app_button.dart

import 'dart:async';
import 'dart:io';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

class AppButton extends StatefulWidget {
  final Widget child;
  final TargetPlatform platform;
  final double height;
  final double width;
  final VoidCallback onPressed;
  final Border border;
  final BorderRadius borderRadius;
  final Color color;
  final Color splashColor;
  final Color highlightColor;
  final EdgeInsets padding;
  final bool enabled;

  AppButton({
    @required this.child,
    @required this.onPressed,
    this.platform,
    this.height = 55.0,
    this.width,
    this.color = const Color(0x00FFFFFF),
    this.highlightColor = const Color(0x22000000),
    this.splashColor = const Color(0x33000000),
    this.border,
    this.borderRadius,
    this.padding,
    this.enabled = true,
  });

  @override
  AppButtonState createState() => new AppButtonState();
}

class AppButtonState extends State<AppButton>
    with SingleTickerProviderStateMixin {
  static const Duration fadeOutDuration = const Duration(milliseconds: 120);
  static const Duration fadeInDuration = const Duration(milliseconds: 1200);
  static const Duration quickFadeInDuration = const Duration(milliseconds: 300);
  static const Duration quickFadeOutDuration =
      const Duration(milliseconds: 200);
  Tween<double> _opacityTween;
  AnimationController _animationController;

  void _setTween() {
    _opacityTween = new Tween<double>(
      begin: 0.0,
      end: 1.0,
    );
  }

  @override
  void initState() {
    super.initState();
    _animationController = new AnimationController(
      duration: const Duration(milliseconds: 800),
      value: 0.0,
      vsync: this,
    );
    _setTween();
  }

  @override
  Widget build(BuildContext context) {
    final enabled = widget.enabled && widget.onPressed != null;
    return new SizedBox(
      height: widget.height,
      width: widget.width,
      child: new Semantics(
        container: true,
        button: true,
        enabled: enabled,
        child: new ClipRRect(
          borderRadius: widget.borderRadius ?? new BorderRadius.circular(0.0),
          child: new Material(
            color: widget.color,
            child: new Container(
              constraints:
                  const BoxConstraints(minWidth: 88.0, minHeight: 36.0),
              decoration: new BoxDecoration(
                border: widget.border,
                borderRadius: widget.borderRadius,
              ),
              child: _platformButton(context, enabled),
            ),
          ),
        ),
      ),
    );
  }

  static TargetPlatform _currentPlatform({BuildContext context}) {
    if (context != null) {
      return Theme.of(context).platform;
    }
    if (Platform.isIOS) return TargetPlatform.iOS;
    if (Platform.isAndroid) return TargetPlatform.android;
    if (Platform.isFuchsia) return TargetPlatform.fuchsia;
    throw Exception("Unsupported platform");
  }

  // widget generation
  _platformButton(BuildContext context, bool isEnabled) {
    final ButtonThemeData buttonTheme = ButtonTheme.of(context);
    final targetPlatform =
        widget.platform ?? _currentPlatform(context: context);
    if (targetPlatform == TargetPlatform.iOS) {
      return _iOSButton(context, buttonTheme, isEnabled);
    } else {
      return _androidButton(context, buttonTheme, isEnabled);
    }
  }

  _androidButton(
      BuildContext context, ButtonThemeData buttonTheme, bool isEnabled) {
    return new InkWell(
      highlightColor: widget.highlightColor,
      splashColor: widget.splashColor,
      onTap: widget.enabled ? widget.onPressed : null,
      child: new Container(
        child: new Center(
          widthFactor: 1.0,
          heightFactor: 1.0,
          child: new Padding(
            padding: widget.padding ?? new EdgeInsets.all(0.0),
            child: widget.child,
          ),
        ),
      ),
    );
  }

  _iOSButton(
      BuildContext context, ButtonThemeData buttonTheme, bool isEnabled) {
    return GestureDetector(
      behavior: HitTestBehavior.opaque,
      onTapDown: isEnabled ? _handleTapDown : null,
      onTapUp: isEnabled ? _handleTapUp : null,
      onTapCancel: isEnabled ? _handleTapCancel : null,
      onTap: isEnabled ? _handleTap : null,
      child: new Stack(
        fit: StackFit.passthrough,
        children: <Widget>[
          new Positioned.fill(
            child: new FadeTransition(
              opacity: _opacityTween.animate(new CurvedAnimation(
                parent: _animationController,
                curve: Curves.decelerate,
              )),
              child: new Material(color: widget.splashColor),
            ),
          ),
          new Container(
            child: new Center(
              widthFactor: 1.0,
              heightFactor: 1.0,
              child: new Padding(
                padding: widget.padding ?? new EdgeInsets.all(0.0),
                child: widget.child,
              ),
            ),
          ),
        ],
      ),
    );
  }

  // animation
  _animate({bool quickHit = false, bool quickOut = false}) {
    if (quickHit) {
      final Future<Null> ticker = !quickOut
          ? _animationController.animateTo(1.0, duration: quickFadeInDuration)
          : _animationController.animateTo(0.0, duration: quickFadeOutDuration);
      ticker.then((Null value) {
        if (mounted && !quickOut) {
          _animate(quickHit: true, quickOut: true);
        }
      });
      return;
    }
    final bool wasHeldDown = _buttonHeldDown;
    if (_animationController.isAnimating) return;
    final Future<Null> ticker = _buttonHeldDown
        ? _animationController.animateTo(1.0, duration: fadeInDuration)
        : _animationController.animateTo(0.0, duration: fadeOutDuration);
    ticker.then((Null value) {
      if (mounted && wasHeldDown != _buttonHeldDown) {
        _animate();
      }
    });
  }

  // gesture detection
  bool _buttonHeldDown = false;

  void _handleTapDown(TapDownDetails event) {
    _animationController.reset();
    if (!_buttonHeldDown) {
      _buttonHeldDown = true;
      _animate();
    }
  }

  void _handleTapUp(TapUpDetails event) {
    if (_buttonHeldDown) {
      _buttonHeldDown = false;
      if (_animationController.value <= 0.2) {
        _animate(quickHit: true);
      } else {
        _animationController.reset();
      }
    }
  }

  void _handleTap() {
    widget.onPressed();
  }

  void _handleTapCancel() {
    if (_buttonHeldDown) {
      _buttonHeldDown = false;
      _animate();
    }
  }

  // clean up
  @override
  void dispose() {
    _animationController.dispose();
    _animationController = null;
    super.dispose();
  }
}

how about change _animate to this code? i cast it because i get error

  _animate({bool quickHit = false, bool quickOut = false}) {
    if (quickHit) {
      final Future<Null> ticker = (!quickOut
          ? _animationController.animateTo(1.0, duration: quickFadeInDuration)
          : _animationController.animateTo(0.0, duration: quickFadeOutDuration)) as Future<Null>;
      ticker.then((Null value) {
        if (mounted && !quickOut) {
          _animate(quickHit: true, quickOut: true);
        }
      });
      return;
    }
    final bool wasHeldDown = _buttonHeldDown;
    if (_animationController.isAnimating) return;
    final Future<Null> ticker = (_buttonHeldDown
        ? _animationController.animateTo(1.0, duration: fadeInDuration)
        : _animationController.animateTo(0.0, duration: fadeOutDuration)) as Future<Null>;
    ticker.then((Null value) {
      if (mounted && wasHeldDown != _buttonHeldDown) {
        _animate();
      }
    });
  }