DART组件:如何返回异步回调的结果?[英] Dart Component: How to return result of asynchronous callback?

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问题描述

嘿,我是Dart Futures的新手,我有以下情况.

每当用户在UI中键入字母时,我的ui_component中的addressChanged()方法都会被调用.此方法在我的地图组合中调用方法getProposals(),该方法对Google Maps API发出异步请求.一旦结果就在这里,我想将它们返回到UI组件,该组件将填充UI中的提议下拉.

我坚持最后一步:如何(最好的方法)将异步回调函数的结果返回到父组件(同时保留可重复使用的地图组件?).

这是我尝试的:

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组件:

  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;
          }
      );
    }
  }

推荐答案

此方法不返回任何数据

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

将其更改为

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

这也可以工作,但是这里async和await是Redunant

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

对于_getProposals您可以使用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;
  }

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