Kobold

Easy declarative web interfaces.

Key features:

• Declarative view! macro that uses HTML-esque syntax complete with optional closing tags.
• Functional components with optional parameters.
• State management and event handling.
• High performance and consistently the lowest Wasm footprint in the Rust ecosystem.

Zero-Cost Static HTML

The view! macro produces opaque impl View types that by default do no allocations. All static DOM elements compile to inline JavaScript code that constructs them. Expressions are injected into the constructed DOM on first render. Kobold keeps track of the DOM node references for these expressions.

Since the exact types the expressions evaluate to are known to the Rust compiler, update calls can diff them by value (or pointer) and surgically update the DOM should they change. Changing a string or an integer only updates the exact Text node that string or integer was rendered to.

If the view! macro invocation contains DOM elements with no expressions, the constructed View type will be zero-sized, and its View::update method will be empty, making updates of static DOM literally zero-cost.

Hello World!

Components in Kobold are created by annotating a render function with a #[component] attribute.

use kobold::prelude::*;

#[component]
fn hello(name: &str) -> impl View + '_ {
    view! {
        <h1>"Hello "{ name }"!"</h1>
    }
}

fn main() {
    kobold::start(view! {
        <!hello name="Kobold">
    });
}

The component function must return a type that implements the View trait. Since the view! macro produces transient locally defined types the best approach here is to always use the opaque impl View return type.

Everything here is statically typed and the macro doesn't delete any information when manipulating the token stream, so the Rust compiler can tell you when you've made a mistake:

error[E0560]: struct `Hello` has no field named `nam`
  --> examples/hello_world/src/main.rs:12:16
   |
12 |         <!hello nam="Kobold">
   |                 ^^^ help: there is a method with a similar name: `name`

You can even use rust-analyzer to refactor component or field names, and it will change the invocations inside the macros for you.

State management

The stateful function can be used to create views that own and manipulate their state:

use kobold::prelude::*;

#[component]
fn counter(init: u32) -> impl View {
    stateful(init, |count| {
        bind! { count:
            // Create an event handler with access to `&mut u32`
            let onclick = move |_event| *count += 1;
        }

        view! {
            <p>
                "You clicked the "
                // `{onclick}` here is shorthand for `onclick={onclick}`
                <button {onclick}>"Button"</button>
                " "{ count }" times."
            </p>
        }
    })
}

fn main() {
    kobold::start(view! {
        <!counter init={0}>
    });
}

The stateful function takes two parameters:

• State constructor that implements the IntoState trait. Kobold comes with default implementations for most primitive types, so we can use u32 here.
• The anonymous render closure that uses the constructed state, in our case its argument is &Hook<u32>.

The Hook here is a smart pointer to the state itself that allows non-mutable access to the state. The bind! macro can be invoked for any Hook to create closures with &mut references to the underlying state.

For more details visit the stateful module documentation.

Optional parameters

Use #[component(<param>?)] syntax to set a component parameter as default:

// `code` will default to `200` if omitted
#[component(code?: 200)]
fn status(code: u32) -> impl View {
    view! {
        <p> "Status code was "{ code }
    }
}

view! {
    // Status code was 200
    <!status>
    // Status code was 404
    <!status code={404}>
}

For more details visit the #[component] macro documentation.

Conditional Rendering

Because the view! macro produces unique transient types, if and match expressions that invoke the macro will naturally fail to compile.

Using the auto_branch flag on the #[component] attribute Kobold will scan the body of of your component render function, and make all view! macro invocations inside an if or match expression, and wrap them in an enum making them the same type:

#[component(auto_branch)]
fn conditional(illuminatus: bool) -> impl View {
    if illuminatus {
        view! { <p> "It was the year when they finally immanentized the Eschaton." }
    } else {
        view! { <blockquote> "It was love at first sight." }
    }
}

For more details visit the branching module documentation.

Lists and Iterators

To render an iterator use the for keyword:

use kobold::prelude::*;

#[component]
fn iterate_numbers(count: u32) -> impl View {
    view! {
        <ul>
        {
            for (1..=count).map(|n| view! { <li> "Item #"{n} })
        }
    }
}

On updates the iterator is consumed once and all items are diffed with the previous version. No allocations are made by Kobold when updating such a list, unless the rendered list needs to grow past its original capacity.

For more information about keywords visit the keywords module documentation.

Borrowed Values

View types are truly transient and only need to live for the duration of the initial render, or for the duration of the subsequent update. This means that you can easily and cheaply render borrowed state without unnecessary clones:

#[component]
fn users<'a>(names: &'a [&'a str]) -> impl View + 'a {
    view! {
        <ul>
        {
            for names.iter().map(|name| view! { <li> { name } })
        }
    }
}

Components with Children

If you wish to capture children from parent view! invocation, simply change #[component] to #[component(children)]:

use kobold::prelude::*;

#[component(children)]
fn header(children: impl View) -> impl View {
    view! {
        <header><h1>{ children }</h1></header>
    }
}

fn main() {
    kobold::start(view! {
        <!header>"Hello Kobold"</!header>
    });
}

You can change the name of the parameter used and even set it to a concrete:

use kobold::prelude::*;

// Capture children into the argument `n`
#[component(children: n)]
fn add_ten(n: i32) -> i32 {
    // integers implement `View` so they can be passed by value
    n + 10
}

fn main() {
    kobold::start(view! {
        <p>
            "Meaning of life is "
            <!add_ten>{ 32 }</!add_ten>
        </p>
    });
}

More Examples

To run Kobold you'll need to install trunk:

cargo install --locked trunk

You might also need to add the Wasm target to Rust:

rustup target add wasm32-unknown-unknown

Then just run an example:

## Go to an example
cd examples/todomvc

## Run with trunk
trunk serve

Acknowledgements

• Pedrors for the Kobold logo.

License

Kobold is free software, and is released under the terms of the Mozilla Public License version 2.0. See LICENSE.