How to create and use Hooks in Leptos

Introduction

Leptos is a modern Rust framework for building full-stack reactive web applications.  Custom hooks in Leptos can encapsulate reusable logic, similar to React hooks in JavaScript. In this article, we’ll explore how to create and use hooks in Leptos, how to return methods from hooks, and working effectively with signals.

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Prerequisites

• Some familiarity with Leptos
• Familiar with Web concepts

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What are Leptos Hooks?

Hooks in Leptos are reusable functions that encapsulate stateful logic. They allow you to:

• Manage signals and reactive state.
• Interact with side effects (e.g., fetching data or accessing browser APIs).
• Share common logic across components.

Hooks make your code more modular, testable, and easier to maintain.

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Creating a simple hook

Here’s an example of a simple custom hook, use_counter, that manages a counter state:

use leptos::prelude::*;

#[derive(Clone)]
pub struct UseCounterReturn<F: Fn() + Clone + Send + Sync + 'static> {
    pub count: ReadSignal<i32>,
    pub increment: F,
}


pub fn use_counter() -> UseCounterReturn<impl Fn() + Clone + Send + Sync + 'static> {
    let (count, set_count) = signal(0);

    let increment = move || {
        set_count.update(|count: &mut i32| *count += 1);
    };

    UseCounterReturn {
        count,
        increment
    }
}

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Explanation

1. Signals: The create_signal function initializes a reactive signal with an initial value of 0.
2. Method: The increment closure updates the signal by incrementing its value.
3. Return Values: The hook returns the ReadSignal (read-only signal) and  increment function

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Why use impl Fn() + Clone + Send + Sync + 'static?

• Fn(): Specifies that the returned value is a closure that can be called without arguments.
• Clone: Ensures that the closure can be cloned, enabling it to be used in multiple places within the consuming component.
• Send: Marks the closure as safe to transfer between threads.
• Sync: Indicates the closure can be safely shared between threads.
• 'static: Ensures the closure doesn’t borrow any non-static references, making it more flexible to use.

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Usage in a Component

You can use this hook in a component as follows:

use leptos::prelude::*;

use crate::hooks::counter::{use_counter, UseCounterReturn};

#[component]
pub fn Counter() -> impl IntoView {
    // Use the custom hook
    let UseCounterReturn { count, increment }  = use_counter();

    view! {
        <div>
            <p>{move || count.get()}</p>
            <button on:click=move |_| increment()>"Increment"</button>
        </div>
    }
}

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Returning Multiple Closures

Hooks can return multiple closures when more functionality is needed. For example, let’s extend use_counter to include decrement and reset closures:

use leptos::prelude::*;

#[derive(Clone)]
pub struct UseCounterReturn2<
    F: Fn() + Clone + Send + Sync + 'static,
    T: Fn() + Clone + Send + Sync + 'static,
> {
    pub count: ReadSignal<i32>,
    pub increment: F,
    pub decrement: T,
}

pub fn use_counter_2() -> UseCounterReturn2<
    impl Fn() + Clone + Send + Sync + 'static,
    impl Fn() + Clone + Send + Sync + 'static,
> {
    // Create a signal for the counter value
    let (count, set_count) = signal(0);

    let increment = move || set_count.update(|count: &mut i32| *count += 1);
    let decrement = move || set_count.update(|count: &mut i32| *count -= 1);

    UseCounterReturn2 {
        count,
        increment,
        decrement,
    }
}

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Explanation

• Cloneable Closures: Each closure can be cloned and used independently in the component.
• Thread-Safety: The Send and Sync bounds make the closures thread-safe, which is especially important for concurrent operations in a reactive framework.
• Encapsulation: The UseCounterReturn struct ensures all the logic is encapsulated and reusable.

We need a separate generic parameter for each closure. Each closure expression produces a closure value with a unique anonymous type that cannot be written out. Because of monomorphization, generic parameters are limited to one type per implementation.

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Best Practices for Hooks in Leptos

1. Encapsulate logic: Keep your hooks focused on specific functionality.
2. Minimize side effects: Use hooks for state management and let the component handle rendering logic.
3. Test hooks independently: Modular hooks are easier to test and debug.

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Conclusion

Custom hooks in Leptos allow you to reuse logic and manage state effectively. By leveraging signals, you can create hooks that are powerful and efficient. Understanding the nuances of signals ensures your applications remain performant and maintainable. Start building hooks today to streamline your Leptos projects!

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