Programming language: Kotlin
License: MIT License
Tags: Web    
Latest version: v0.8

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fritz2 is an extremely lightweight, well-performing, independent library to build reactive web apps in Kotlin heavily depending on coroutines and flows.

fritz2 includes an intuitive way to build and render html-elements and styles using a type-safe dsl. You can easily create lightweight reactive html-components that are bound to an underlying model and automatically change whenever the model data changes:

val model = storeOf("init value")

render {
    div("some-css-class") {
        input {
            changes.values() handledBy model.update 
        p {
            +"model value = "

fritz2 implements precise data binding. This means that when parts of your data model change, exactly those and only those DOM-nodes depending on the changed parts will automatically change as well. No intermediate layer (like a virtual DOM) is needed. fritz2 requires no additional methods to decide which parts of your component have to be re-rendered:

State management in fritz2

Utilizing Kotlin's multiplatform-abilities, you'll write the code of your data classes only once and use it on your client and server (i.e. in a SpringBoot- or Ktor-Backend). This is also true for your model-validation-code, which can quickly become far more complex than your data model.

Key Features

  • easy reactive one- and two-way-databinding (even for lists and deep nested structures)
  • hassle-free redux-like state-handling
  • model-validation and message handling
  • integrated styling-dsl (outdated)
  • component-library (outdated, will be replaced by a headless approach)
  • routing (for SPAs, hash-based)
  • backend-repositories (Rest APIs, WebSockets, LocalStorage, etc.)
  • history / undo
  • processing state ("spinning wheel")
  • easy to learn
  • documentation
  • examples i.e. implementing the specification of TodoMVC

How to try it?

Overall Goals

  • staying lightweight (a few hundred lines of code for the core)
  • keeping dependencies as low as possible (zero up to now!)
  • generating elements, attributes, events for html from specification (w3c, mozilla, ...)
  • make it as easy as possible to write reactive pwas in pure kotlin


fritz2 is hugely inspired by the great Binding.scala. Later we discovered that a lot of those concepts are described independently in Meiosis.

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If you like the idea of a lightweight pure Kotlin implementation for building reactive web apps, please give us a star on github. Thank you.