A List of Components

A list of components is a common interface pattern. In mogwai we can express this using two component definitions. In this example we'll use a List component and an Item component.



The List defines its view to include a button to create new items and a ul to hold each item. Each item will have a unique id that will help us determine which items to remove. This id isn't strictly neccessary but it my experience it's a foolproof way to maintain a list of items with frequent splices.

Each item contains a button to remove the item from the list. Click events on this button must be bound to the parent since it is the job of the parent to patch its child views. Each item will also maintain a count of clicks - just for fun.

Both List and Item must define their own model and view messages - ListIn, ListOut, ItemIn and ItemOut, respectively. These messages encode all the interaction between the user/operator, the List and each Item.

When the operator clicks an item's remove button the item's view sends an ItemIn::Remove message. The item's Component::update function then sends an ItemOut::Remove(item.id) message - which is bound to its parent List and mapped to ListIn::Remove(id). This triggers the parent's Component::update function, which will search for the item that triggered the event, remove it from its items and also send an ItemOut::PatchItem(...) patch message to remove the item's view from the list's view.

This is a good example of how mogwai separates component state from component views. The item gizmos don't own the view - the window does! Using the item gizmos we can communicate to the item views. Conversly the item views will communicate with our item gizmos, which will trickle up into the parent.


In this example you can see that unlike other vdom based libraries, mogwai's state is completely separate from its views. Indeed they even require separate handling to "keep them in sync". This is by design. In general a mogwai app tends to be more explicit and less magical than its vdom counterparts.


use log::Level;
use mogwai::prelude::*;
use std::panic;
use wasm_bindgen::prelude::*;

// When the `wee_alloc` feature is enabled, use `wee_alloc` as the global
// allocator.
#[cfg(feature = "wee_alloc")]
static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;

/// One item - keeps track of clicks.
struct Item {
    id: usize,
    clicks: u32,

/// An item's model messages.
enum ItemIn {
    /// The user clicked
    /// The user requested this item be removed

/// An item's view messages.
enum ItemOut {
    /// Change the number of clicks displayed in the view
    /// Remove the item from the parent view

impl Component for Item {
    type ModelMsg = ItemIn;
    type ViewMsg = ItemOut;
    type DomNode = HtmlElement;

    fn update(
        &mut self,
        msg: &Self::ModelMsg,
        tx: &Transmitter<Self::ViewMsg>,
        _sub: &Subscriber<Self::ModelMsg>,
    ) {
        match msg {
            ItemIn::Click => {
                self.clicks += 1;
            ItemIn::Remove => {

    fn view(
        tx: &Transmitter<Self::ModelMsg>,
        rx: &Receiver<Self::ViewMsg>,
    ) -> ViewBuilder<Self::DomNode> {
        let clicks_to_string = |clicks| match clicks {
            1 => "1 click".to_string(),
            n => format!("{} clicks", n),
        builder! {
                <button style:cursor="pointer" on:click=tx.contra_map(|_| ItemIn::Click)>"Increment"</button>
                <button style:cursor="pointer" on:click=tx.contra_map(|_| ItemIn::Remove)>"Remove"</button>
                " "
                    rx.branch_filter_map(move |msg| match msg {
                        ItemOut::Clicked(clicks) => Some(clicks_to_string(*clicks)),
                        _ => None

struct List {
    next_id: usize,
    items: Vec<Gizmo<Item>>,

enum ListIn {
    /// Create a new item
    /// Remove the item at the given index

enum ListOut {
    /// Patch the list of items

impl Component for List {
    type ModelMsg = ListIn;
    type ViewMsg = ListOut;
    type DomNode = HtmlElement;

    fn update(&mut self, msg: &ListIn, tx: &Transmitter<ListOut>, sub: &Subscriber<ListIn>) {
        match msg {
            ListIn::NewItem => {
                let item: Item = Item { id: self.next_id, clicks: 0 };
                self.next_id += 1;

                let gizmo: Gizmo<Item> = Gizmo::from(item);
                sub.subscribe_filter_map(&gizmo.recv, |child_msg: &ItemOut| match child_msg {
                    ItemOut::Remove(index) => Some(ListIn::RemoveItem(*index)),
                    _ => None

                let view: View<HtmlElement> = View::from(gizmo.view_builder());
                tx.send(&ListOut::PatchItem(Patch::PushBack { value: view }));
            ListIn::RemoveItem(id) => {
                let mut may_index = None;
                'find_item_by_id: for (item, index) in self.items.iter().zip(0..) {
                    if &item.state_ref().id == id {
                        may_index = Some(index);
                        tx.send(&ListOut::PatchItem(Patch::Remove{ index }));
                        break 'find_item_by_id;
                if let Some(index) = may_index {

    fn view(&self, tx: &Transmitter<ListIn>, rx: &Receiver<ListOut>) -> ViewBuilder<HtmlElement> {
        builder! {
                <legend>"A List of Gizmos"</legend>
                <button style:cursor="pointer" on:click=tx.contra_map(|_| ListIn::NewItem)>
                    "Create a new item"
                    <ol patch:children=rx.branch_map(|ListOut::PatchItem(patch)| patch.clone())>

pub fn main(parent_id: Option<String>) -> Result<(), JsValue> {

    let gizmo = Gizmo::from(List { items: vec![], next_id: 0 });
    let view = View::from(gizmo.view_builder());
    if let Some(id) = parent_id {
        let parent = utils::document()
    } else {

Notice that the main of this example takes an optional string. This allows us to pass the id of an element that we'd like to append our list/parent component to. This allows us to load the example on the page right here.

Play with it