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

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


2 Answers
2

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 usr = User.fromJson(json.decode(contents));
String name = usr.NameLast + ", " + usr.NameFirst;
print(name);
l.add(name);
}
return l;
}


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

l

forEach(...)

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 usr = User.fromJson(json.decode(contents));
String name = usr.NameLast + ", " + usr.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.

await

forEach

(filename) async {...}

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/…
– Jerry
Jun 29 at 18:29

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
Jun 29 at 20:04

files

forEach(f)

f

files

forEach()

list.map(f)

Future

f

files

await Future.all(files.map(f))

for(...) { ... }

It's later here. Let me know if you want further explanations and i'll try to elaborate more tomorrow.
– Günter Zöchbauer
Jun 29 at 20:04

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.

list.map(f)

forEach

Incorrectly assumes forEach will wait on futures:

forEach

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

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

return results;
}


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

Future.wait

.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;
}


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