Flutter: Unraveling the "setState() or markNeedsBuild() called during build" Error in Provider Classes
Flutter's Provider package is a powerful tool for managing state in your application. However, it can sometimes lead to unexpected errors, like the dreaded "setState() or markNeedsBuild() called during build" message. This article will break down the root cause of this error when working with Provider classes and provide clear solutions to overcome it.
Understanding the Error
The "setState() or markNeedsBuild() called during build" error indicates that you're trying to update the UI within the build method of your widget, which is generally not allowed. Flutter uses a single thread for UI updates. If you try to modify the state during the build phase, you're essentially interrupting the UI rendering process, leading to this error.
The Scenario: Provider and setState()
Let's imagine you have a simple counter example using Provider:
class CounterModel with ChangeNotifier {
int _counter = 0;
int get counter => _counter;
void increment() {
_counter++;
notifyListeners();
}
}
class CounterWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text("Counter")),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(
'You have pushed the button this many times:',
),
Consumer<CounterModel>(
builder: (context, counterModel, child) {
return Text(
'${counterModel.counter}',
style: TextStyle(fontSize: 30),
);
},
),
ElevatedButton(
onPressed: () {
// Error occurs here
context.read<CounterModel>().increment();
setState(() {}); // Incorrect usage
},
child: Text('Increment'),
),
],
),
),
);
}
}
In this code, you're using context.read<CounterModel>().increment() to update the counter. However, the setState() call within the button's onPressed callback is the culprit. This attempt to rebuild the widget during the build phase triggers the error.
The Root Cause: Asynchronous Operations
The core reason for this error lies in the asynchronous nature of how Flutter handles UI updates. When increment() is called, it triggers notifyListeners() which rebuilds widgets listening to the CounterModel. This process is asynchronous, meaning it doesn't happen immediately. If you call setState() within this asynchronous operation, Flutter encounters the error because the widget is already in the process of being built.
The Solutions:
-
Avoid setState() during Build: The most straightforward solution is to remove the
setState()call entirely. You don't need it in this case because theConsumerwidget already handles the rebuild automatically when theCounterModelnotifies listeners. -
Use FutureBuilder: For more complex scenarios involving asynchronous operations, use
FutureBuilder. It allows you to handle the loading state and update the UI accordingly after the asynchronous operation completes. -
Leverage Provider's Listeners: The Provider package provides a convenient
addListenermethod. You can attach a listener to your Provider and trigger UI updates within the listener's callback. This ensures that the UI updates happen after the asynchronous operation has finished.
Revised Code:
class CounterWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text("Counter")),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(
'You have pushed the button this many times:',
),
Consumer<CounterModel>(
builder: (context, counterModel, child) {
return Text(
'${counterModel.counter}',
style: TextStyle(fontSize: 30),
);
},
),
ElevatedButton(
onPressed: () {
context.read<CounterModel>().increment();
},
child: Text('Increment'),
),
],
),
),
);
}
}
Conclusion
Understanding the asynchronous nature of Flutter UI updates is crucial for avoiding the "setState() or markNeedsBuild() called during build" error. By removing unnecessary setState() calls, utilizing FutureBuilder, or leveraging Provider's listeners, you can ensure smooth and error-free UI updates in your Flutter applications.
