Dart Component: How to return result of asynchronous callback?

Question

Hey there I am quite new to Dart Futures and I have the following situation.

Whenever a user types a letter in the UI the addressChanged() method in my ui_component is called. This method calls the method getProposals() in my maps componenet which does an asynchronous request to the google maps API. As soon as the results are here I want to return them to the UI Component which is going to populate the propasals dropdown in the UI.

I am stuck with the last step: How (and whats the best way) to return the results of an asynchronous callback function to a parent component (while keeping an reusable maps component?).

This is what I have tried:

1) UI_Component:

// I get called if a user typed a new letter
     Future addressChanged(dynamic event) async {
        String id = event.target.id;
        String address = event.target.value;
          if(id=="pickup") {
              this.pickup = address;
          } else if(id=="destination") {
              this.destination = address;
          }
        // this is where I call the subcomponent and want to get the address propasals
        String proposals = await googleMap.getProposals(address,id);
        print(proposals);
        populateProposalDropdown();
      }

2) Google Map component:

  Future getProposals(String address,String id) async {
    await _getProposals(address,id);
  }

  Future _getProposals(String address,String id) async {

    if(address != "") {
      autocompleteService.getPlacePredictions(
          new AutocompletionRequest()
            ..input = address
          ,
          (predictions,status) {
            List<String> result = [];
            if(status == PlacesServiceStatus.OK) {
              predictions.forEach(
                  (AutocompletePrediction prediction) =>
                      result.add(prediction.description)
              );
            }

            // HERE is the problem: How do I return this result from the callback as a result of the getProposals method?
            return result;
          }
      );
    }
  }

Is the `AutoCompleteService` your code? It could be rewritten to return a `Future` itself rather than invoke a callback, which would relieve you of having to use a `Completer` when you invoke it. — Argenti Apparatus, Feb 14 '17 at 16:54
Nah unfortunatly its not: https://pub.dartlang.org/packages/google_maps . They should really implement the library like this ;-) — Blackbam, Feb 14 '17 at 17:06

score 9 · Accepted Answer · answered Feb 14 '17 at 14:41

This method doesn't return any data

  Future getProposals(String address,String id) async {
    await _getProposals(address,id);
  }

Change it to

  Future getProposals(String address,String id) {
    return _getProposals(address,id);
  }

This would also work, but here async and await is redunant

  Future getProposals(String address,String id) async {
    return await _getProposals(address,id);
  }

For _getProposals you can use a Completer

  Future _getProposals(String address,String id) async {
    if(address != "") {
      Completer completer = new Completer();

      autocompleteService.getPlacePredictions(
          new AutocompletionRequest()
            ..input = address
          ,
          (predictions,status) {
            List<String> result = [];
            if(status == PlacesServiceStatus.OK) {
              predictions.forEach(
                  (AutocompletePrediction prediction) =>
                      result.add(prediction.description)
              );
            }

            // HERE is the problem: How do I return this result from the callback as a result of the getProposals method?
            completer.complete(result);
          }
      );
      return completer.future;
    }
    return null;
  }

Thx thats exactly what i needed :-) – Blackbam Feb 14 '17 at 14:47 — Blackbam, Feb 14 '17 at 14:47

Dart Component: How to return result of asynchronous callback?

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