Creating Custom Gesture Recognizers in Flutter

Flutter, Google’s UI toolkit for building natively compiled applications for mobile, web, and desktop from a single codebase, provides a rich set of built-in gesture recognizers. These cover most common interactions like taps, swipes, and long presses. However, for more specialized or intricate UI interactions, you might need to create custom gesture recognizers. Custom gesture recognizers enable you to detect gestures that Flutter’s default set doesn’t support out of the box, giving you more control over your app’s user experience.

What are Gesture Recognizers?

Gesture recognizers are classes that detect specific sequences of pointer events and translate them into semantic gestures, such as a tap or a drag. They listen for input from the user, analyze it, and determine when a specific gesture has occurred. Flutter provides various built-in recognizers like TapGestureRecognizer, LongPressGestureRecognizer, PanGestureRecognizer, and more.

Why Create Custom Gesture Recognizers?

• Specialized Interactions: Implement interactions not covered by built-in recognizers (e.g., a two-finger rotation or a specific swipe pattern).
• Customizable Logic: Fine-tune gesture recognition logic to match specific app requirements.
• Enhanced User Experience: Create unique and intuitive user interactions.

How to Implement Custom Gesture Recognizers in Flutter

Creating a custom gesture recognizer in Flutter involves the following steps:

Step 1: Extend GestureRecognizer

Create a class that extends GestureRecognizer. This base class provides the foundation for gesture recognition in Flutter.

import 'package:flutter/gestures.dart';

class CustomGestureRecognizer extends GestureRecognizer {
  // Implement gesture recognition logic here
}

Step 2: Implement acceptGesture

Override the acceptGesture method. This method is called when a pointer has contacted the screen. It should return true if the recognizer wants to track the pointer, and false otherwise.

import 'package:flutter/gestures.dart';

class CustomGestureRecognizer extends GestureRecognizer {
  @override
  void acceptGesture(int pointer) {
    // Logic to accept or reject the gesture
  }
}

Step 3: Implement handleEvent

Override the handleEvent method. This method is called for each pointer event (e.g., pointer down, pointer move, pointer up). Analyze the events to detect your custom gesture.

import 'package:flutter/gestures.dart';
import 'package:flutter/services.dart';

class CustomGestureRecognizer extends GestureRecognizer {
  @override
  void acceptGesture(int pointer) {
    // Logic to accept or reject the gesture
  }

  @override
  void handleEvent(PointerEvent event) {
    // Logic to handle pointer events and detect the gesture
  }
}

Step 4: Dispatch the Gesture

Once the custom gesture is detected, invoke the invokeCallback method to notify listeners. You can pass any relevant data to the callback.

import 'package:flutter/gestures.dart';
import 'package:flutter/services.dart';

class CustomGestureRecognizer extends GestureRecognizer {
  GestureTapCallback? onTap; // Custom callback

  @override
  void acceptGesture(int pointer) {
    // Logic to accept or reject the gesture
  }

  @override
  void handleEvent(PointerEvent event) {
    if (event is PointerUpEvent) {
      // Custom gesture detected (e.g., tap)
      invokeCallback('onTap', () => onTap?.call());
    }
  }
}

Step 5: Create a Custom Gesture Detector

Use RawGestureDetector to incorporate your custom gesture recognizer into the Flutter widget tree. This allows the widget to listen for and process the specified gestures.

import 'package:flutter/widgets.dart';
import 'package:flutter/gestures.dart';

class CustomGestureDetector extends StatelessWidget {
  final Widget child;
  final GestureTapCallback? onTap;

  const CustomGestureDetector({Key? key, required this.child, this.onTap}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      gestures: {
        CustomGestureRecognizer: GestureRecognizerFactoryWithHandlers(
              () => CustomGestureRecognizer(),
              (CustomGestureRecognizer instance) {
            instance.onTap = onTap;
          },
        ),
      },
      child: child,
    );
  }
}

Example: Implementing a Two-Finger Tap Recognizer

Let’s create a gesture recognizer that detects a tap with two fingers.

CustomGestureRecognizer Class

import 'package:flutter/gestures.dart';
import 'package:flutter/services.dart';

class TwoFingerTapGestureRecognizer extends GestureRecognizer {
  GestureTapCallback? onTap;
  int _trackedPointers = 0;

  @override
  void acceptGesture(int pointer) {
    _trackedPointers++;
    if (_trackedPointers == 2) {
      // Two fingers are down; accept the gesture
      invokeCallback('onTap', () => onTap?.call());
      _trackedPointers = 0; // Reset count
    }
  }

  @override
  void rejectGesture(int pointer) {
    _trackedPointers--;
    if (_trackedPointers < 0) _trackedPointers = 0;
  }

  @override
  void handleEvent(PointerEvent event) {
    if (event is PointerUpEvent) {
      //A pointer has been lifted, reduce count and reset the state if required
        _trackedPointers--;
         if (_trackedPointers < 0) {
           _trackedPointers = 0;
         }

    } else if (event is PointerDownEvent) {
      // Logic for PointerDownEvent moved to acceptGesture
    }
  }

  @override
  void dispose() {
    _trackedPointers = 0;
    super.dispose();
  }

  @override
  String get debugDescription => 'two_finger_tap';
}

Explanation:

• _trackedPointers keeps track of the number of fingers currently pressing on the screen.
• The acceptGesture() method increments the tracked pointers count.
• Once two fingers are down (_trackedPointers == 2), the onTap callback is invoked and the count is reset.
• rejectGesture() decrements the number of tracked pointers.
• handleEvent now manages `PointerUpEvent` by decrementing tracked pointers. If the gesture fails after this event, pointer count is managed to ensure gesture recognition stability.
• `dispose()` clears any lingering tracked pointer states.

CustomGestureDetector Class

import 'package:flutter/widgets.dart';
import 'package:flutter/gestures.dart';

class TwoFingerTapDetector extends StatelessWidget {
  final Widget child;
  final GestureTapCallback? onTap;

  const TwoFingerTapDetector({Key? key, required this.child, this.onTap}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      gestures: {
        TwoFingerTapGestureRecognizer: GestureRecognizerFactoryWithHandlers(
              () => TwoFingerTapGestureRecognizer(),
              (TwoFingerTapGestureRecognizer instance) {
            instance.onTap = onTap;
          },
        ),
      },
      child: child,
    );
  }
}

Usage

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Custom Gesture Example'),
        ),
        body: Center(
          child: TwoFingerTapDetector(
            onTap: () {
              print('Two-finger tap detected!');
              // Show a dialog or perform any other action
            },
            child: Container(
              width: 200,
              height: 200,
              color: Colors.blue,
              child: Center(
                child: Text(
                  'Tap with two fingers',
                  style: TextStyle(color: Colors.white),
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Considerations

• Performance: Custom gesture recognizers can be computationally expensive. Optimize your logic to avoid performance bottlenecks.
• Conflict Resolution: Be aware of potential conflicts with other gesture recognizers. Implement proper disambiguation logic.
• Testing: Thoroughly test your custom gesture recognizers to ensure they behave as expected in various scenarios.

Conclusion

Creating custom gesture recognizers in Flutter empowers you to implement unique and specialized user interactions. By extending the GestureRecognizer class and handling pointer events, you can detect complex gestures beyond Flutter’s built-in offerings. Proper implementation and optimization are essential for ensuring a smooth and responsive user experience.