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

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();
});
});
}
});

Answer 1

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.

Answer 2

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

Answer 3

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;
}

Answer 4

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();
});

Answer 5

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