TodoMVC - Update

Let's fire and handle messages!

And don't forget to check that everything works after each step as usual.

Msg::ToggleTodo(Ulid)

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
       ...
        Msg::ToggleTodo(id) => {
            if let Some(todo) = model.todos.get_mut(&id) {
                todo.completed = not(todo.completed);
            }
        } 
...

fn view_todo_list(todos: &BTreeMap<Ulid, Todo>, selected_todo: Option<&SelectedTodo>) -> Node<Msg> {
ul![C!["todo-list"],
    todos.values().map(|todo| {
        let id = todo.id;
        let is_selected = Some(id) == selected_todo.map(|selected_todo| selected_todo.id);
        ...
                input![C!["toggle"], 
                    attrs!{At::Type => "checkbox", At::Checked => todo.completed.as_at_value()},
                    ev(Ev::Change, move |_| Msg::ToggleTodo(id))
                ],
...

We can't write move |_| Msg::ToggleTodo(todo.id) because we can't close (and move) the referenced todo. And we can't write |_| Msg::ToggleTodo(id) because without move we only close the reference to id. We need to move the value into the closure so the closure is 'static and can be used inside a listener. Fortunately our id implements Copy so move isn't a real move but copy - otherwise we would need to clone the id.

Msg::RemoveTodo(Ulid)

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
       ...
        Msg::RemoveTodo(id) => {
            model.todos.remove(&id);
        }
...

fn view_todo_list(todos: &BTreeMap<Ulid, Todo>, selected_todo: Option<&SelectedTodo>) -> Node<Msg> {
    ...
                    button![C!["destroy"],
                        ev(Ev::Click, move |_| Msg::RemoveTodo(id))
                    ],
...

Msg::NewTodoTitleChanged(String)

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
       ...
        Msg::NewTodoTitleChanged(title) => {
            model.new_todo_title = title;
        }
...

fn view_header(new_todo_title: &str) -> Node<Msg> {
        input![C!["new-todo"],
            ...
            input_ev(Ev::Input, Msg::NewTodoTitleChanged),
...

There aren't any changes from the user point of view, but the main goal was to store changed input value to Model.

Note:

input_ev(Ev::Input, Msg::NewTodoTitleChanged)

is almost the same as

input_ev(Ev::Input, |title| Msg::NewTodoTitleChanged(title))

However there are cases where you have to use the latter one, because Rust can't apply all coercion rules without explicitly written variables.

Msg::CreateTodo
New todo
New todos are entered in the input at the top of the app. The input element should be focused when the page is loaded, preferably by using the autofocus input attribute. Pressing Enter creates the todo, appends it to the todo list, and clears the input. Make sure to .trim() the input and then check that it's not empty before creating a new todo.
```
use ulid::Ulid;

const ENTER_KEY: &str = "Enter";

// ------ ------
//     Init
// ------ ------
...

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
        ...
        Msg::CreateTodo => {
            let title = model.new_todo_title.trim();
            if not(title.is_empty()) {
                let id = Ulid::new();
                model.todos.insert(id, Todo {
                    id,
                    title: title.to_owned(),
                    completed: false,
                });
                model.new_todo_title.clear();
            }
        }
...

fn view_header(new_todo_title: &str) -> Node<Msg> {
    ...
            input_ev(...),
            keyboard_ev(Ev::KeyDown, |keyboard_event| {
                IF!(keyboard_event.key() == ENTER_KEY => Msg::CreateTodo)
            }),
...
```
Notes:
- We used const ENTER_KEY instead of static ENTER_KEY. const is generally preferable because it's more expressive (it's clear that we don't want to mutate const) and because it's inlined and therefore faster in the most cases. However it's relatively easy to make the application (*.wasm file size) too big (it may even crash in runtime) if your const is complex. static is more suitable for such cases.
- There is cloning hidden behind title: title.to_owned(). There isn't a safe and simple way how to "pour" the trimmed string slice from the original String (new_todo_title) into Todo's title. So we have to clone the trimmed string slice and then clear the original String.
- We used Ev::KeyDown because keypress event is deprecated. And .key() because:
  - keyCode, which and keyIdentifier are deprecated.
  - .code() returns different values for "classic" Enter (Enter) and Enter on the numeric keyboard (NumpadEnter).
  You can test inputs in Keyboard Event Viewer.

Msg::ClearCompleted

use std::collections::BTreeMap;
use std::mem;
...

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
        ...
        Msg::ClearCompleted => {
            model.todos.retain(|(_, todo)| not(todo.completed));
        }
...

fn view_footer(todos: &BTreeMap<Ulid, Todo>, selected_filter: Filter) -> Node<Msg> {
    ...
            button![C!["clear-completed"],
                "Clear completed",
                ev(Ev::Click, |_| Msg::ClearCompleted),
            ]
...

Msg::CheckOrUncheckAll

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
        ...
        Msg::CheckOrUncheckAll => {
            let all_checked = model.todos.values().all(|todo| todo.completed);
            for todo in model.todos.values_mut() {
                todo.completed = not(all_checked);
            }
        }
...

fn view_toggle_all(todos: &BTreeMap<Ulid, Todo>) -> Vec<Node<Msg>> {
    let all_completed = todos.values().all(|todo| todo.completed);
    ...
        input![C!["toggle-all"], 
            ...
            ev(Ev::Change, |_| Msg::CheckOrUncheckAll),
        ],
...

Note: You may be tempted to pass all_completed along the message to replace all_checked with it to eliminate one loop. Don't do it. view often contains old data and you may accidentally introduce a hard-to-debug bug.

Msg::SelectTodo(Option<Ulid>)

Item
A todo item has three possible interactions:
Clicking the checkbox marks the todo as complete by updating its completed value and toggling the class completed on its parent <li>
Double-clicking the <label> activates editing mode, by toggling the .editing class on its <li>
Hovering over the todo shows the remove button (.destroy)
Editing
When editing mode is activated it will hide the other controls and bring forward an input that contains the todo title, which should be focused (.focus()). The edit should be saved on both blur and enter, and the editing class should be removed. Make sure to .trim() the input and then check that it's not empty. If it's empty the todo should instead be destroyed. If escape is pressed during the edit, the edit state should be left and any changes be discarded.

use std::mem;
use std::convert::TryFrom;
...

const ENTER_KEY: &str = "Enter";
const ESCAPE_KEY: &str = "Escape";
...

fn update(msg: Msg, model: &mut Model, orders: &mut impl Orders<Msg>) {
    match msg {
        ...
        Msg::SelectTodo(Some(id)) => {
            if let Some(todo) = model.todos.get(&id) {
                let input_element = ElRef::new();
                
                model.selected_todo = Some(SelectedTodo {
                    id,
                    title: todo.title.clone(),
                    input_element: input_element.clone(),
                });

                let title_length = u32::try_from(todo.title.len()).expect("title length as u32");
                orders.after_next_render(move |_| {
                    let input_element = input_element.get().expect("input_element");

                    input_element
                        .focus()
                        .expect("focus input_element");
                        
                    input_element
                        .set_selection_range(title_length, title_length)
                        .expect("move cursor to the end of input_element");
                });
            }
        },
        Msg::SelectTodo(None) => {
            model.selected_todo = None;
        },
...

fn view_todo_list(todos: &BTreeMap<Ulid, Todo>, selected_todo: Option<&SelectedTodo>) -> Node<Msg> {
    ...
                    label![
                        ...
                        ev(Ev::DblClick, move |_| Msg::SelectTodo(Some(id))),
                    ],
    ...
                    input![C!["edit"], 
                        ...
                        keyboard_ev(Ev::KeyDown, |keyboard_event| {
                            IF!(keyboard_event.key() == ESCAPE_KEY => Msg::SelectTodo(None))
                        }),
...

Notes:

orders.after_next_render registers a callback that is invoked after the next view invocation. The callback receives RenderInfo - it's useful for animations but we don't need it here (see example animation).
input_element.get() returns Option<E> where E is a specific DOM element reference like web_sys::HtmlInputElement. It returns None when the element doesn't exists in the DOM or has an incompatible type => all ElRef methods are safe to use.
There are many .expect(..) calls because DOM operations are dangerous - any JS library or browser extension can modify the DOM "under our hands", browsers have bugs and don't support all features in official specs, etc. So we want to get as much information as possible when our app panics for one of these reasons. Descriptions inside expect calls help with readability.
as for casting is an anti-pattern in most cases. You should write xx::from(yy) or xx::try_from(yy) instead. E.g.
```
u32::try_from(todo.title.len()))
```
Alternatives are xx = yy.into() and xx = yy.try_into() - they are as safe as their (Try)From counterparts however they make the code LESS READABLE because you often have to guess the type.

Msg::SelectedTodoTitleChanged(String)

fn update(msg: Msg, model: &mut Model, orders: &mut impl Orders<Msg>) {
    match msg {
        ...
        Msg::SelectedTodoTitleChanged(title) => {
            if let Some(selected_todo) = &mut model.selected_todo {
                selected_todo.title = title;
            }
        },
...

fn view_todo_list(todos: &BTreeMap<Ulid, Todo>, selected_todo: Option<&SelectedTodo>) -> Node<Msg> {
    ...
                    input![C!["edit"], 
                        ...
                        input_ev(Ev::Input, Msg::SelectedTodoTitleChanged),
                    ]
...

Msg::SaveSelectedTodo

Editing
When editing mode is activated it will hide the other controls and bring forward an input that contains the todo title, which should be focused (.focus()). The edit should be saved on both blur and enter, and the editing class should be removed. Make sure to .trim() the input and then check that it's not empty. If it's empty the todo should instead be destroyed. If escape is pressed during the edit, the edit state should be left and any changes be discarded.

fn update(msg: Msg, model: &mut Model, orders: &mut impl Orders<Msg>) {
    match msg {
        ...
        Msg::SaveSelectedTodo => {
            if let Some(selected_todo) = model.selected_todo.take() {
                let title = selected_todo.title.trim();
                if title.is_empty() {
                    model.todos.remove(&selected_todo.id);
                } else {
                    if let Some(todo) = model.todos.get_mut(&selected_todo.id) {
                        todo.title = title.to_owned();
                    }
                }
            }
        }
...

fn view_todo_list(todos: &BTreeMap<Ulid, Todo>, selected_todo: Option<&SelectedTodo>) -> Node<Msg> {
    ...
                        keyboard_ev(Ev::KeyDown, |keyboard_event| {
                            Some(match keyboard_event.key().as_str() {
                                ESCAPE_KEY => Msg::SelectTodo(None),
                                ENTER_KEY => Msg::SaveSelectedTodo,
                                _ => return None
                            })
                        }),
                        ev(Ev::Blur, |_| Msg::SaveSelectedTodo),
                    ]
...

Notes:

selected_todo.take() - it's "a common trick" how to take ownership of the chosen variable. It's basically equivalent to mem::take however you can be sure that creating a default value is cheap and it's idiomatic Rust. It has two benefits in our case: It implicitly deselects the todo and we don't have to clone anything.
Some(match... - Some is used as a wrapper here so we don't have to wrap all Msgs in the match arms - e.g. Some(Msg::SelectTodo(None)).

Msg::UrlChanged(subs::UrlChanged)
- It's the last one but we'll implement it once we know routing.
We no longer need the method Model::add_mock_data.
- Delete it and remove the .add_mock_data() call from your init function, too.

That's it! We'll store todos in LocalStorage during the next chapter and the app is almost complete!

TodoMVC - Update

New todo

Item

Editing

Editing