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TagTree is an experimental UI framework written in Dart, inspired by React.


This is not an official Google product. TagTree is unsupported and not meant for use in a production Dart application, unless you're willing to fork the code and fix any bugs you find yourself.


The three classes at the heart of TagTree are Tag, Animator, and Place. You might think of them as an alternative to Model-View-Controller.


A Tag is just a class that implements a record:

class GridView extends Tag {
  final Grid grid;
  final List<String> palette;
  final Function onPaint;
  const GridView({this.grid, this.palette, this.onPaint});

  get animator => null; // TODO: explain

In this example, the GridView tag has three attributes, which are just Dart fields. The grid and palette attributes contain data to be used by the tag, and onPaint is an event handler. Unlike an HTML element, these attributes can be various Dart types, not just strings.

Like an HTML element, a Tag can have children. GridView doesn't have any children, but if it did, they would go in another field named inner. But that's just a convention; a Tag can have any fields you like.

Unlike an HTML element, a Tag should never change. Neither should its descendants; the tag trees in TagTree should be deeply immutable, with the exception of event handlers that can point to functions that operate on mutable objects. (Dart doesn't enforce this, but you can give your Tags a const constructor as a hint.)


To give a Tag some behavior, you need to write an Animator.

class ButtonDemo extends Tag {
  const ButtonDemo();
  get animator => const MyButtonAnimator();

class MyButtonAnimator extends Animator<ButtonDemo, int> {
  const MyButtonAnimator();

  Place start(_) => new Place(0);

  Tag renderAt(Place<int> p, _) {

    onClick(_) {
      print("button clicked");
      p.nextState = p.nextState + 1;

    return $.Div(inner: [
      $.H1(inner: "Button Demo"),
      $.Div(inner: "Clicks: ${p.state}"),
      $.Button(onClick: onClick, inner: "Click here"),

As you'd expect, an animator generates an animation. An animation is just a stream of Tags, or rather a stream of tag trees, since a Tag can have descendants. You can think of each of these trees as one frame of the animation. Tag Tree calls the renderAt method to generate each frame.

The input to an animator is another stream, usually a tag. Or rather, a stream of tags, which is how TagTree represents a tag that changes.

In the above example, the input (a ButtonDemo tag) has no fields and doesn't change, so MyButtonAnimator ignores it and generates new animation frames at its own pace, by setting the nextState property on a Place.

This shows the difference between an Animator and a regular HTML template. A template is passive (stateless); its output doesn't change unless its input changes. An Animator could act as a template that expands each input Tag. But it can also get input in other ways and may have internal state. An Animator's output stream has a variable frame rate that's independent of its input stream.

The result is sort of like a slide show where some slides contain animated gifs or videos. The screen changes when you advance to the next slide, but the slides themselves can move on their own, too.

In a similar way, each custom Tag in TagTree is rendered as a separate animation. These animations can be nested to any number of levels and they form a dynamic tree structure, similar to the HTML elements in a web page.

So a TagTree can be thought of as a React-like UI component library, implementing a virtual DOM. Like in React, the virtual DOM has tags representing regular HTML elements and custom tags that you write yourself. But TagTree uses a different metaphor to implement similar functionality.


Since the nested animations in TagTree can move on their own, we need a way to keep track of their state. In TagTree, the state of an animation is always stored in a Place. This allows TagTree to freely share animators between multiple animations running in different places.

If you want to use a more traditional object-oriented style, you can subclass Place and think of it as a widget object. TagTree isolates each Place within an Animator, so this is just an implementation detail. All communication between animations happens using Tag streams and event callbacks.

Optional Features

Actually, there's a third way that an animation can move: its Theme can change. A Theme provides Animators for a bunch of Tags at once and switching Themes will often restart any animations in progress. (A Theme can be thought of as a way to do dependency injection.)

TagTree provides a simple JSON-based codec and basic support for RPC, so that tag trees and their callback events can be sent over the network. This allows a user interface to be split between client and server, using a WebSocket. (This makes an interesting demo, but it will need more work to be secure and reliable enough to use on something other than localhost.)

TemplateTag and AnimatedTag are convenience classes that combine a Tag and its Animator. They're a useful shortcut when you don't care about Themes or RPC, as is the case in most of the examples.


For comparison, I translated a couple of Dart examples to TagTree, along with all the examples from React's front page. PixelPaint is a slightly larger example that can run either entirely in the browser or client-server.

I don't have an online demo page set up yet, so for now you will need to check out the repository and launch the demos from inside the Dart Editor.

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