What we have learned about reuse and components in web front-end development.

15 min read


Since early 2020, we have been developing a frontend library at and for Öffentliche Versicherung for Web UI development in Kotlin: fritz2.

fritz2 is a reactive UI framework. At its core, it provides functions for reactively linking a data model to a corresponding user interface. This means that changes to the data model immediately lead to changes to the user interface and events from the browser in turn change the data model. fritz2 provides the necessary tools to manage the data model, the UI and the cyclic, reactive connection between both aspects using a declarative syntax.

fritz2 sees itself as a lightweight framework, which is why the core relies on a few Kotlin features basic for reactive programming like Coroutines and especially Flows. The central fritz2 concepts, like stores, handler, events and tags rely on and integrate these Kotlin building blocks and do not hide them, but rather expose them on the contrary as part of the framework API. So a user does not primarily have to learn fritz2, but much more the universal Kotlin core concepts of coroutines and flows.

By focusing on the reactive connection of model and UI, the few concepts necessary for this and the resulting lightweight API, fritz2 allows to easily and elegantly develop modern browser-based UIs.

Motivation for reuse

A web application is of course based on HTML and CSS at its core. However, for many applications you need the same or very similar UI patterns and structures, commonly called components. Such components form self-contained units that encapsulate function, presentation and, in some cases, data handling.

The use - and thus also the development of such components - is worthwhile itself therefore due to the following aspects:

  • Scarce resources: The focus of an application usually lies on the domain and thus on a higher level than the small-scale generation of a good and functional DOM structure. When developing multiple applications within an organization, it is even more worthwhile to reuse basic building blocks.
  • Minimization of maintenance: In case of errors or new features, not all places in an application or in all applications that implement an affected UI feature need to be touched; it is sufficient to improve the components in a central location.
  • Uniform UX: Many organizations wish for or even demand a uniform appearance, structures and operation within their own applications. Centrally built and maintained components are therefore a simple and efficient way to implement such requirements.

Naive approach

It was therefore obvious to us to integrate such a component library directly into fritz2, in order to use it as the basis for the applications of the "Öffentliche Versicherung".

We started with version 0.8, to integrate a component library into fritz2, which then - including a custom declarative language for styling - has been extended in the following versions.

This library consisted of 20 different components, which cover important ranges from the simple layout, over typical, simple form elements, up to complex generic data tables and whole page frames.

A basic idea was that each component must still be individually styleable in order to adapt it to the specific context of use within the application.

In addition, we tried to incorporate a theme structure as the basis for the look and feel. This made it possible to adapt the components for all applications of an organization to a uniform corporate identity design on the basis of a central styling document (style system). In addition, we were even able to go one level below that, and derive from the central organization theme, to create and use customized themes for different areas of the company. This actually made it possible to implement two different design concepts with the same components.

Problems of the naive approach

Unfortunately, we now consider this approach as a failure!

We have identified the following problems with it:

  • Our components allow their appearance to be customized via the theme. This includes to a large extent styling information, but corresponds to certain HTML tags or an entire DOM substructure. Because of this strong relation between structure and presentation, it is difficult to make components really flexible:
    • If there are different variants of the structure, there must also be styling definitions for each variant. This increases the complexity of the components and the theme - even if only one variant is used within an application. An example is the position of the label in a FormControl: Either always above the input element or always to the left of it.
    • Often you have to apply styling for a special state to several tags to achieve a desired effect. An example of this is the deactivation of form elements, where you can style the label and input element in different ways for such an effect. There is a great risk that when the user is customizing the theme, he will undo such special styling by his specifications.
    • It also gets complicated if the styling is dependent on a status. This must be made available accordingly in the sections in the theme. A component specific type can not be passed on directly, because it is not available at the package level of the theme. So you must fall back to some base types as workaround, with the cost of loosing expressive semantic types and thus a loss of intention revealing.
    • The user will always face the problem of not being able to make really meaningful adjustments to the theme without in-depth knowledge of the final DOM structure of a component, because he has to understand to which tag a styling from the theme would be applied. This cannot be achieved by good naming alone and is anything but trivial.
    • If it comes to fixing bugs or adding new features that require a change of the styling in the theme, the worst case scenario is that a theme that has been customized by a user no longer works properly. Finally, the user must carefully analyze the changes made to the default theme in order to adapt the new, relevant styling definitions to his own theme.
  • The components consist of code that is relatively difficult to understand. Much is, as already indicated, dependent on the theme and thus only comprehensible by reading the code there.
  • Despite the components having been planned and implemented from ground up to be flexible and customizable, the need to build "completely different" components for a similar use case has arisen soon. The adaptability is in practice by far not as high as we had had assumed. (Despite the complexity of the code. See previous item).
  • The focus during creation was primarily on the design, which is in general no core competence of the developers, instead of focusing on usability (ARIA, keyboard support, standard functions, etc.).
  • It means an immense effort to provide and maintain alone the 20 components in this flexibility with good defaults. There would still be a need for further, elementary components, which are not yet even implemented! In addition, urgently needed functions or features of existing components are still missing (keyboard support, ARIA attributes, and much more).

The most important conclusion that can be derived from these problems is: Presentation and functionality of components strongly dependent on each other!

Trying to separate both aspects in a way that the component encapsulates the functionality and the structure, but exposes the styling in a fully customizable way via a public API in the theme, is ultimately just a fallacy. In truth, both parts are still strongly coupled and not independent of each other.

In our view, this makes it impossible to create components that guarantee perfect functionality on the one hand, and on the other hand are really flexible and adaptable.

But what is the solution to this dilemma? To build innumerable, rigid special components, which, in addition to the immense initial effort, would also result in a high level of code duplication and thus even greater maintenance effort? Go without central components at all, but build everything from scratch for every application?

We finally came across a concept that seems to be the solution to the dilemma of combining both extreme alternatives in an elegant way: Headless Components!


In search of a replacement for the previous component library of fritz2, we found the brilliant headlessui which we picked up as a solution for us and which serves as inspiration and template at the same time.

The idea behind it is as simple as the benefit is hard to grasp:

A component is solely defined by its functionality and not by DOM structure and presentation!

This requires explanation: If one considers only the basic function in the core, but the representation, thus the DOM structure and the styling, the myriads of UI components out there can be reduced to a few like:

  • a single selection from a list of options
  • a multiple selection from a list of options
  • showing and hiding panels like dialogs, popovers or action menus
  • the fading in of associated content to a selected option
  • the typing of text

The headless concept takes advantage of this fact and defines components solely by their functionality. Thus, the number of such disjunctive components is reduced to relatively few: Currently, we have identified 15 of them that we have already implemented or are about to implement.

These components are composed of smaller, well-defined building blocks (we call them bricks). Thus, the minimal functional structure of a simple selection can be decomposed as follows:

- single selection
- two-way data-binding (→ What is selected? → Preselection)
// for each option
- option (→ Is this option selected?)
- toggle (→ changes selextion)
- label

In practice, we supplement the functionality with a few often useful, but still optional additional functions:

- single selection
- two-way data-binding
- label (optional)
- description (optional)
- validation (optional) (→ Is this selection valid? → Messages)
// for each option
- option
- toggle
- label
- description (optional)

Using these functions you can build a classic RadioGroup based on native <input type="radio"> tags, but also any other variants can be realized with it.

They differ only in the selection and structure of the HTML tags and styling!

fritz2's headless components consist of such context-sensitive component and module functions. These must be combined in a way that suits the application and, if necessary, supplemented with standard HTML elements to create the desired structure. Special fields are used to manipulate statuses and functions in the context of the module.

The following (partly reduced) examples shows two possible implementations of a RadioGroup.

The first example uses all tags created by default, is based upon the native <input type="radio"> tag and is generally minimal:

val frameworks = listOf("fritz2", "Spring", "flask")
val storedChoice = storeOf(frameworks.first())
// headless start
radioGroup<String>() { // <div>
value(storedChoice) // databinding
frameworks.forEach { option ->
radioGroupOption(option) { // <div>
radioGroupOptionToggle() { // <input type=radio>
radioGroupOptionLabel() { // <label>

As a second example, a selection is shown that replaces some tags created by default with other types - e.g. it dispenses with <input> elements as a basis for the toggle - and in addition supports a special data type for the selection:

data class Framework(val name: String, val info: String)
val frameworks = listOf(
Framework("fritz2", "Kotlin based, reactive, frontend based web framework"),
// ... and some more...
val storedChoice = storeOf(null)
// headless start
radioGroup<Framework?>() { // <div>
value(storedChoice) // databinding
radioGroupLabel() { // <label>
+"Choose some framework please"
frameworks.forEach { option ->
radioGroupOption(option) { // <div>
radioGroupOptionToggle(tag = RenderContext::div) { // <div>
div { // mix HTML tags as custom structure inside!
radioGroupOptionLabel(tag = RenderContext::span) { // <span>
+" * "
radioGroupOptionDescription(tag = RenderContext::span) { // <span>
selected.render { // use some headless function: access current state!
if(it) span { +"(selected)" }

The key insight here is the following: We can build two variants of a single selection aka RadioGroup with the same building blocks. The effort is relatively low in relation to the functionality: Between ten and 25 lines of code are necessary. But both components support keyboard operation, set appropriate ARIA attributes and, of course, deliver the guaranteed functionality of the single selection by passing a 'store'. All the heavy work of the state handling is done by the headless component itself, like reacting to user actions for selecting, setting a preselected option, indicating the selected or active (mouse hovered) option and so on. This is a huge benefit compared to crafting this by yourself!

The individual components help the user with their expressive naming and thus also foster a better understanding of the code too.

In addition, one still would have to style each building block accordingly within the code, in order to have a finished and fully functional surface unit, which was omitted here for reasons of space and simplicity! In principle one injects the styling into the function block as the first parameter, analogous to the known tag factory functions. The source code of the example for a RadioGroup from the headless-demo project clearly shows that the styling specifications make the whole thing only slightly more complex.

To emphasize it again: If you do not pass any styling to the brick functions, the resulting HTML tags will be rendered without any styling!

The deliberate omission of internal styling for headless components has yet another advantage, which is immense in the ecosystem of the web: Such components are open for any kind of styling and CSS framework! Be it tailwindcss, Bootstrap or native CSS styling. Each user can choose the technology he is used to and that suits him.

And where are the components now?

As you can easily guess from the problems described in the introduction, from now on fritz2 will no longer ship styled components ready to use!

Instead, as described before, fritz2 provides the headless components in the new module headless, which can be used as a basis for creating your own components.

To develop a ready styled and reusable component from the headless components and their bricks, fritz2 provides additional tools in the package dev.fritz2.headless.foundation, which also form the foundation for the headless components themselves. Particularly noteworthy are the concepts of properties and hooks. For the component construction these aids support the user to create an outwardly understandable and at the same time comfortable API, thereby preserve so much flexibility and control in the component itself.

For the conception of an individual, styled and ready to use component, one should rely therefore on these tools. They can also be optimally combined with the headless components: Properties have a dedicated mechanism to pass on data to an underlying property of a headless component.


In addition to the headless components implemented so far, we already have two more in the pipeline that will further complement our portfolio and cover missing functionalities:

  • Toasts: Serve to display volatile messages, whose display and (automatic) hiding is handled by the component itself.
  • TypeAhead: (aka ComboBox) Serve as basis for (single) selections with large data set that can be filtered by typing.

We hope that we have made you curious and that you give the concept a chance! Of course, we would also be very happy about your feedback.

To get started, please refer to the documentation of the headless components, which also explains some of the most relevant basic concepts. Furthermore, have a look at the headless-demo module which contains all the demos from the docs.

For an easier introduction to the combination of fritz2 and tailwindcss we have created a dedicated template project in addition to the well known one. We encourage you to use those templates as basis for your own fritz2 based applications!

In the future there will be more blog articles around the topic "component building with headless".

Warm words about the retired components

As already clearly mentioned, the old components (and the styling API) are no longer part of fritz2.

But we will continue to offer patches for serious issues on the 0.14 branch until end of 2022, so there is much time left to migrate to the new headless approach.

We would love to hear some feedback about the migration path you take!

If you have specific questions about the headless approach, its documentation so far, or probably you create your own headless based components with fritz2, please let us know!

For now, we wave goodbye to the old components and welcome the exciting new headless components!