有个 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?
这就要用到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];
});
=================================================================
另外的解决思路,没有实践,供大家参考:
1,ReactiveCocoa
2,combineLatest
3,promisekit 库
4,NSOperation depends
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/)。