depends