My async call is returning before list is populated in forEach loop

Question

I have a routine which gets a list of filenames from the device, then reads the file(s) to build a list. However, the calling routine always returns with zero items. I print the filenames, so I know they exist, however, it appears that the async is returning before I read the files. I used similar code when making an HTTP call. But, something here is causing the routine to return the list even though it hasn't completed. Perhaps, it is possible that I am calling it at the wrong time? I am calling retrieveItems here:

@override
  void initState() {
    super.initState();
    retrieveItems();
  }

Eventually I will have a refresh button, but for now I'd simply like the list to populate with the data from the files...

--------------------

Callee

Future<List<String>> readHeaderData() async {
  List<String> l = new List();
  List<String> files = await readHeaders(); // Gets filenames
  files.forEach((filename) async {
    final file = await File(filename);
    String contents = await file.readAsString();
    User usr = User.fromJson(json.decode(contents));
    String name = usr.NameLast + ", " + usr.NameFirst;
    print(name);
    l.add(name);
  }
  return l;

Caller

void retrieveItems() async {
  LocalStorage storage = new LocalStorage();
  await storage.readHeaderData().then((item) {
      try {
        if ((item != null ) &&(item.length >= 1)) {
          setState(() {
            users.clear();
            _users.addAll(item);
          });
        } else {
          setState(() {
            _users.clear();
            final snackbar = new SnackBar(
              content: new Text('No users found.'),
            );
            scaffoldKey.currentState.showSnackBar(snackbar);
          });
        }
      } on FileNotFoundException catch (e) {
        print(e.toString()); //For debug only
        setState(() {
          _users.clear();
        });
      });
    }
  });

HBG · Accepted Answer · 2019-02-16 15:04:52Z

122

This code

Future<List<String>> readHeaderData() async {
  List<String> l = new List();
  List<String> files = await readHeaders(); // Gets filenames
  files.forEach((filename) async {
    final file = await File(filename);
    String contents = await file.readAsString();
    User user = User.fromJson(json.decode(contents));
    String name = user.NameLast + ", " + user.NameFirst;
    print(name);
    l.add(name);
  }
  return l;
}

returns the list l and then processes the asyc forEach(...) callbacks

If you change it to

Future<List<String>> readHeaderData() async {
  List<String> l = new List();
  List<String> files = await readHeaders(); // Gets filenames
  for(var filename in files) {  /// <<<<==== changed line
    final file = await File(filename);
    String contents = await file.readAsString();
    User user = User.fromJson(json.decode(contents));
    String name = user.NameLast + ", " + user.NameFirst;
    print(name);
    l.add(name);
  }
  return l;
}

the function will not return before all filenames are processed.

files.forEach((filename) async {

means that you can use await inside the callback, but forEach doesn't care about what (filename) async {...} returns.

edited Feb 16, 2019 at 15:04

HBG

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answered Jun 29, 2018 at 17:45

Günter Zöchbauer

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5 Comments

Jerry Over a year ago

That worked. I appreciate the quick response. May I ask where in the docs I could find that the difference between for and foreach, since it doesn't seem to be clearly defined when it comes to async calls. I read through webdev.dartlang.org/articles/performance/event-loop and I thought that I clearly understood futures, however, there appears to be times that the "await" makes sense, and times it doesn't. Though, now I will make sure to use for in this scenario. This shows it, but not why... stackoverflow.com/questions/24437673/…

Günter Zöchbauer Over a year ago

There is a list files with a method forEach(f) that internally calls the function f you pass to it for every element in files. forEach() doesn't return anything. If you use instead list.map(f) then you will get one Future returned from f for each item in files. You can then use await Future.all(files.map(f)) which will await all Futures and only then continue with the code that follows below. The loop for(...) { ... } is not just a method that calls functions passed to it, it is a language construct where statements and expressions are executed sequentially.

Günter Zöchbauer Over a year ago

It's later here. Let me know if you want further explanations and i'll try to elaborate more tomorrow.

Eradicatore Over a year ago

Thanks! I had a Map instead of a list, so this answer helped as well:stackoverflow.com/questions/52276412/…

Asbah Riyas Over a year ago

what about Future<void> functions...creating a simple list from forEach and passing that list to firebase...

iLobanov · Accepted Answer · 2019-12-10 04:08:29Z

44

Also possible

await Future.forEach(yourList, (T elem) async { ...async staff });

answered Dec 10, 2019 at 4:08

iLobanov

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Comments

Duncan Jones · Accepted Answer · 2018-06-30 07:35:38Z

To expand on Günter's comment regarding using list.map(f), here's an example of converting a forEach call so that it works correctly.

Broken example

Incorrectly assumes forEach will wait on futures:

Future<void> brokenExample(List<String> someInput) async {    
  List<String> results;

  someInput.forEach((input) async {
    String result = await doSomethingAsync(input);
    results.add(result);
  });

  return results;
}

Corrected example

Waits on the async functions to complete, using Future.wait and .map():

Future<void> correctedExample(List<String> someInput) async {
  List<String> results;

  await Future.wait(someInput.map((input) async {
    String result = await doSomethingAsync(input);
    results.add(result);
  }));

  return results;
}

Dharman · Accepted Answer · 2021-10-17 02:18:34Z

6

I encountered the similar issue. The problem is that dart will NOT wait for "forEach" contrary to public believe. There are two solutions:

1) Convert forEach to for loop as indicated by others. Another is use Future:
2) await Future.forEach(list, (item) async {
   // your code
    final result = await getMyResult();
   });

edited Oct 17, 2021 at 2:18

Dharman♦

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answered Oct 17, 2021 at 2:13

us_david

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Comments

Evan Chu · Accepted Answer · 2020-04-13 19:55:19Z

0

Another option

Future.wait(someList.map((item) => something_returns_future(item)));

answered Apr 13, 2020 at 19:55

Evan Chu

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Comments

Matt · Accepted Answer · 2023-08-05 14:40:42Z

0

Future.forEach() will execute futures sequentially. The iteration waits until the future is completed before continuing with the next element.

Future.wait() will execute futures in parallel. Returns a future which will complete once all the provided futures have completed.

See Future class

answered Aug 5, 2023 at 14:40

Matt

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