Flutter Provider^4.3.2+3 踩坑记录

════════ Exception caught by gesture ═══════════════════════════════════════════════════════════════
The following assertion was thrown while handling a gesture:
Tried to listen to a value exposed with provider, from outside of the widget tree.

This is likely caused by an event handler (like a button's onPressed) that called
Provider.of without passing `listen: false`.

To fix, write:
Provider.of<Counter>(context, listen: false);

It is unsupported because may pointlessly rebuild the widget associated to the
event handler, when the widget tree doesn't care about the value.

The context used was: HomePage(dependencies: [_InheritedProviderScope<Counter>], state: _HomePageState#aa95a)
'package:provider/src/provider.dart':
Failed assertion: line 262 pos 7: 'context.owner.debugBuilding ||
          listen == false ||
          debugIsInInheritedProviderUpdate'

When the exception was thrown, this was the stack: 
#2      Provider.of (package:provider/src/provider.dart:262:7)
#3      _HomePageState.build.<anonymous closure> (package:flutter_provider_test/main.dart:42:34)
#4      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:993:19)
#5      _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:1111:38)
#6      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:183:24)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#5c9f6
  debugOwner: GestureDetector
  state: possible
  won arena
  finalPosition: Offset(63.4, 33.0)
  finalLocalPosition: Offset(63.4, 27.0)
  button: 1
  sent tap down

原代码如下

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'model.dart';
void main(){
    
    
  runApp(ChangeNotifierProvider(
    create: (_) => Counter(0),
      child: MyApp())
  );
}

class MyApp extends StatefulWidget {
    
    
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
    
    
  @override
  Widget build(BuildContext context) {
    
    
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
    
    
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
    
    
  @override
  Widget build(BuildContext context) {
    
    
    
    int a = context.watch<Counter>().a;
    
    return Scaffold(
      body: Container(
        child: Column(
          children: [
            RaisedButton(onPressed: (){
    
    
                Provider.of<Counter>(context).change() ;
            },child: Text('增加'),),
            Text('$a'),
          ],
          
        ),
      ),
    );
  }
}

import 'package:flutter/material.dart';

class Counter extends ChangeNotifier{
    
    
  int a;
  Counter(this.a);

  void change(){
    
    
    this.a++;
    notifyListeners();
  }
}

根据错误提示，代码中监听了不该监听的值。

所以根据提示把RaisedButton的onPressed方法中的

Provider.of<Counter>(context).change() ;

改成

Provider.of<Counter>(context,listen: false).change() ;