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

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.

Also possible

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

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;}