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Lists as a Model

Like for any other type, you can create a Store that holds a list:

val listStore = RootStore<List<String>>(listOf("a","b","c"))

It is perfectly valid to handle this as seen before: render the data by iterating over the List (using forEach() inside the map {} function, for example). Keep in mind that this means re-rendering your whole List whenever an element in your model changes. This might be exactly what you want for small Lists, for Lists that changes rarely or not at all, or for Lists with a small representation in HTML (just a test per item), etc. However, for Lists that change more often and/or result in complex HTML-trees per item, this does not perform well.

For those cases, fritz2 offers the method renderEach {} which creates a RenderContext and mounts its result to the DOM. renderEach {} works analogously to render {} and renderElement {}, but it creates a specialized mount-point in order to identify elements for re-rendering. This mount-point compares the last version of your model with the new one on every change and applies the minimum necessary patches to the DOM.

val seq = object : RootStore<List<String>>(listOf("one", "two", "three")) {
    var count = 0

    val addItem = handle { list ->
        list + "yet another item no. $count"
    val deleteItem = handle<String> { list, current ->

render {
    section {
        ul {
   { s ->
                li {
                    button("btn", id = "delete-btn") {
               { console.log("deleting $s"); s } handledBy seq.deleteItem
        button("button") {
            +"add an item"
            clicks handledBy seq.addItem

renderEach {} allows you to use one-way-databinding when working with the elements in your List out of the box. If you need two-way-databinding (to edit the single elements in a form, for example), just call renderEach {} on the Store<List<T> instead. This gives you a SubStore for each element of your List. You can use them just like any other Store to build a form and bind your data.

in commonMain:

data class ToDo(
    val id: String = uniqueId(),
    val text: String,
    val completed: Boolean = false

in jsMain:

val defaultToDos = listOf(ToDo(text = "foo"), ToDo(text = "bar"))
object ToDosStore: RootStore<List<ToDo>>(defaultToDos) {
    // handlers here...

fun main() {
    render {
        section {
            ToDosStore.renderEach(ToDo::id) { toDo ->
                val textStore = toDo.sub(L.ToDo.text)
                val completedStore = toDo.sub(L.ToDo.completed)

If you have to manipulate your Store’s data-flow before rendering the elements you can do so and call renderEach {} on the resulting Flow and get your Store by simply calling myListStore.sub(myElement) in your renderEach {} block.

There are four flavours of renderEach {} to chose from to fit your use-case:

  • use Flow<T>.renderEach { } to render each instance of T. It uses Kotlin’s equality function to determine whether two elements are the same, and therefore re-renders the whole content you mapped when an element is changed or moved.

  • Flow<T>.renderEach(idProvider: (T) -> I) also renders each instance of T, but it uses the given idProvider to determine whether two elements are the same. In your mapping, you can get a SubStore for an element using listStore.sub(element, idProvider), so only the parts of your sub-model that actually changed will be re-rendered. Caution: when using this flavour without the SubStore, an element with the same id but different content will be treated as unchanged and therefore not be re-rendered.

  • with Store<List<T>>.renderEach(idProvider: (T) -> I) { } you get a SubStore<T> to render your items. It uses the given idProvider to determine whether two elements are the same`, so only the parts that are bound and actually changed will be re-rendered. Use this whenever you work on entities that can be identified using some sort of constant id and need two-way-databinding.

  • use Store<List<T>>.renderEach() to render your items using a SubStore<T> whenever you want to use the position of a list element to determine whether two elements are the same. This means that when inserting something into the middle of the list, the changed element and all following elements will be re-rendered. Use this when working on elements that are not identifiable using a constant id (like simple [String]s) and still need two-way-databinding.

Now you know how to handle all kinds of data and structures in your Stores. Next, you might want to check whether your model is valid. In fritz2 this is done with Validation.

Distributed by a MIT license.