可拖数据类型与颤振中的DragTarget动态candidateData不匹配

Question

我有一个可拖式包含数据类型int。当我将拖放到DragTarget上时，List candidateData不接受int数据。

当我更改为List candidateData时，它就被接受了。

我以为dynamic接受所有类型的数据。这是窃听器还是我误解了什么。

class _DragTargetDataState extends State<DragTargetData> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Container(
          child: Stack(
            children: <Widget>[
              Positioned(
                top: 100,
                left: 50,
                child: Draggable(
                  feedback: buildBox('Draggable', Colors.cyanAccent),
                  child: buildBox('Draggable', Colors.cyanAccent),

                  data: 1, // -> data Draggable bring along

                  onDragEnd: (details) {
                    print('wasAccepted: ${details.wasAccepted.toString()}');
                  },
                ),
              ),
              Positioned(
                top: 300,
                left: 120,
                child: DragTarget(
                  builder: (BuildContext context, 

                      List<dynamic> candidateData, // List dynamic type of candidate data

                      List<dynamic> rejectedData) {
                    return buildBox('Drag Target', Colors.deepOrange);
                  },
                  onWillAccept: (data) {
                    print('onWillAccept: $data');
                    return true;
                  },
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  Container buildBox(String title, Color color) {
    return Container(
      width: 100,
      height: 100,
      color: color,
      child: Center(
        child: Text(
          title,
          style: TextStyle(
              fontSize: 15,
              color: Colors.black,
              fontStyle: FontStyle.normal,
              fontWeight: FontWeight.normal,
              decoration: TextDecoration.none),
        ),
      ),
    );
  }
}

Answer 1

您可以复制粘贴，运行下面的完整代码

要与dynamic一起使用，可以转换数据类型data: 1 as dynamic

转换后，系统知道T是动态的。

 typedef DragTargetBuilder<T> = Widget Function(BuildContext context, List<T> candidateData, List<dynamic> rejectedData);

代码段

Draggable(
              feedback: buildBox('Draggable', Colors.cyanAccent),
              child: buildBox('Draggable', Colors.cyanAccent),
              data: 1 as dynamic,

输出

I/flutter (14993): onWillAccept: 1
I/flutter (14993): wasAccepted: true 

工作演示

​

​

全码

import 'package:flutter/material.dart';

class DragTargetData extends StatefulWidget {
  @override
  _DragTargetDataState createState() => _DragTargetDataState();
}

class _DragTargetDataState extends State<DragTargetData> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Container(
          child: Stack(
            children: <Widget>[
              Positioned(
                top: 100,
                left: 50,
                child: Draggable(
                  feedback: buildBox('Draggable', Colors.cyanAccent),
                  child: buildBox('Draggable', Colors.cyanAccent),
                  data: 1 as dynamic, // -> data Draggable bring along
                  onDragEnd: (details) {
                    print('wasAccepted: ${details.wasAccepted.toString()}');
                  },
                ),
              ),
              Positioned(
                top: 300,
                left: 120,
                child: DragTarget<dynamic>(
                  builder: (BuildContext context,
                      List<dynamic>
                          candidateData, // List dynamic type of candidate data
                      List<dynamic> rejectedData) {
                    return buildBox('Drag Target', Colors.deepOrange);
                  },
                  onWillAccept: (data) {
                    print('onWillAccept: $data');
                    return true;
                  },
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  Container buildBox(String title, Color color) {
    return Container(
      width: 100,
      height: 100,
      color: color,
      child: Center(
        child: Text(
          title,
          style: TextStyle(
              fontSize: 15,
              color: Colors.black,
              fontStyle: FontStyle.normal,
              fontWeight: FontWeight.normal,
              decoration: TextDecoration.none),
        ),
      ),
    );
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: DragTargetData(),
    );
  }
}