Leptos: Building Web Apps with Fine-Grained Reactivity

Leptos is a full-stack, isomorphic Rust web framework leveraging fine-grained reactivity to build declarative user interfaces. Inspired by SolidJS and React, Leptos embraces a reactive programming model, offering exceptional performance and a developer-friendly experience. This article delves deep into the framework’s architecture, features, benefits, and how it empowers developers to build dynamic and efficient web applications.

I. Introduction to Fine-Grained Reactivity

At the heart of Leptos lies the concept of fine-grained reactivity. Unlike traditional coarse-grained approaches where entire components re-render on state changes, fine-grained reactivity tracks dependencies at a granular level. This means only the specific parts of the DOM affected by a state change are updated, leading to significant performance improvements, especially in complex applications.

Leptos achieves this through signals, which are reactive variables that hold application state. When a signal’s value changes, Leptos automatically re-evaluates only the computations and DOM updates that depend on that specific signal. This eliminates unnecessary re-renders and optimizes update performance.

II. Leptos Architecture: A Deep Dive

Leptos’ architecture is built around several key components:

• Signals: The foundation of reactivity. Signals encapsulate application state and notify dependent computations when their value changes. Leptos provides primitives like create_signal, create_memo, and create_resource to manage reactive state, derived state, and asynchronous operations, respectively.

• Effects: Functions that react to signal changes. Effects are executed whenever a dependency changes, allowing for side effects like DOM manipulation, network requests, or logging. The create_effect function creates these reactive side effects.

• Components: Reusable units of UI logic. Leptos components are functions that accept props (arguments) and return a reactive view. These components can be composed to build complex user interfaces.

• Templates: HTML-like syntax for defining the structure of the UI. Leptos leverages a template macro that compiles down to efficient reactive code. This macro seamlessly integrates reactive signals within the template, making it easy to dynamically update the DOM.

• Server-Side Rendering (SSR): Leptos supports SSR, allowing the initial rendering of the application on the server. This improves initial load times and SEO performance. The same codebase can be used for both server and client rendering, simplifying development.

• Hydration: After the initial SSR, Leptos hydrates the static HTML on the client, attaching event listeners and making the application interactive. This seamless transition from server to client provides a smooth user experience.

III. Building Blocks of Leptos Applications

Let’s explore the core building blocks in more detail:

A. Signals:

Signals are the fundamental units of reactivity. create_signal creates a signal with an initial value and returns a getter and a setter.

“`rust
use leptos::*;

[component]

fn Counter(cx: Scope) -> impl IntoView {
let (count, set_count) = create_signal(cx, 0);

view! { cx,
    <div>
        <p>"Count: " {count}</p>
        <button on:click=move |_| set_count.update(|c| *c += 1)>"Increment"</button>
    </div>
}

}
“`

B. Memos:

Memos represent derived state. They are created using create_memo and automatically update whenever their dependencies change.

“`rust
use leptos::*;

[component]

fn DoubleCounter(cx: Scope) -> impl IntoView {
let (count, set_count) = create_signal(cx, 0);
let doubled = create_memo(cx, move |_| count() * 2);

view! { cx,
    <div>
        <p>"Count: " {count}</p>
        <p>"Doubled: " {doubled}</p>
        <button on:click=move |_| set_count.update(|c| *c += 1)>"Increment"</button>
    </div>
}

}
“`

C. Resources:

Resources manage asynchronous operations. create_resource handles loading, error, and data states.

“`rust
use leptos::*;

[component]

async fn UserData(cx: Scope) -> impl IntoView {
let user = create_resource(cx, || (), async move |_| {
// Fetch user data
reqwest::get(“/api/user”).await.unwrap().json::().await.unwrap()
});

view! { cx,
    <div>
        {move || match user.read() {
            Some(user_data) => view! { cx, <p>"User: " {user_data.name}</p> },
            None => view! { cx, <p>"Loading..."</p> },
        }}
    </div>
}

}
“`

D. Components and Templates:

Components encapsulate UI logic and structure. Leptos uses a JSX-like syntax within the view! macro for defining templates.

“`rust
use leptos::*;

[component]

fn Greeting(cx: Scope, name: String) -> impl IntoView {
view! { cx,

“Hello, ” {name} “!”

}
}

[component]

fn App(cx: Scope) -> impl IntoView {
view! { cx,

}
}
“`

IV. Benefits of Using Leptos

• Performance: Fine-grained reactivity minimizes DOM updates, resulting in exceptional performance.

• Developer Experience: The reactive programming model simplifies state management and UI updates. The Rust compiler provides strong type safety and early error detection.

• Isomorphic Capabilities: The same codebase can be used for both client and server rendering, streamlining development and improving SEO.

• WebAssembly (Wasm) Target: Leptos can compile to Wasm, enabling efficient execution in the browser.

• Interoperability: Leptos can interoperate with existing JavaScript libraries and frameworks.

V. Building a Real-World Application with Leptos

Building a todo list application showcases Leptos’ capabilities:

“`rust
use leptos::*;

[component]

fn TodoItem(cx: Scope, todo: String, completed: bool, on_toggle: Callback<()>) -> impl IntoView {
view! { cx,

{todo}

}
}

[component]

fn TodoApp(cx: Scope) -> impl IntoView {
let (todos, set_todos) = create_signal(cx, Vec::new());
let (new_todo, set_new_todo) = create_signal(cx, String::new());

let add_todo = move |_| {
    let new_todo_value = new_todo.get();
    if !new_todo_value.is_empty() {
        set_todos.update(move |todos| todos.push((new_todo_value.clone(), false)));
        set_new_todo.set(String::new());
    }
};

view! { cx,
    <div>
        <h1>"Todo App"</h1>
        <input type="text"
            prop:value=new_todo
            on:input=move |ev| set_new_todo.set(event_target_value(&ev))
        />
        <button on:click=add_todo>"Add Todo"</button>

        <ul>
            {move || todos.get().iter().enumerate().map(|(index, (todo, completed))| {
                 let on_toggle = move |_| {
                    set_todos.update(move |todos| todos[index].1 = !todos[index].1);
                 };
                 view! { cx,  <TodoItem todo=todo.clone() completed=*completed on_toggle=on_toggle/> }
            }).collect_view(cx)}
        </ul>
    </div>
}

}

//Needed to run with trunk or cargo leptos watch

[wasm_bindgen(start)]

pub fn main() {
_ = console_log::init_with_level(log::Level::Debug);
console_error_panic_hook::set_once();

mount_to_body(|cx| {
    view! { cx,  <TodoApp/> }
});

}

“`

This example demonstrates how to manage state with signals, create components, handle user input, and dynamically update the DOM.

VI. Conclusion

Leptos empowers developers to build high-performance web applications using Rust and fine-grained reactivity. Its component-based architecture, coupled with powerful primitives like signals, memos, and resources, simplifies complex state management and UI updates. With SSR support and Wasm compilation, Leptos offers a robust and efficient solution for modern web development. As the framework matures, its growing community and active development promise a bright future for building sophisticated and performant web apps with Rust.