Objective-C 中，如何等待多个异步函数完成后，再执行某段代码？

+ (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]);
        }
    }];
}

一次调用如下：

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

但如果需要多次调用后再执行回调：

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

    }];
}

如何在所有 updateByUser:completion: 执行完后，再执行 [self.tableView reloadData]; ？

JavaScript 可以用 promise 库（比如 bluebird ）解决这样的问题：

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，如何解决？

22 条回复 • 2015-10-15 20:09:22 +08:00

hustlzp

2015-10-15 13:49:55 +08:00

已解决，原来 GCD 中有个 dispatch_group 是专门解决这种问题的...

看来得找个时间专门学习 GCD ...

```objc
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];
});
```

参考：

* [Waiting on Groups of Queued Tasks]( https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW25)
* [Using dispatch groups to wait for multiple web services]( http://commandshift.co.uk/blog/2014/03/19/using-dispatch-groups-to-wait-for-multiple-web-services/)。