Replace Future.then() with async/await

user_9367924 0
Viewed 2766

I've always considered async/await more elegant/sexy over the Futures API, but now I'm faced with a situation where the Future API implementation is very short and concise and the async/await alternative seems verbose and ugly.

I marked my two question #1 and #2 in the comments:

class ItemsRepository
{
  Future<dynamic> item_int2string;

  ItemsRepository() {
    // #1
    item_int2string = 
     rootBundle.loadString('assets/data/item_int2string.json').then(jsonDecode);
  }

  Future<String> getItem(String id) async {
    // #2
    return await item_int2string[id];
  }
}

#1: How do I use async/await here instead of Future.then()? What's the most elegant solution?

#2: Is this efficient if the method is called a lot? How much overhead does await add? Should I make the resolved future an instance variable, aka 

completedFuture ??= await item_int2string;
return completedFuture[id];
async-await dart flutter
2 Answers

1: How do I use async/await here instead of Future.then()? What's the most elegant solution?

async methods are contagious. That means your ItemsRepository method has to be async in order to use await inside. This also means you have to call it asynchronously from other places. See example:

Future<dynamic> ItemsRepository() async {
    // #1
    myString = await rootBundle.loadString('assets/data/item_int2string.json');
    // do something with my string here, which is not in a Future anymore...
  }

Note that using .then is absolutely the same as await in a async function. It is just syntactic sugar. Note that you would use .then differently than in your example though:

  ItemsRepository() {
    // #1
    
     rootBundle.loadString('assets/data/item_int2string.json').then((String myString) {
       // do something with myString here, which is not in a Future anymore...
     });
  }

And for #2 don't worry about a performance impact of async code. The code will be executed at the same speed as synchronous code, just later whenever the callback happens. The only reason async exists is for having an easy way of allowing code to continue running while the system waits for the return of the asynchronously called portion. For example not block the UI while waiting for the disk to load a file.

I recommend you read the basic docs about async in Dart.

user_78036500

then and await are different. await will stop the program there until the Future task is finished. However then will not block the program. The block within then will be executed when the Future task is finished afterwards.

If you want your program to wait for the Future task, then use await. If you want your program to continue running and the Future task do it things "in the background", then use then.

user_24857080
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