async is snowballing to callers, can't make constructor async

Question

I have a function loadData that loads some text from a file:

Future<String> loadAsset() async {   return await rootBundle.loadString('assets/data/entities.json'); } 

The loadString method is from Flutter SDK, and is asynchronous.

The loadAsset method is then called in another method, that must me marked as async, since loadAsset is async and I need to use await:

Future<List<Entity>> loadEntities() async {   String jsonData = await loadAsset();   return parseData(jsonData); } 

The parseData method is not async, it receives a String, parse it, and return a list of objects:

List<Entity> parseData(String jsonString) {   ... } 

But since loadEntities must be marked with async, this requires that it returns a Future, but in practice, it's not a Future because since I use await, it awaits for the loadAsset method to finish, then call the parseData funcion using the result.

This easily turns into a snowball of async call, because every method that uses loadEntities must be marked as async too.

Also, I can't use loadEntities in a class constructor, because the constructor should be marked as async, which is not allowed in Dart.

Am I using the async/await pattern in Dart wrong? How could I use the loadEntities method in a class constructor?

Answer 1

No, async is contagious and there is no way to go back from async to sync execution.

async/await is only syntactic sugar for methodThatReturnsFuture().then(...)

Marking a method with async is only to allow you to use await inside its body. Without async you would still need to return a Future for calling code to only execute after the result of loadAsset() becomes available.

Answer 2

You can use the Future returned from the async call directly. This would look something like this:

class HasAsync {   HasAsync() {     asyncFunction().then((val) {       print(val);     });   }    Future<int> asyncFunction() async {      int val = await otherFunction();      return val;   } } 

You just can't use await within the non-async function.

As you've tagged this with 'flutter', I'm going to guess this is within a flutter app. If that's the case look at the docs for FutureBuilder - it might help with what you're trying to do.