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...

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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; 

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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.