SetState() on flutter does not update the drawing

I make my own widget using SingleChildRenderObjectWidget, RenderBox.
I want to update the drawing in setState() when certain actions are performed in the parent widget of this self-widget, but it may not work.

Below is the sample code.
I expect the color to change from black to red when I press the button, but it doesn't really change.

import'package:flutter/material.dart';

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

class MyApp extensions StatelessWidget {
  @override
  Widget build (BuildContext context) {
    US>return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwitch:Colors.blue,
        visualDensity —VisualDensity.adaptivePlatformDensity,
      ),
      home:MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

US>class MyRenderData{
  Color color;

  MyRenderData(this.color);
}

class MyRenderBox extensions RenderBox {
  MyRenderData;

  MyRenderBox (this.data);
  @override
  boole get sizedByParent=>true;

  @override
  void performResize() {
    size = constructs.biggest;
  }

  @override
  void paint (PaintingContext context, Offset offset) {
    Canvas c=context.canvas;
    c.drawRect(
        Rect.fromLTRB (offset.dx, offset.dy, offset.dx+100, offset.dy+100),
        Paint()..color=data.color);
  }
}

class MyRenderWidget extensions SingleChildRenderObjectWidget{
  MyRenderData;

  MyRenderWidget(this.data);

  @override
  RenderObject createRenderObject (BuildContext context) {
    return MyRenderBox(data);
  }
}

class MyHomePage extensions StatefulWidget{
  MyHomePage({Key, this.title}):super(key:key);

  // This widget is the home page of your application.It is stateful, meeting
  // that has a State object (defined below) that contains fields that effect
  // How it looks.

  // This class is the configuration for the state.It holds the values (in this)
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState()=>_MyHomePageState();
}

class_MyHomePageState extensions State<MyHomePage>{
  MyRenderData;

  @override
  void initState() {
    super.initState();
    This.data=MyRenderData(Colors.black);
  }

  @override
  Widget build (BuildContext context) {
    US>return Scaffold(
      appBar —AppBar(
        title:Text(widget.title),
      ),
      body: Center(
        child —MyRenderWidget(data),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed:() {
          setState((){
            // This side moves.
            // data.color=Colors.red;

            // It doesn't work here.
            This.data=MyRenderData(Colors.red);
          });
        },
      ),
    );
  }
}

The sample project was created as follows:

flutter create-t app render_object_test

This is a sample, so it is quite small, but
There is actually a list and a self-made widget for selecting items.
When you select an item from a list, the contents of the item appear in your own widget.

There is a similar problem with your code.
It seems that overwriting the object instead of changing it will cause this problem.
I don't understand why it works like that.
Also, if I want to update the state, is it the right implementation to change the existing instance instead of overwriting it?

flutter dart

2022-09-30 21:50

2 Answers

< p > < code > as follows My Rent Reader Widget < / code > ni < code > Union Square I expect that the street. get your hopes up could actually hand these SAML-HolderOfKey < / code > < / >

class MyRenderWidget extensions SingleChildRenderObjectWidget{
  final MyRenderData;

  const MyRenderWidget(this.data, {Keykey}):super(key:key);

  @override
  RenderObject createRenderObject (BuildContext context) {
    return MyRenderBox(data);
  }
}

child:MyRenderWidget(data, key:UniqueKey()) ,

2022-09-30 21:50

I think it will be easier to understand if you display the dart version.(Because the result depends on whether it is nullsafe or not)

There are two possibilities:

For one thing, Flutter's Widget change detection failed to detect the change.
Two errors occurred in lambda in nullsafe

setState(){
            // If it's nullsafe, you can't substitute the value.
            This.data=MyRenderData(Colors.red);
          });

2022-09-30 21:50

If you have any answers or tips

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