How to create a draggable element that expands to full screen

Issue

I want to build a typical audio player in flutter, that has a miniplayer visible at the bottom of the screen, that can be dragger or tapped to become fullscreen, exactly like the built in iOS music app or spotify:

enter image description here

The element at the top should not be able to stop in between so, if the user drags halfway, then it should either go back down again, or complete the transition (depending on where it currently is).

It should also be possible to drag it down again.

Sidenote: I’m using CupertinoApp not MaterialApp… not sure if that makes a difference.

Solution

Have you tried backdrop element? You can check out this really cool article to see how to implement it. You can customise it and use it according to your needs. I implemented it roughly with the example below, and I am using CupertinoApp (which is my first time) 🙂

Example

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'dart:math' as math;

import 'package:meta/meta.dart';
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CupertinoApp(
      title: 'Flutter Demo',
      routes: {
        '/': (context) => SimpleExample(),
      },
    );
  }
}

class SimpleExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) =>
      Scaffold(body: SafeArea(child: Panels()));
}

class Panels extends StatelessWidget {
  final frontPanelVisible = ValueNotifier<bool>(false);

  @override
  Widget build(BuildContext context) {
    return Backdrop(
      frontLayer: FrontPanel(),
      backLayer: BackPanel(
        frontPanelOpen: frontPanelVisible,
      ),
      frontHeader: FrontPanelTitle(),
      panelVisible: frontPanelVisible,
      frontPanelOpenHeight: 40.0,
      frontHeaderHeight: 48.0,
      frontHeaderVisibleClosed: true,
    );
  }
}

class FrontPanelTitle extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.only(top: 16.0, left: 16.0),
      child: Text(
        'Tap Me',
        style: Theme.of(context).textTheme.subhead,
      ),
    );
  }
}

class FrontPanel extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
        color: Theme.of(context).cardColor,
        child: Center(child: Text('Hello world')));
  }
}

class BackPanel extends StatefulWidget {
  BackPanel({@required this.frontPanelOpen});
  final ValueNotifier<bool> frontPanelOpen;

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

class _BackPanelState extends State<BackPanel> {
  bool panelOpen;

  @override
  initState() {
    super.initState();
    panelOpen = widget.frontPanelOpen.value;
    widget.frontPanelOpen.addListener(_subscribeToValueNotifier);
  }

  void _subscribeToValueNotifier() =>
      setState(() => panelOpen = widget.frontPanelOpen.value);

  /// Required for resubscribing when hot reload occurs
  @override
  void didUpdateWidget(BackPanel oldWidget) {
    super.didUpdateWidget(oldWidget);
    oldWidget.frontPanelOpen.removeListener(_subscribeToValueNotifier);
    widget.frontPanelOpen.addListener(_subscribeToValueNotifier);
  }

  @override
  Widget build(BuildContext context) {
    return Column(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Center(
              child: Padding(
                padding: const EdgeInsets.only(
                  top: 10.0,
                ),
                child: Text('Front panel is ${panelOpen ? "open" : "closed"}'),
              )),
          Center(
              child: RaisedButton(
                child: Text('Tap Me'),
                onPressed: () {
                  widget.frontPanelOpen.value = true;
                },
              )),
          // will not be seen; covered by front panel
          Center(child: Text('Bottom of Panel')),
        ]);
  }
}

const _kFlingVelocity = 2.0;

class _BackdropPanel extends StatelessWidget {
  const _BackdropPanel({
    Key key,
    this.onTap,
    this.onVerticalDragUpdate,
    this.onVerticalDragEnd,
    this.title,
    this.child,
    this.titleHeight,
    this.padding,
  }) : super(key: key);

  final VoidCallback onTap;
  final GestureDragUpdateCallback onVerticalDragUpdate;
  final GestureDragEndCallback onVerticalDragEnd;
  final Widget title;
  final Widget child;
  final double titleHeight;
  final EdgeInsets padding;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: padding,
      child: Material(
        elevation: 12.0,
        borderRadius: BorderRadius.only(
          topLeft: Radius.circular(16.0),
          topRight: Radius.circular(16.0),
        ),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            GestureDetector(
              behavior: HitTestBehavior.opaque,
              onVerticalDragUpdate: onVerticalDragUpdate,
              onVerticalDragEnd: onVerticalDragEnd,
              onTap: onTap,
              child: Container(height: titleHeight, child: title),
            ),
            Divider(
              height: 1.0,
            ),
            Expanded(
              child: child,
            ),
          ],
        ),
      ),
    );
  }
}

/// Builds a Backdrop.
///
/// A Backdrop widget has two panels, front and back. The front panel is shown
/// by default, and slides down to show the back panel, from which a user
/// can make a selection. The user can also configure the titles for when the
/// front or back panel is showing.
class Backdrop extends StatefulWidget {
  final Widget frontLayer;
  final Widget backLayer;
  final Widget frontHeader;
  final double frontPanelOpenHeight;
  final double frontHeaderHeight;
  final bool frontHeaderVisibleClosed;
  final EdgeInsets frontPanelPadding;
  final ValueNotifier<bool> panelVisible;

  Backdrop(
      {@required this.frontLayer,
        @required this.backLayer,
        this.frontPanelOpenHeight = 0.0,
        this.frontHeaderHeight = 48.0,
        this.frontPanelPadding = const EdgeInsets.all(0.0),
        this.frontHeaderVisibleClosed = true,
        this.panelVisible,
        this.frontHeader})
      : assert(frontLayer != null),
        assert(backLayer != null);

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

class _BackdropState extends State<Backdrop>
    with SingleTickerProviderStateMixin {
  final _backdropKey = GlobalKey(debugLabel: 'Backdrop');
  AnimationController _controller;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      duration: Duration(milliseconds: 300),
      // value of 0 hides the panel; value of 1 fully shows the panel
      value: (widget.panelVisible?.value ?? true) ? 1.0 : 0.0,
      vsync: this,
    );

    // Listen on the toggle value notifier if it's not null

    widget.panelVisible?.addListener(_subscribeToValueNotifier);

    // Ensure that the value notifier is updated when the panel is opened or closed
    if (widget.panelVisible != null) {
      _controller.addStatusListener((status) {
        if (status == AnimationStatus.completed)
          widget.panelVisible.value = true;
        else if (status == AnimationStatus.dismissed)
          widget.panelVisible.value = false;
      });
    }
  }

  void _subscribeToValueNotifier() {
    if (widget.panelVisible.value != _backdropPanelVisible)
      _toggleBackdropPanelVisibility();
  }

  /// Required for resubscribing when hot reload occurs
  @override
  void didUpdateWidget(Backdrop oldWidget) {
    super.didUpdateWidget(oldWidget);
    oldWidget.panelVisible?.removeListener(_subscribeToValueNotifier);
    widget.panelVisible?.addListener(_subscribeToValueNotifier);
  }

  @override
  void dispose() {
    _controller.dispose();
    widget.panelVisible?.dispose();
    super.dispose();
  }

  bool get _backdropPanelVisible =>
      _controller.status == AnimationStatus.completed ||
          _controller.status == AnimationStatus.forward;

  void _toggleBackdropPanelVisibility() => _controller.fling(
      velocity: _backdropPanelVisible ? -_kFlingVelocity : _kFlingVelocity);

  double get _backdropHeight {
    final RenderBox renderBox = _backdropKey.currentContext.findRenderObject();
    return renderBox.size.height;
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    if (!_controller.isAnimating)
      _controller.value -= details.primaryDelta / _backdropHeight;
  }

  void _handleDragEnd(DragEndDetails details) {
    if (_controller.isAnimating ||
        _controller.status == AnimationStatus.completed) return;

    final double flingVelocity =
        details.velocity.pixelsPerSecond.dy / _backdropHeight;
    if (flingVelocity < 0.0)
      _controller.fling(velocity: math.max(_kFlingVelocity, -flingVelocity));
    else if (flingVelocity > 0.0)
      _controller.fling(velocity: math.min(-_kFlingVelocity, -flingVelocity));
    else
      _controller.fling(
          velocity:
          _controller.value < 0.5 ? -_kFlingVelocity : _kFlingVelocity);
  }

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(builder: (context, constraints) {
      final panelSize = constraints.biggest;
      final closedPercentage = widget.frontHeaderVisibleClosed
          ? (panelSize.height - widget.frontHeaderHeight) / panelSize.height
          : 1.0;
      final openPercentage = widget.frontPanelOpenHeight / panelSize.height;

      final panelDetailsPosition = Tween<Offset>(
        begin: Offset(0.0, closedPercentage),
        end: Offset(0.0, openPercentage),
      ).animate(_controller.view);

      return Container(
        key: _backdropKey,
        child: Stack(
          children: <Widget>[
            widget.backLayer,
            SlideTransition(
              position: panelDetailsPosition,
              child: _BackdropPanel(
                onTap: _toggleBackdropPanelVisibility,
                onVerticalDragUpdate: _handleDragUpdate,
                onVerticalDragEnd: _handleDragEnd,
                title: widget.frontHeader,
                titleHeight: widget.frontHeaderHeight,
                child: widget.frontLayer,
                padding: widget.frontPanelPadding,
              ),
            ),
          ],
        ),
      );
    });
  }
}

Answered By – salihgueler

Answer Checked By – Mary Flores (FlutterFixes Volunteer)

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