TodoMVC - Project Setup

There are two reasons why we are creating the project now, when the Model and Msg are prepared:

It forces us to think and design before we start to write code. It leads in the most cases to better architecture.
update and especially view is better to write and test "live" because it's fun and fast feedback loop reveals design and implementation issues quickly.

How to setup TodoMVC project

I assume you've already tried to setup a new project for the counter example. If not, open seed-quickstart repo and read its README.md.

Please, create and start a new Seed app if you want to follow the steps below.

Remove unnecessary comments from your lib.rs.

lib.rs without comments

#![allow(clippy::wildcard_imports)]

use seed::{prelude::*, *};

// ------ ------
//     Init
// ------ ------

fn init(_: Url, _: &mut impl Orders<Msg>) -> Model {
    Model::default()
}

// ------ ------
//     Model
// ------ ------

type Model = i32;

// ------ ------
//    Update
// ------ ------

#[derive(Copy, Clone)]
enum Msg {
    Increment,
}

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
        Msg::Increment => *model += 1,
    }
}

// ------ ------
//     View
// ------ ------

#[allow(clippy::trivially_copy_pass_by_ref)]
fn view(model: &Model) -> Node<Msg> {
    div![
        "This is a counter: ",
        C!["counter"],
        button![model, ev(Ev::Click, |_| Msg::Increment),],
    ]
}

// ------ ------
//     Start
// ------ ------

#[wasm_bindgen(start)]
pub fn start() {
    App::start("app", init, update, view);
}

Add our designed Model and Msg. (We'll resolve compilation errors in next steps.)

Updated Model and Msg

// ------ ------
//     Model
// ------ ------

struct Model {
    todos: BTreeMap<Ulid, Todo>,
    new_todo_title: String,
    selected_todo: Option<SelectedTodo>,
    filter: Filter,
    base_url: Url,
}

struct Todo {
    id: Ulid,
    title: String,
    completed: bool,
}

struct SelectedTodo {
    id: Ulid,
    title: String,
    input_element: ElRef<web_sys::HtmlInputElement>,
}

enum Filter {
  All,
  Active,
  Completed,
}

// ------ ------
//    Update
// ------ ------

#[derive(Copy, Clone)]
enum Msg {
    UrlChanged(subs::UrlChanged),
    NewTodoTitleChanged(String),

    // ------ Basic Todo operations ------

    CreateTodo,
    ToggleTodo(Ulid),
    RemoveTodo(Ulid),
    
    // ------ Bulk operations ------

    CheckOrUncheckAll,
    ClearCompleted,
    
    // ------ Selection ------

    SelectTodo(Option<Ulid>),
    SelectedTodoTitleChanged(String),
    SaveSelectedTodo,
}

Add dependency ulid into Cargo.toml.
```
[dependencies]
ulid = "0.3.3"
...
```

Import BTreeMap and Ulid into lib.rs.

use seed::{prelude::*, *};
use std::collections::BTreeMap;
use ulid::Ulid;

Remove the line
```
#[derive(Copy, Clone)]
```
from lib.rs, because subs::UrlChanged and String don't implement Copy and standalone Clone for Msg is an anti-pattern.
Remove allow attribute and div! content from view. You can write simple alternative content (e.g. "I'm a placeholder") to check in your browser that everything works once we fix all compilation errors.
```
// ------ ------
//     View
// ------ ------

fn view(model: &Model) -> Node<Msg> {
    div![
        "I'm a placeholder"
    ]
}
```

Write update skeleton with all match arms. Each arm contains log! with Msg data - it'll help us to write and test view.

update skeleton

fn update(msg: Msg, model: &mut Model, _: &mut impl Orders<Msg>) {
    match msg {
        Msg::UrlChanged(subs::UrlChanged(url)) => {
            log!("UrlChanged", url);
        }
        Msg::NewTodoTitleChanged(title) => {
            log!("NewTodoTitleChanged", title);
        }
    
        // ------ Basic Todo operations ------

        Msg::CreateTodo => {
            log!("CreateTodo");
        }
        Msg::ToggleTodo(id) => {
            log!("ToggleTodo");
        }
        Msg::RemoveTodo(id) => {
            log!("RemoveTodo");
        }
        
        // ------ Bulk operations ------

        Msg::CheckOrUncheckAll => {
            log!("CheckOrUncheckAll");
        }
        Msg::ClearCompleted => {
            log!("ClearCompleted");
        }
        
        // ------ Selection ------

        Msg::SelectTodo(opt_id) => {
            log!("SelectTodo", opt_id);
        },
        Msg::SelectedTodoTitleChanged(title) => {
            log!("SelectedTodoTitleChanged", title);
        },
        Msg::SaveSelectedTodo => {
            log!("SaveSelectedTodo");
        }
    }
}

Let's create the most simple Model instance in our init.

fn init(_: Url, _: &mut impl Orders<Msg>) -> Model {
    Model {
        todos: BTreeMap::new(),
        new_todo_title: String::new(),
        selected_todo: None,
        filter: Filter::All,
        base_url: Url::new(),
    }
}

Temporarily disable compiler linters dead_code and unused_variables. They are very useful in later stages of the project however they cause a sea of warnings in the terminal now and it's annoying when you want to read compilation errors.
```
#![allow(clippy::wildcard_imports)]
// TODO: Remove
#![allow(dead_code, unused_variables)]
```

Implement Model method add_mock_data. It'll allow us to test view function until our update function is complete.

Updated init and add_mock_data

fn init(_: Url, _: &mut impl Orders<Msg>) -> Model {
    Model {
        ...
    }.add_mock_data()
}

...

struct Model {
    ...
}

// TODO: Remove
impl Model {
    fn add_mock_data(mut self) -> Self {
        let (id_a, id_b) = (Ulid::new(), Ulid::new());
        
        self.todos.insert(id_a, Todo {
            id: id_a,
            title: "I'm todo A".to_owned(),
            completed: false,
        });

        self.todos.insert(id_b, Todo {
            id: id_b,
            title: "I'm todo B".to_owned(),
            completed: true,
        });

        self.new_todo_title = "I'm a new todo title".to_owned();

        self.selected_todo = Some(SelectedTodo {
            id: id_b,
            title: "I'm better todo B".to_owned(),
            input_element: ElRef::new(),
        });
        self
    }
}

Make sure there aren't any errors or warnings in your terminal and you see something like I'm a placeholder in your browser.
- If you don't see anything, make sure your dev server is running (consult quickstart's README.md) and that you didn't forget to refresh you browser tab.

Note: There are .to_owned() calls instead of to_string() or into() in the code above and in other chapters.

"foo".to_string() wouldn't make any sense if you don't know Rust: "Why we are casting string to string??". If you understand &str as a kind of text/string, then it makes more sense to write to_owned(). to_owned() better expresses the operation: "promoting" a string reference to the owned string. And you can accidentally introduce more expensive to_string operation when you replace a_str.to_string() with a_complex_item.to_string().
into() has similar problems like to_string but they are worse because into is even more general than to_string. For example, you have no idea what type bar has in the expression let bar = "foo".into().

Done! We are ready to write view in the next chapter!