Link Search Menu Expand Document


A Lens is basically a way to describe the relation between an outer and inner entity in a structure. It focuses on the inner entity from the viewpoint of the outer entity, which is how it got its name. Lenses are especially useful when using immutable data-types like fritz2 does. A Lens needs to handle the following:

  • Getting the value of the inner entity from a given instance of the outer entity
  • Creating a new instance of the outer entity (immutable!) as a copy of a given one with a different value only for the inner entity

In fritz2, a Lens is defined by the following interface:

interface Lens<P,T> {
    val id: String
    fun get(parent: P): T
    fun set(parent: P, value: T): P

You can easily use this interface by just implementing get() and set(). fritz2 also offers the method buildLens() for a short-and-sweet-experience:

val nameLens = buildLens("name", { }, { person, value -> person.copy(name = value) })

No magic there. The first parameter sets an id for the Lens. When using Lenses with SubStores, the id will be used to generate a valid html-id representing the path through your model. This can be used to identify your elements semantically (for automated ui-tests for example).

If you have deep nested structures or a lot of them, you may want to automate this behavior. fritz2 offers an annotation @Lenses you can add to your data-classes in the commonMain source-set of your multiplatform project:

data class Person(val name: String, val value: String)

Using an annotation-processor, fritz2 builds a L-object per package from these annotations which contains all the Lenses you need. They are named exactly like the entities and properties, so it’s easy to use:

val nameLens =

You can see it in action at our nestedmodel-example.

Keep in mind that your annotated classes have to be in your commonMain source-set otherwise the automatic generation of the lenses will not work!

Have a look at the validation-example to see how to set it up.

This will also help you define a multiplatform project for sharing your model and validation code between the browser and backend.

Distributed by a MIT license.