小编典典

Dart组件:如何返回异步回调的结果?

dart

嘿,我对Dart Futures还是陌生的,我有以下情况。

每当用户在UI中键入字母时,addressChanged()都会调用ui_component中的方法。此方法调用getProposals()我的地图componenet中的方法,该方法向google maps API发出异步请求。结果到这里后,我想将它们返回到UI组件,该组件将填充UI中的propasals下拉列表。

我停留在最后一步:如何(以及最好的方法是什么)将异步回调函数的结果返回给父组件(同时保留可重用的map组件?)。

这是我尝试过的:

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;
          }
      );
    }
  }

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2020-08-30

共1个答案

小编典典

此方法不返回任何数据

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

更改为

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

这也将工作,但在这里async和await是redunant

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

对于_getProposals你可以使用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;

}
```

2020-08-30