Explaining what mind conceives..

How to master the Liquid Cards Vignette (by gskinner team)…Hmmm


For Flutter Interact ’19, Google challenged gskinner to showcase the Flutter framework’s capabilities to build beautiful apps.

What are we covering today…?

Flutter Vignette — Liquid Cards
Flutter Vignette — Liquid Cards

Liquid Cards : This demo aims to utilize the Flutter’s list of widgets, specially AnimtedContainer and finally Flutter’s Canvas to simulate a liquid effect..


This tutorial is divided into 3 steps :

  1. Article UI as per Vignette
  2. Implement card opening/closing animation
  3. Show the liquid effect animation

Step 1 :

Data part is static and besides ,is taken from the original source code….

We replicate the data for drinks (static data)….

Flutter Vignette — Liquid Cards
Flutter Vignette — Liquid Cards

Body is moreover a Stack of widgets (top -> My rewards, bottom -> list of cards/drinks)

Top section (My rewards):

  • Column of texts, (middle one being row)..
  • Overlapped by an image (in a widget) of the same size as point 1

Bottom section (List of Cards)

  • Parent widget as Stack
  • Inside it’s a Column of widgets. In a nutshell, basically everything is wrapped inside SingleChildScrollView..

Things to Note (Basically):

  • NeverScrollableScrollPhysics : Denies user scrolling behavior..
  • ButtonTheme : Used to create the Redeem button, specify default properties like minWidth etc inside it…
  • MediaQuery : Use properties like aspectRatio, from the device, to take decisions..
  • BorderRadius : For specifying borders around the card…
  • Using list reduce functionality : Basically returns a single value by iterating all elements of given list..
var lst = [1,2,3,4,5];
var res = lst.reduce((i, j) => i + j);
print('res is ${res}'); // res is 15

Source Code for Step 1

Step 2 :

Flutter Vignette — Liquid Cards
Flutter Vignette — Liquid Cards

In this step, we have kept the card closed (as per vignette).

Moreover, upon the opening or closing of each card, an animation gets triggered……Also, there is a border (rounded) for each card….

Things to Note :

Each card is wrapped inside AnimatedContainer. We detect whether the card is clicked and then animate the height of the card…

  • ElasticOutCurve : Curve property of AnimatedContainer uses ElasticOutCurve(0.9), by default it’s
const ElasticOutCurve([this.period = 0.4]);

Hence, meaning we can even alter the curves 🙂

  • RoundedShadow : Each card is given a rounded border, with the help of RoundedShadow (custom widget)..
RoundedShadow custom widget….
RoundedShadow custom widget….

Pro-tip :

In case you want your widget to be bordered exactly,

  • Set the Container’s decoration -> borderRadius
  • Wrap the widget in ClipRRect with the same borderRadius

Source Code for Step 2

Final Step :

Flutter Vignette — Liquid Cards
Flutter Vignette — Liquid Cards

We integrate the liquid effect animation….

Things to Note (Moreover):

  • AnimatedOpacity : Wave background is wrapped inside this widget. Once a card is tapped / opened, this triggers the animation
opacity: widget.isOpen ? 1 : 0,
duration: Duration(milliseconds: 500),
child: _buildLiquidBackground(_maxFillLevel, fillLevel),
  • _buildLiquidBackground : This part is responsible for the liquid effect animation. In addition to this, below is the lucid explaination for this function….

Liquid Effect Animation (LEA)

return Stack(
fit: StackFit.expand,
children: <Widget>[
offset: Offset(
DrinkListCard.nominalHeightOpen * 1.2 -
DrinkListCard.nominalHeightOpen *
_fillTween.value *
_maxFillLevel *
child: CustomPaint(
painter: LiquidPainter(
waveHeight: 100,

_buildLiquidBackground(code above) is made up of

  • Stack with 1 child (Transform.translate widget)…

We need to start the wave from bottom to top, meaning from one offset to another..Hail, this widget…:)

  • It has 2 properties : offset and child

Offset : Takes 2 params (dx, dy) ….dx = 0.0

dy -> depends on the points left (as per card) and the current animation value..

  • Child -> CustomPaint and the painter property is set to a custom painter (LiquidPainter)


We override the paint method of this painter, to include 2 waves…

void paint(Canvas canvas, Size size) {

// DEF. OF _drawLiquidSim

void _drawLiquidSim(
LiquidSimulation simulation,
Canvas canvas,
Size size,
double offsetY,
Color color

In _drawLiquidSim, we create a path from left side of the card to the right. Before rendering the path, we modify it in the form of quadraticBezierCurve

  • Create a straight line path
  • Divide into 4 sections
  • Each section is converted into a quadraticBezierTo(property from Canvas path)
  • Finally, draw the path…

For each, quadraticBezierTo, we require control points

path.quadraticBezierTo(ctrlPt.dx, ctrlPt.dy, endPt.dx, endPt.dy);

Hence, we created a custom class, LiquidSimulation and outsource the control points calculation inside it….

LiquidSimulation class

In this class, we

  • Calculate control points for the quadraticBezierCurve
  • Create animation with non linear progression..
var animSequence = TweenSequence([
tween: Tween<double>(begin: 0, end: 0),
weight: 10.0,
tween: Tween<double>(begin: 0, end: height).chain(
CurveTween(curve: Curves.linear),
weight: 10.0,
tween: Tween<double>(begin: height, end: 0).chain(
CurveTween(curve: _ease),
weight: 60.0,

Introducing chaining of animations. Using TweenSequenceItem, we direct our animation’s curve to stay same for the desired weight (begin and end)

Tween<double>(begin: 0, end: height)

This in turn, hence, creates a non linear progression of animations….

Source code here……

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