Flutter Async 一个接一个地加载多个函数

Question

如何让 Flutter 知道等待一个函数（以及未来）完成，直到调用下一个函数？ load() 函数本身似乎在 loadFeed() Future 完成之前完成，我该如何解决？

这是我当前的代码：

ValueNotifier<List> data = ValueNotifier([]);
ValueNotifier<bool> loading = ValueNotifier(true);

loadAll() async {
  await load(0, 'url1');
  print("0 finished");
  await load(1, 'url2');
  print("1 finished");
  await load(2, 'url3');
  print("2 finished");
}

load(int categoryIndex, String _url) async {
  loadFeed(_url).then((result) {
    if (null == result || result.toString().isEmpty) {
      print("error");
      return;
    }
    data.value = [[],[],[],];
    for (var i = 0; i < result.items.length; i++) {
      data.value[categoryIndex].add(result.items[i]);
    }
    data.notifyListeners();
  });
}

Future<RssFeed> loadFeed(String _url) async {
  loading.value = true;
  try {
    final client = http.Client();
    final response = await client.get(Uri.parse(_url));

    return RssFeed.parse(response.body);
  } catch (e) {
    print("error");
  } finally {
    loading.value = false;
  }
  return null;
}

Answer 1

在未来的功能中，您可以使用 whencomplete，来自文档：

"WhenComplete", Registers a function to be called when this future completes.

The [action] function is called when this future completes, whether it does so with a value or with an error.

This is the asynchronous equivalent of a "finally" block.

将load改成未来的函数，并添加一个return值让dart知道你完成了，像这样：

Future load(int categoryIndex, String _url) async {
   loadFeed(_url).then((result) {
     if (null == result || result.toString().isEmpty) {
       print("error");
       return;
     }
     data.value = [[],[],[],];
     for (var i = 0; i < result.items.length; i++) {
       data.value[categoryIndex].add(result.items[i]);
     }
    return data.notifyListeners(); //add a return here, to tell dart that this function is done.
   });
 }

把你的loadAll改成这样：

loadAll() async { 
await load(0, 'url1')
.whenComplete(() async => await load(1, 'url2')
.whenComplete(() async => await load(2, 'url3')));}

Answer 2

It seems like the load() function itself completes before the loadFeed() Future is finished, how can I fix that?

这正是问题所在。你这样做：


load(int categoryIndex, String _url) async {
  loadFeed(_url).then((result) {
    // ... bunch of code...
  });
}

您的 load 函数调用 loadFeed，注册一个 Future.then 回调，忽略新 returned Future，并同步 return s 回调注册后的调用者。它不使用await等待Future完成，也不returnFuture给调用者让调用者等待Future.

如果你使用 async/await 而不是将其与原始 Future 混合使用会简单得多 API:

Future<void> load(int categoryIndex, String _url) async {
  var result = await loadFeed(_url);
  // ... bunch of code...
}

此外，启用 unawaited_futures lint in your analysis_options.yaml file 会在静态分析期间发现此问题。

Flutter Async 一个接一个地加载多个函数

Flutter Async Load Multiple Functions One After The Other

asynchronous

dart

flutter

flutter-future