Animation API and dynamic values for Flutter widgets. Allows building interactive apps using purely Stateless Widgets. Inspired by react-recollect.

How to use

  • Add with Floop at the end of the widget class definition
  • Retrieve a value from floop and the widget will reactively update on changes to it

Extra step:

  • Use transition(ms) within the build method to have a value transition from 0 to 1 in ms milliseconds.


-class Clicker extends StatelessWidget {
+class Clicker extends StatelessWidget with Floop {
  Widget build(BuildContext context) {
    return Scaffold(
      // Text widget always displays the current value of #clicks.
      body: Center(
        child: Opacity(
          opacity: transition(2000),
          child: Text('${floop[#clicks]}'),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          // Change #clicks from anywhere in the app (except build methods).
          // Control transitions using [TrasitionGroup] methods.
          TransitionGroup(context: context).restart();

Other options:

  • DynamicWidget is a Floop widget that carries it's own state in the form of a map of dynamic values.
  • ... extends StatelessWidget with Floop is equivalent to ... extends FloopWidget.
  • ...extends StatefulWidget with FloopStateful or extend ... extends FloopStatefulWidget for stateful widgets.
  • Maps of dynamic values like floop can be instantiated using DynMap.

Suggested use cases

  • Store data that affects many widgets (eg. user data).

  • Asynchronous operations like data fetching. Conditionally check floop['myData'] == null ? LoadingWidget() : DisplayDataWidget(floop['myData']).

  • Animate widgets.


Add floop dependency to the project's pubspec.yaml


Run flutter pub get in the root directory of the project.

Transitions and Animations

All floop widgets can be animated using transition, which returns a double that will go from 0 to 1 in the specified time:

Widget build(BuildContext context) {
  return Opacity(
    opacity: transition(3000, key: #myKey), // transitions a number from 0 to 1
    child: Container(
      Text('Text will completely appear after 3 seconds')),

Transitions of the same refresh periodicity are synchronized.

TransitionGroup controls transitions.

Resume, reverse, shift time, pause, restart or cancel. They cannot be controlled from inside build methods.

// filter transitions with `key = #myKey` (there can be at most one).
var transitionGroup = TransitionGroup(key: #myKey);
// filter transitions with `bindContext = context` and `tag = #myTag`.
transitionGroup = TransitionGroup(context: context, tag: #myTag);

transitionGroup.reverse();  // reverses the time direction.
transitionGroup.shiftTime(shiftMillis=1000);  // advances the time by 1 second.

// Resumes transitions of the group that belong to the widget tree that starts
// at `rootContext`.
transitionGroup.resume(rootContext: context);

transitionOf retrieves the current value

// retrieves the current value of transition with key #myKey.
double t = transitionOf(#myKey);

Transition provide patterns over the output of transition

These patterns are only useful inside build methods, they do not change the transition output like transitionEval does.

// an int transitions from 5 to 20 over 2 seconds.
Transition.integer(5, 20, 2000);
// a string starts empty and finishes as 'My app' over 3 seconds.
Transition.string('My App', 3000);
// a value that oscillates between 0 and 1.
Transition.sin(2000, repeatAfterMillis=0);

transitionEval receives an evaluate function as parameter

It cannot be used inside build methods. It can be useful to create from UI interactions and reference it with transitionOf. The evaluate function is invoked on every update cycle.

// The transition output is scaled by 10.
transitionEval(3000, (value) => 10*value, key: #myKey);
// reference it somewhere else (for example inside a build method):

TransitionsConfig to set default parameters

TransitionsConfig.refreshPeriodicityMillis = 100;  // sets the default refresh periodicity of transitions to 100ms.
TransitionsConfig.timeDilationFactor = 0.5;  // the progress time of transitions advances at half the speed.

Refresh rate

double refreshRate = TransitionGroup.currentRefreshRateDynamic();

Special Considerations


Dynamic values do not work inside Builder functions. A workaround is to read the dynamic values outside of the builder definition. Example:

Works fine:

  Widget build(BuildContext context) {
    final opacity = transition(3000);
    final text = floop[myText];
    return Builder(
      builder: (context) => Opacity(
        opacity: opacity,
        child: Container(

Should not do:

Widget build(BuildContext context) {
  return Builder(
    builder: (context) => Opacity(
      // Error.
      opacity: transition(3000),
      child: Container(
        // The widget will not update if floop[myText] changes.

The builder function is a callback that executes outside of the encompassing Floop widget's build method and Floop does not listen to these builds.

Transitions and Keys in Stateless Widgets

Use keys on widgets that invoke transition when the widgets belong to the same list of children widgets ...children: [widget1, widget2,...], and this list can change. These are the same conditions when keys should be used on Stateful widgets.



floop is an instance of DynMap, which implements Map. Other instances can be created in the same way as any map is created, e.g: Map<String, int> myDynInts = DynMap().

Widgets subscribe to the values retrieved during their last build.

Maps and Lists

Map and List values are not stored as they are when using []= operator, they are copied into a new DynMap or DynList unless they are already of type Dyn. This behavior ensures that changes on maps or list are detected in a deep sense. Maps and lists can still be stored as they are by using the method DynMap.setValue. This method also receives optional parameter to prevent triggering updates to widgets.

Initializing and Disposing a Context

Floop.initContext is invoked by the BuildContext instance when it's added for the first time to the element tree.

Floop.disposeContext is invoked by the BuildContext instance when it's unmounted (removed from tree to never be used again).

These methods are the equivalent to State.init and State.dispose. They can be overriden to initialize or dispose dynamic values.


Performance rules of thumb:

  • Including Floop on a widget is less impactful than wrapping a widget with another widget.
  • Reading one value from floop inside a build method is like reading five Map values

The only impact Floop has on a widget is to its build time and it does not go beyond x1.2 on minimal widgets (a container, a button and a text).

These build time increases can be considered as rough references when comparing reading data from a DynMap in Floop widgets, to reading the same data from a LinkedHashMap in widgets without Floop. Only integer numbers were used as keys and values. It was also assumed that the same context would access the same keys on every invocation to StatelessWidget.build. It's more expensive when there are different keys read (that should be an uncommon case).

On small Widgets (less than 10 lines in the build method), including Floop implies these build time performance hits:

  • x1.15 when 0 values are read.
  • x1.9 when up to 5 values are read.
  • x2.9 when up to 20 values are read.

On medium Widgets:

  • x1.1 when 0 values are read.
  • x1.6 when up to 5 values are read.
  • x2.5 when up to 20 values are read.

DynMap in comparison to a regular LinkedHashMap:


  • x1.1 using the map like a regular map (outside build methods).
  • x3 while Floop is on 'listening' mode (when a Widget is building).
  • x5 considering the whole preprocessing (start listening) and post processing (stop listening).


  • x1.6.


Writing code, reporting bugs, giving advice or ideas to improve the library is appreciated.