我有许多 Draggables 和 DragTargets。在 Draggables 上，我指定了 child 和 feedback，但是我也希望它在 Draggable 进入 DragTarget 时改变它的外观。我看不出有什么办法。
每当我拖动 Draggable 时，我都会将其颜色更改为红色，但是一旦它进入 DragTarget 我想将 Draggable 颜色更新为绿色。
我知道 DragTarget.OnWillAccept，每当 Draggable 进入 DragTarget 时都会调用此方法，但我只有 数据。我尝试用新颜色更新数据，然后调用 setState，但这似乎不起作用。
关于如何获得这种行为的任何建议？
我想要类似下面的回调 Draggable.onEnteringDragTarget 和 Draggable.onLeavingDragTarget。
【问题讨论】：
标签： drag-and-drop flutter draggable flutter-layout
我能想到的唯一方法是使用一个流构建器和一个流，该流将携带有关可拖动对象是否在拖动目标上的信息。这段代码提供了一个基本的解决方案。
class _MyHomePageState extends State<MyHomePage> {
BehaviorSubject<bool> willAcceptStream;
@override
void initState() {
willAcceptStream = new BehaviorSubject<bool>();
willAcceptStream.add(false);
super.initState();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Column(
children: <Widget>[
Container(
height: 400,
width: double.infinity,
child: Container(
width: 100,
height: 100,
child: Center(
child: Draggable(
feedback: StreamBuilder(
initialData: false,
stream: willAcceptStream,
builder: (context, snapshot) {
return Container(
height: 100,
width: 100,
color: snapshot.data ? Colors.green : Colors.red,
);
},
),
childWhenDragging: Container(),
child: Container(
height: 100,
width: 100,
color: this.willAcceptStream.value ?? false
? Colors.green
: Colors.blue,
),
onDraggableCanceled: (v, f) => setState(
() {
this.willAcceptStream.add(false);
},
),
),
),
),
),
DragTarget(
builder: (context, list, list2) {
return Container(
height: 50,
width: double.infinity,
color: Colors.blueGrey,
child: Center(child: Text('TARGET ZONE'),),
);
},
onWillAccept: (item) {
debugPrint('will accept');
this.willAcceptStream.add(true);
return true;
},
onLeave: (item) {
debugPrint('left the target');
this.willAcceptStream.add(false);
},
),
],
),
);
}
}
编辑：第一个示例不会同时处理多个拖动，并且它更简洁。
import 'package:rxdart/rxdart.dart';
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Draggable Test',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
MyDraggableController<String> draggableController;
@override
void initState() {
this.draggableController = new MyDraggableController<String>();
super.initState();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Draggable Test'),
),
body: Column(
children: <Widget>[
Container(
height: 400,
width: double.infinity,
child: Container(
width: 100,
height: 100,
child: Center(
child: Stack(
children: <Widget>[
Positioned(
left: 30,
top: 30,
child: MyDraggable<String>(draggableController, 'Test1'),
),
Positioned(
left: 230,
top: 230,
child: MyDraggable<String>(draggableController, 'Test2'),
)
],
),
),
),
),
DragTarget<String>(
builder: (context, list, list2) {
return Container(
height: 50,
width: double.infinity,
color: Colors.blueGrey,
child: Center(
child: Text('TARGET ZONE'),
),
);
},
onWillAccept: (item) {
debugPrint('draggable is on the target');
this.draggableController.onTarget(true, item);
return true;
},
onLeave: (item) {
debugPrint('draggable has left the target');
this.draggableController.onTarget(false, item);
},
),
],
),
);
}
}
class MyDraggable<T> extends StatefulWidget {
final MyDraggableController<T> controller;
final T data;
MyDraggable(this.controller, this.data);
@override
_MyDraggableState createState() =>
_MyDraggableState<T>(this.controller, this.data);
}
class _MyDraggableState<T> extends State<MyDraggable> {
BehaviorSubject<DraggableInfo<T>> willAcceptStream;
MyDraggableController<T> controller;
T data;
_MyDraggableState(this.controller, this.data);
@override
void initState() {
willAcceptStream = this.controller._isOnTarget;
willAcceptStream.add(new DraggableInfo<T>(false, this.data));
super.initState();
}
@override
Widget build(BuildContext context) {
return Draggable<T>(
data: this.data,
feedback: StreamBuilder<DraggableInfo<T>>(
initialData: DraggableInfo<T>(false, this.data),
stream: willAcceptStream,
builder: (context, snapshot) {
return Container(
height: 100,
width: 100,
color: snapshot.data.isOnTarget && snapshot.data.data == this.data ? Colors.green : Colors.red,
);
},
),
childWhenDragging: Container(),
child: Container(
height: 100,
width: 100,
color: (this.willAcceptStream.value.isOnTarget ?? this.willAcceptStream.value.data == this.data)
? Colors.green
: Colors.blue,
),
onDraggableCanceled: (v, f) => setState(
() {
this.willAcceptStream.add(DraggableInfo(false, null));
},
),
);
}
}
class DraggableInfo<T> {
bool isOnTarget;
T data;
DraggableInfo(this.isOnTarget, this.data);
}
class MyDraggableController<T> {
BehaviorSubject<DraggableInfo<T>> _isOnTarget;
MyDraggableController() {
this._isOnTarget = new BehaviorSubject<DraggableInfo<T>>();
}
void onTarget(bool onTarget, T data) {
_isOnTarget.add(new DraggableInfo(onTarget, data));
}
}
编辑 2：不使用流和流构建器的解决方案。重要的部分是反馈小部件是有状态的。
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Draggable Test',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
MyDraggableController<String> draggableController;
@override
void initState() {
this.draggableController = new MyDraggableController<String>();
super.initState();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Draggable Test'),
),
body: Column(
children: <Widget>[
Container(
height: 400,
width: double.infinity,
child: Container(
width: 100,
height: 100,
child: Center(
child: Stack(
children: <Widget>[
Positioned(
left: 30,
top: 30,
child: MyDraggable<String>(
draggableController,
'Test1',
),
),
Positioned(
left: 230,
top: 230,
child: MyDraggable<String>(
draggableController,
'Test2',
),
),
],
),
),
),
),
DragTarget<String>(
builder: (context, list, list2) {
return Container(
height: 100,
width: double.infinity,
color: Colors.blueGrey,
child: Center(
child: Text('TARGET ZONE'),
),
);
},
onWillAccept: (item) {
debugPrint('draggable is on the target $item');
this.draggableController.onTarget(true, item);
return true;
},
onLeave: (item) {
debugPrint('draggable has left the target $item');
this.draggableController.onTarget(false, item);
},
),
],
),
);
}
}
class MyDraggable<T> extends StatefulWidget {
final MyDraggableController<T> controller;
final T data;
MyDraggable(this.controller, this.data, {Key key}) : super(key: key);
@override
_MyDraggableState createState() =>
_MyDraggableState<T>(this.controller, this.data);
}
class _MyDraggableState<T> extends State<MyDraggable> {
MyDraggableController<T> controller;
T data;
bool isOnTarget;
_MyDraggableState(this.controller, this.data);
FeedbackController feedbackController;
@override
void initState() {
feedbackController = new FeedbackController();
this.controller.subscribeToOnTargetCallback(onTargetCallbackHandler);
super.initState();
}
void onTargetCallbackHandler(bool t, T data) {
this.isOnTarget = t && data == this.data;
this.feedbackController.updateFeedback(this.isOnTarget);
}
@override
void dispose() {
this.controller.unSubscribeFromOnTargetCallback(onTargetCallbackHandler);
super.dispose();
}
@override
Widget build(BuildContext context) {
return Draggable<T>(
data: this.data,
feedback: FeedbackWidget(feedbackController),
childWhenDragging: Container(
height: 100,
width: 100,
color: Colors.blue[50],
),
child: Container(
height: 100,
width: 100,
color: (this.isOnTarget ?? false) ? Colors.green : Colors.blue,
),
onDraggableCanceled: (v, f) => setState(
() {
this.isOnTarget = false;
this.feedbackController.updateFeedback(this.isOnTarget);
},
),
);
}
}
class FeedbackController {
Function(bool) feedbackNeedsUpdateCallback;
void updateFeedback(bool isOnTarget) {
if (feedbackNeedsUpdateCallback != null) {
feedbackNeedsUpdateCallback(isOnTarget);
}
}
}
class FeedbackWidget extends StatefulWidget {
final FeedbackController controller;
FeedbackWidget(this.controller);
@override
_FeedbackWidgetState createState() => _FeedbackWidgetState();
}
class _FeedbackWidgetState extends State<FeedbackWidget> {
bool isOnTarget;
@override
void initState() {
this.isOnTarget = false;
this.widget.controller.feedbackNeedsUpdateCallback = feedbackNeedsUpdateCallbackHandler;
super.initState();
}
void feedbackNeedsUpdateCallbackHandler(bool t) {
setState(() {
this.isOnTarget = t;
});
}
@override
Widget build(BuildContext context) {
return Container(
height: 100,
width: 100,
color: this.isOnTarget ?? false ? Colors.green : Colors.red,
);
}
@override
void dispose() {
this.widget.controller.feedbackNeedsUpdateCallback = null;
super.dispose();
}
}
class DraggableInfo<T> {
bool isOnTarget;
T data;
DraggableInfo(this.isOnTarget, this.data);
}
class MyDraggableController<T> {
List<Function(bool, T)> _targetUpdateCallbacks = new List<Function(bool, T)>();
MyDraggableController();
void onTarget(bool onTarget, T data) {
if (_targetUpdateCallbacks != null) {
_targetUpdateCallbacks.forEach((f) => f(onTarget, data));
}
}
void subscribeToOnTargetCallback(Function(bool, T) f) {
_targetUpdateCallbacks.add(f);
}
void unSubscribeFromOnTargetCallback(Function(bool, T) f) {
_targetUpdateCallbacks.remove(f);
}
}
【讨论】：