Provider: Tried to use `context.read<Counter>` inside either a `build` method or the `update` callback of a provider.

Created on 15 Sep 2020 · 6Comments · Source: rrousselGit/provider

Describe the bug

As in the documentation, I tried to use the context.read<Counter> just to fetch a count value but throws an exception:

The following assertion was thrown building ProviderPractice(dirty): Tried to usecontext.readinside either abuildmethod or theupdatecallback of a provider. This is unsafe to do so. Instead, consider usingcontext.watch. If you usedcontext.readvoluntarily as a performance optimisation, the solution is instead to usecontext.select. 'package:provider/src/provider.dart': Failed assertion: line 584 pos 9: 'debugIsInInheritedProviderCreate || (!debugDoingBuild && !debugIsInInheritedProviderUpdate)' The relevant error-causing widget was: ProviderPractice

To Reproduce

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (_) => Counter(),
      child: MaterialApp(
        title: 'Custom Page Transitions',
        home: ProviderPractice(),
        theme: ThemeData.light().copyWith(
          floatingActionButtonTheme: FloatingActionButtonThemeData(
            backgroundColor: Colors.deepOrange,
          ),
        ),
      ),
    );
  }
}

class ProviderPractice extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print("I am rebuilding");
    return Scaffold(
      floatingActionButton: FloatingActionButton(
          onPressed: () => context.read<Counter>().increment(),
          child: Icon(Icons.add)),
      body: Center(
          child: Text(context.read<Counter>().count.toString(),
              style: TextStyle(fontSize: 50.0))),
    );
  }
}

class Counter extends ChangeNotifier {
  int count = 0;

  void increment() {
    count++;
    notifyListeners();
  }
}

Expected behavior

I was expected '0' to be shown on the screen even when I incremented the count variable.

bug

Source

chinkysight

Most helpful comment

@chinkysight , check out this link, contains the explanation for this.

That helped me, Thank You So Much @vinodpahumalani

chinkysight on 15 Sep 2020

👍2

All 6 comments

Hey @chinkysight ,

Full Error states the following:

 The following assertion was thrown building ProviderPractice(dirty):
 Tried to use `context.read<Counter>` inside either a `build` method or the `update` callback of a provider.
 This is unsafe to do so. Instead, consider using `context.watch<Counter>`.
 If you used `context.read` voluntarily as a performance optimisation, the solution is instead to use `context.select`.

Now the reason you are getting this error is that you are using context.read<Counter> in StatelessWidget.

So; here as from the above error note, you can either use context.watch<Counter> or
context.select((Counter counter)=>counter.count) for performance optimization.

We can use context.read in StatefulWidget.
You can place it inside of initState.

Go to the documentation you will find examples of context.read with only StatefulWidget.

I hope, i was helpful here.