Using loops with Futures in Dart

Question

Okay, so I have a List of Files and I need to run a function on each member of the list. I essentially want to do something like this:

for(File file in files) {
    functionThatReturnsAFuture(file);
}

But obviously this won't work, since the function that returns a Future fires of asynchronously. Is my only option something like this?

List<File> files = new List<File>();
// Add files somewhere

Future processFile(int i) {
   return new Future.sync(() {
       //Do stuff to the file
       if(files.length>i+1) {
           return processFile(i+1);
       }
   });
}

processFile(0);

EDIT: I suppose more context is important. The end goal is to combine several files into a single JSON object for submitting to a server. The FileReader object uses events to perform a read, so I used a Completer to create a wrapper function that provides a Future. I could let all of these run asynchronously, and then fire off an event that submits it when they are all done, but that's comparatively a lot of setup vs. a for-each-loop that makes it all work (if one exists, anyway). The core issue is needing to run a Future-returning function on a List of files and then perform an action that depends on them all having completed.

Cusp

What's the problem with your first example? What does it matter if the function runs asynchronously or not? – Ply

I edited the question with more information. – Cusp

Answer 1

Dart supports async/await since quite some time which allows it to write as

someFunc() async {
  for(File file in files) {
    await functionThatReturnsAFuture(file);
  }
}

Stumpage

I'm changing the accepted answer to this, because this is how it should be done now. – Cusp

I was so struggled, to understand how this works. I am having more inner methods. like inside functionThatReturnsAFuture method another method. So not worked initially. Then i added the same combination of async/ await for all inner method. Then started working as expected. – Winebaum

await in front of method call and async after method signature. Don't miss these. – Winebaum

Answer 2

When you need to wait for multiple Futures to complete and you don't care about the order, you can use Future.wait():

Future.wait(files.map(functionThatReturnsAFuture))
  .then((List response) => print('All files processed'));

If order is important you can use Future.forEach() instead which waits for each Future to be completed before moving to the next element:

Future.forEach(files, functionThatReturnsAFuture)
  .then((response) => print('All files processed'));

Ply

THAT is perfect. Thank you very much! Might I ask what you would do if the order did matter? – Cusp

Glad to help. I've added some more info to address that case. – Ply

And with that, I finally have enough reputation to +1 answers. You get my first +1! – Cusp

The Fjuture.wait takes an Iterable, so there is no need to make a List first. Just do Future.wait(files.map(functionThatReturnsAFuture)). – Amari

@Amari Ah thanks, it was bothering me that the Future.wait code was longer than Future.forEach. – Ply

Answer 3

This library can help https://pub.dartlang.org/packages/heavylist

HeavyList<File> abc = new HeavyList<File>([new File(), new File(), ]);
abc.loop(new Duration(seconds: 1), (List<File> origin) {
print(origin);
}, (File item, Function resume) {
  //simulating an asynchronous call
  print(item);
  //move to next item
  resume();
});

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