objective-c 等待多個非同步函式完成後,再執行某段程式碼--GCD Group 的使用
阿新 • • 發佈:2019-01-22
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 *userin 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];
});
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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
參考:
* [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/)。