Flutter Execute Method so long the button pressed

Question

I want to execute a method while a user is pressing down on a button. In pseudocode:

while (button.isPressed) {
  executeCallback();
}

In other words, the executeCallback method should fire repeatedly as long as the user is pressing down on the button, and stop firing when the button is released. How can I achieve this in Flutter?

Answer 1

Use a Listener and a stateful widget. I also introduced a slight delay after every loop:

import 'dart:async';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(brightness: Brightness.dark),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  bool _buttonPressed = false;
  bool _loopActive = false;

  void _increaseCounterWhilePressed() async {
    // make sure that only one loop is active
    if (_loopActive) return;

    _loopActive = true;

    while (_buttonPressed) {
      // do your thing
      setState(() {
        _counter++;
      });

      // wait a bit
      await Future.delayed(Duration(milliseconds: 200));
    }

    _loopActive = false;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Listener(
          onPointerDown: (details) {
            _buttonPressed = true;
            _increaseCounterWhilePressed();
          },
          onPointerUp: (details) {
            _buttonPressed = false;
          },
          child: Container(
            decoration: BoxDecoration(color: Colors.orange, border: Border.all()),
            padding: EdgeInsets.all(16.0),
            child: Text('Value: $_counter'),
          ),
        ),
      ),
    );
  }
}

Answer 2

Building on the solution from ThinkDigital, my observation is that InkWell contains all the events necessary to do this without an extra GestureDetector (I find that the GestureDetector interferes with the ink animation on long press). Here's a control I implemented for a pet project that fires its event with a decreasing delay when held (this is a rounded button with an icon, but anything using InkWell will do):

/// A round button with an icon that can be tapped or held
/// Tapping the button once simply calls [onUpdate], holding
/// the button will repeatedly call [onUpdate] with a
/// decreasing time interval.
class TapOrHoldButton extends StatefulWidget {
  /// Update callback
  final VoidCallback onUpdate;

  /// Minimum delay between update events when holding the button
  final int minDelay;

  /// Initial delay between change events when holding the button
  final int initialDelay;

  /// Number of steps to go from [initialDelay] to [minDelay]
  final int delaySteps;

  /// Icon on the button
  final IconData icon;

  const TapOrHoldButton(
      {Key? key,
      required this.onUpdate,
      this.minDelay = 80,
      this.initialDelay = 300,
      this.delaySteps = 5,
      required this.icon})
      : assert(minDelay <= initialDelay,
            "The minimum delay cannot be larger than the initial delay"),
        super(key: key);

  @override
  _TapOrHoldButtonState createState() => _TapOrHoldButtonState();
}

class _TapOrHoldButtonState extends State<TapOrHoldButton> {
  /// True if the button is currently being held
  bool _holding = false;

  @override
  Widget build(BuildContext context) {
    var shape = CircleBorder();
    return Material(
      color: Theme.of(context).dividerColor,
      shape: shape,
      child: InkWell(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Icon(
            widget.icon,
            color:
                Theme.of(context).textTheme.headline1?.color ?? Colors.white70,
            size: 36,
          ),
        ),
        onTap: () => _stopHolding(),
        onTapDown: (_) => _startHolding(),
        onTapCancel: () => _stopHolding(),
        customBorder: shape,
      ),
    );
  }

  void _startHolding() async {
    // Make sure this isn't called more than once for
    // whatever reason.
    if (_holding) return;
    _holding = true;

    // Calculate the delay decrease per step
    final step =
        (widget.initialDelay - widget.minDelay).toDouble() / widget.delaySteps;
    var delay = widget.initialDelay.toDouble();

    while (_holding) {
      widget.onUpdate();
      await Future.delayed(Duration(milliseconds: delay.round()));
      if (delay > widget.minDelay) delay -= step;
    }
  }

  void _stopHolding() {
    _holding = false;
  }
}

Here it is in action: