Handle changes to a text field

In some cases, it’s useful to run a callback function every time the text in a text field changes. For example, you might want to build a search screen with autocomplete functionality where you want to update the results as the user types.

How do you run a callback function every time the text changes? With Flutter, you have two options:

  1. Supply an onChanged() callback to a TextField or a TextFormField.
  2. Use a TextEditingController.

1. Supply an onChanged() callback to a TextField or a TextFormField

The simplest approach is to supply an onChanged() callback to a TextField or a TextFormField. Whenever the text changes, the callback is invoked.

In this example, print the current value of the text field to the console every time the text changes.

TextField(
  onChanged: (text) {
    print("First text field: $text");
  },
);

2. Use a TextEditingController

A more powerful, but more elaborate approach, is to supply a TextEditingController as the controller property of the TextField or a TextFormField.

To be notified when the text changes, listen to the controller using the addListener() method using the following steps:

  1. Create a TextEditingController.
  2. Connect the TextEditingController to a text field.
  3. Create a function to print the latest value.
  4. Listen to the controller for changes.

Create a TextEditingController

Create a TextEditingController:

// Define a custom Form widget.
class MyCustomForm extends StatefulWidget {
  @override
  _MyCustomFormState createState() => _MyCustomFormState();
}

// Define a corresponding State class.
// This class holds data related to the Form.
class _MyCustomFormState extends State<MyCustomForm> {
  // Create a text controller. Later, use it to retrieve the
  // current value of the TextField.
  final myController = TextEditingController();

  @override
  void dispose() {
    // Clean up the controller when the widget is removed from the
    // widget tree.
    myController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    // Fill this out in the next step.
  }
}

Connect the TextEditingController to a text field

Supply the TextEditingController to either a TextField or a TextFormField. Once you wire these two classes together, you can begin listening for changes to the text field.

TextField(
  controller: myController,
);

Create a function to print the latest value

You need a function to run every time the text changes. Create a method in the _MyCustomFormState class that prints out the current value of the text field.

_printLatestValue() {
  print("Second text field: ${myController.text}");
}

Listen to the controller for changes

Finally, listen to the TextEditingController and call the _printLatestValue() method when the text changes. Use the addListener() method for this purpose.

Begin listening for changes when the _MyCustomFormState class is initialized, and stop listening when the _MyCustomFormState is disposed.

class _MyCustomFormState extends State<MyCustomForm> {
  @override
  void initState() {
    super.initState();

    // Start listening to changes.
    myController.addListener(_printLatestValue);
  }
}

Interactive example

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Retrieve Text Input',
      home: MyCustomForm(),
    );
  }
}

// Define a custom Form widget.
class MyCustomForm extends StatefulWidget {
  @override
  _MyCustomFormState createState() => _MyCustomFormState();
}

// Define a corresponding State class.
// This class holds data related to the Form.
class _MyCustomFormState extends State<MyCustomForm> {
  // Create a text controller and use it to retrieve the current value
  // of the TextField.
  final myController = TextEditingController();

  @override
  void initState() {
    super.initState();

    myController.addListener(_printLatestValue);
  }

  @override
  void dispose() {
    // Clean up the controller when the widget is removed from the widget tree.
    // This also removes the _printLatestValue listener.
    myController.dispose();
    super.dispose();
  }

  _printLatestValue() {
    print("Second text field: ${myController.text}");
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Retrieve Text Input'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Column(
          children: <Widget>[
            TextField(
              onChanged: (text) {
                print("First text field: $text");
              },
            ),
            TextField(
              controller: myController,
            ),
          ],
        ),
      ),
    );
  }
}