Picture by means of Judith Browne on Unsplash

We proceed reviewing animations in Flutter, and nowadays it’s flip for tween animations. Are we able to use them to construct a foldable widget? Let’s see!


A foldable widget is a part with some fancy transformation that bends the thing over any of its axis.

When it comes right down to animations, Flutter provides a wide variety of probabilities. So there are a number of tactics to construct a part like this one, maximum of them involving specific animations, animation controllers, mixins… However, the purpose of this text is to offer a more effective implementation, in accordance with tweens but in addition making use of a few previous buddies, like change into.

Flutter API

As same old, the query is… that are the categories to be had to construct a widget like this?


A “Tween” (abbreviation for “in-between”) is an object that shops a collection of values. For instance, the usage of a tween you’ll be able to retailer a record of consecutive numbers. However extra summary knowledge varieties can also be saved as smartly: positions, offsets, colours, and many others.

Since tweens are somewhat versatile, this magnificence is outlined as generic, so it takes as a parameter the knowledge form of the items that it is going to comprise:

//XXX: a tween of doubles...
Tween<double> rotationTween = Tween<double>();

//XXX: any other one for colours...
Tween<Colour> colorTween = Tween<Colour>();

When it comes right down to animations, tweens will do a lot of heavy-lifting, as a result of they’re answerable for traversing the entire record of things and calculate the mid values in between. Tween’s sequential output can also be carried out on any widget that calls for an enter that adjustments through the years.

Internally, a tween transforms the preliminary collection to any other one, at all times throughout the vary [0.0 – 1.0]. Amongst different homes, the category announces:

  • start: preliminary worth for the collection
  • finish: ultimate worth for the collection
Tween<double> rotationTween = Tween<double>(start: 0.0, finish: 180.0);


Prior to speaking about curves, let’s consider some vital ideas:

  • After we animate a part, we’re simply converting the price of a few of its homes. Within the following instance, we animate a container by means of converting its peak:
Animated container.
Supply: medium.com
  • Animations at all times have a length, even if they’re performed in loop mode.

Making an allowance for those issues, the curve of an animation can also be described because the “amount of change” that we practice through the years whilst the animation is operating.

Curves are incorporated in lots of programming languages. In Flutter, they’re represented with the “Curve” hierarchy, that comprises a number of categories. They all let us customise the behaviour of our animations. You’ll see to be had curves in motion here.

The next video, as an example, depicts a linear curve. X axis represents time (length of the animation) and Y axis comprises animation values. Since this curve is linear, the “amount of change” carried out at each and every time is continuous.

Linear curve. Supply: api.flutter.dev


“TweenAnimationBuilder” is a comfort magnificence used to animate a widget the usage of the values supplied by means of a tween collection.

It announces the next fields:

  • tween: collection of values used for the animation.
  • length: transformation operating time.
  • builder: means known as for each and every worth traversed within the related tween.
  • onEnd: callback invoked routinely when animation ends. It lets in us to hook customized movements, like chaining one animation with any other.

Turn out to be

As we noticed within the previous article, this widget is used to practice adjustments (in form, dimension, place or viewpoint) to its kid widget.

“Transform” is the closing vital piece required to construct our widget. However how will we have compatibility they all in combination?

The foldable widget

Which might be the options we need to put into effect this time?

  1. Fold the widget. This impact, as we discussed prior to, is simply a rotation, so a easy transformation will do.
  2. Get a supply of values to change the homes of the widget. As chances are you’ll wager, tween items will clear up this one. We will be able to configure them to vary from 0 to 180, since our rotation should take in part a circle.
  3. Chain consecutive operations. Since tweens additionally notify us when they have got traversed the entire record of values (so the animation is completed), we will release one animation after any other.
  4. Crop the contents of a widget, so we most effective see the “unfolded” portions.
PI Rotation (180 levels in radians).
Supply: wikipedia

At this level, nearly all necessities are beautiful straight-forward. For the reason that closing one might appear a little bit tough, let’s discover it intimately.

Cropping content material

Crop impact can also be applied by means of the usage of “ClipRect” and “Align” categories in collusion, nesting them. The previous lets in us to crop any widget the usage of a oblong dimension. Alternatively, the latter can also be given a partial width or a peak plus an alignment, thus growing some kind of viewport over a part.

The next snippet displays the use of this widgets mixture:

go back ClipRect(
        kid: Align(
          alignment: Alignment.topRight,
          width: 0.5, //XXX: part the w
          peak: 0.5, //XXX: part the h   
          kid: Symbol(...),

…and this image presentations the end result when making use of the crop impact over a picture widget:

Authentic symbol and cropped symbol the usage of ClipRect and Align

Some extra implementation main points

In our instance, the objective widget can be folded first over its horizontal axis, from backside to height. After that, folding can be executed at the vertical axis, from proper to left.

So as to stay issues easy, the structure can be a mixture of rows and columns:

Foldable widget structure

Each and every this sort of parts are boxes that come with as its kid the objective widget (the picture that can be folded), however “cropping” the content material accordingly to the present place.

So the primary mobile, as an example, takes part the width and part the peak, and presentations most effective top-left contents:

go back Container(
      kid: ClipRect(
        kid: Align(
          alignment: Alignment.topLeft,
          kid: ...,

In a similar way, the decrease row takes the entire width to be had, however most effective part the peak, and makes use of the ground a part of the contained component.

go back Container(
      kid: ClipRect(
        kid: Align(
          alignment: Alignment.bottomCenter,
          heightFactor: 0.5,
          widthFactor: 1.0,
          kid: Symbol(...),

Moreover, each and every mobile that bends over any other one is enclosed with some animation builder, so we will be able to practice the corresponding transformation through the years by means of feeding the tween’s present worth to change into:

   length: Period(seconds: ANIM_DURATION_IN_SECS),
   //XXX: keep away from storing tweens as magnificence props
   tween: Tween<double>(start: 0.0, finish: 3.141592),
   builder: (BuildContext cntxt, double worth, Widget kid) {
      go back Turn out to be(
         change into: Matrix4.identification()..rotateY(worth),
         kid: ...

How will we release the entire procedure? On this case, for the reason that primary part is a stateful widget, animations are caused the usage of an integer worth this is incremented each and every time a fold is finished:

static const int NO_FOLDING = 0;
static const int FOLDING_HORIZONTAL = 1;
static const int FOLDING_VERTICAL = 2;
void _startMotion() {
    this.setState(() {
      this._numFolds = FOLDING_HORIZONTAL;

And after all, the chaining of operations: when the primary animation is completed, the fold counter is larger once more, inflicting the widget to rebuild and launching the following animation:

onEnd: () {

This video presentations the general end result:

Our foldable widget containing a picture


In order that can be all! Combining tweens and animation developers, we will be able to arrange a supply of continuing knowledge that brings movement to another widget. And including transformations into the combination, we will be able to rotate the thing to succeed in the specified impact.

Piece of cake! In any case, a foldable part can have been a lot tougher to “bend”, proper…?

Bender bending the onerous method…

Write you subsequent time. As same old, complete supply code is to be had at:



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