Flutter setstate() called after dispose()

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Explanation of Flutter’s `setState()` called after `dispose()` Error

The error message “setState() called after dispose()” in Flutter occurs when you try to update the state of a widget after it has already been disposed. This typically happens when an async operation completes after the widget has been removed from the widget tree.

To understand this better, let’s take an example: 

    
      class MyWidget extends StatefulWidget {
        @override
        _MyWidgetState createState() => _MyWidgetState();
      }
      
      class _MyWidgetState extends State {
        @override
        void dispose() {
          // Clean up resources
          super.dispose();
        }
        
        @override
        Widget build(BuildContext context) {
          return Container();
        }
        
        void fetchData() async {
          final result = await API.getData(); // Assume an async API call
          
          setState(() {
            // Update widget's state with fetched data
          });
        }
        
        @override
        void initState() {
          super.initState();
          fetchData();
        }
      }

In this example, we have a widget called MyWidget which fetches data from an API using the fetchData() method during its initState() lifecycle. When the data is fetched and returned, we call setState() to update the widget’s state with the new data.

However, there is a problem with this code. If the data fetching operation takes longer than expected and the MyWidget widget gets removed from the widget tree, the dispose() method will be called. This is because dispose() is called when a widget is being removed from the tree and is no longer needed.

Now, if the fetchData() function completes after the widget has been disposed, and we try to call setState(), we will encounter the “setState() called after dispose()” error because the widget is no longer in a valid state to be updated.

To solve this issue, you can add a mounted check before calling setState(). The mounted check ensures that the widget is still mounted to the widget tree before updating its state. Here’s an updated version of the code: 

    
      class MyWidget extends StatefulWidget {
        @override
        _MyWidgetState createState() => _MyWidgetState();
      }
      
      class _MyWidgetState extends State {
        bool _mounted = false;
        
        @override
        void dispose() {
          _mounted = false; // Trigger the mounted flag to false
          super.dispose();
        }
        
        void fetchData() async {
          final result = await API.getData(); // Assume an async API call
          
          if (_mounted) {
            setState(() {
              // Update widget's state with fetched data
            });
          }
        }
        
        @override
        void initState() {
          super.initState();
          _mounted = true; // Set the mounted flag to true
          fetchData();
        }
        
        @override
        Widget build(BuildContext context) {
          return Container();
        }
      }

By adding the _mounted flag and checking its value before calling setState(), we can avoid the “setState() called after dispose()” error. The flag ensures that the widget is still available and not disposed before updating its state.

Remember to set the flag to true in initState() and false in dispose() to properly handle the widget’s lifecycle.

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