How to update Draggable child when entering DragTarget in Flutter? - drag-and-drop

Question

I have a number of Draggables and DragTargets. On the Draggables I have specified child and feedback, however I also want it to change it's look when the Draggable enters the DragTarget. I can't see any way to do this.
Whenever I drag the Draggable around, I change it's color to be red, however as soon as it enters the DragTarget I want to update the Draggable color to green.
I am aware of DragTarget.OnWillAccept, this method is called whenever the Draggable enters the DragTarget, but I only have the data. I tried updating the data with new color and then calling setState, but that seemed not to work.
Any suggestions on how to get this behaviour?
I want something like the following callback Draggable.onEnteringDragTarget and Draggable.onLeavingDragTarget.

Answer 1

The only way I can think of is using a streambuilder and a stream that will carry the information of whether the draggable is on a drag target. This code provides a basic solution.
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);
},
),
],
),
);
}
}
Edit: The first example doesn't handle multiple drags at the same time this on does and it's a little cleaner.
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));
}
}
Edit 2: Solution without using streams and streambuilder. The important part is the feedback widget is stateful.
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);
}
}