Components and Props

So far, we’ve been building our whole application in a single component. This is fine for really tiny examples, but in any real application you’ll need to break the user interface out into multiple components, so you can break your interface down into smaller, reusable, composable chunks.

Let’s take our progress bar example. Imagine that you want two progress bars instead of one: one that advances one tick per click, one that advances two ticks per click.

You could do this by just creating two <progress> elements:

let (count, set_count) = create_signal(0);
let double_count = move || count() * 2;

view! {
    <progress
        max="50"
        value=count
    />
    <progress
        max="50"
        value=double_count
    />
}

But of course, this doesn’t scale very well. If you want to add a third progress bar, you need to add this code another time. And if you want to edit anything about it, you need to edit it in triplicate.

Instead, let’s create a <ProgressBar/> component.

#[component]
fn ProgressBar() -> impl IntoView {
    view! {
        <progress
            max="50"
            // hmm... where will we get this from?
            value=progress
        />
    }
}

There’s just one problem: progress is not defined. Where should it come from? When we were defining everything manually, we just used the local variable names. Now we need some way to pass an argument into the component.

Component Props

We do this using component properties, or “props.” If you’ve used another frontend framework, this is probably a familiar idea. Basically, properties are to components as attributes are to HTML elements: they let you pass additional information into the component.

In Leptos, you define props by giving additional arguments to the component function.

#[component]
fn ProgressBar(
    progress: ReadSignal<i32>
) -> impl IntoView {
    view! {
        <progress
            max="50"
            // now this works
            value=progress
        />
    }
}

Now we can use our component in the main <App/> component’s view.

#[component]
fn App() -> impl IntoView {
    let (count, set_count) = create_signal(0);
    view! {
        <button on:click=move |_| { set_count.update(|n| *n += 1); }>
            "Click me"
        </button>
        // now we use our component!
        <ProgressBar progress=count/>
    }
}

Using a component in the view looks a lot like using an HTML element. You’ll notice that you can easily tell the difference between an element and a component because components always have PascalCase names. You pass the progress prop in as if it were an HTML element attribute. Simple.

Reactive and Static Props

You’ll notice that throughout this example, progress takes a reactive ReadSignal<i32>, and not a plain i32. This is very important.

Component props have no special meaning attached to them. A component is simply a function that runs once to set up the user interface. The only way to tell the interface to respond to changes is to pass it a signal type. So if you have a component property that will change over time, like our progress, it should be a signal.

optional Props

Right now the max setting is hard-coded. Let’s take that as a prop too. But let’s add a catch: let’s make this prop optional by annotating the particular argument to the component function with #[prop(optional)].

#[component]
fn ProgressBar(
    // mark this prop optional
    // you can specify it or not when you use <ProgressBar/>
    #[prop(optional)]
    max: u16,
    progress: ReadSignal<i32>
) -> impl IntoView {
    view! {
        <progress
            max=max
            value=progress
        />
    }
}

Now, we can use <ProgressBar max=50 progress=count/>, or we can omit max to use the default value (i.e., <ProgressBar progress=count/>). The default value on an optional is its Default::default() value, which for a u16 is going to be 0. In the case of a progress bar, a max value of 0 is not very useful.

So let’s give it a particular default value instead.

default props

You can specify a default value other than Default::default() pretty simply with #[prop(default = ...).

#[component]
fn ProgressBar(
    #[prop(default = 100)]
    max: u16,
    progress: ReadSignal<i32>
) -> impl IntoView {
    view! {
        <progress
            max=max
            value=progress
        />
    }
}

Generic Props

This is great. But we began with two counters, one driven by count, and one by the derived signal double_count. Let’s recreate that by using double_count as the progress prop on another <ProgressBar/>.

#[component]
fn App() -> impl IntoView {
    let (count, set_count) = create_signal(0);
    let double_count = move || count() * 2;

    view! {
        <button on:click=move |_| { set_count.update(|n| *n += 1); }>
            "Click me"
        </button>
        <ProgressBar progress=count/>
        // add a second progress bar
        <ProgressBar progress=double_count/>
    }
}

Hm... this won’t compile. It should be pretty easy to understand why: we’ve declared that the progress prop takes ReadSignal<i32>, and double_count is not ReadSignal<i32>. As rust-analyzer will tell you, its type is || -> i32, i.e., it’s a closure that returns an i32.

There are a couple ways to handle this. One would be to say: “Well, I know that a ReadSignal is a function, and I know that a closure is a function; maybe I could just take any function?” If you’re savvy, you may know that both these implement the trait Fn() -> i32. So you could use a generic component:

#[component]
fn ProgressBar<F>(
    #[prop(default = 100)]
    max: u16,
    progress: F
) -> impl IntoView
where
    F: Fn() -> i32 + 'static,
{
    view! {
        <progress
            max=max
            value=progress
        />
    }
}

This is a perfectly reasonable way to write this component: progress now takes any value that implements this Fn() trait.

This generic can also be specified inline:

#[component]
fn ProgressBar<F: Fn() -> i32 + 'static>(
    #[prop(default = 100)] max: u16,
    progress: F,
) -> impl IntoView {
    view! {
        <progress
            max=max
            value=progress
        />
    }
}

Note that generic component props can’t be specified with an impl yet (progress: impl Fn() -> i32 + 'static,), in part because they’re actually used to generate a struct ProgressBarProps, and struct fields cannot be impl types. The #[component] macro may be further improved in the future to allow inline impl generic props.

Generics need to be used somewhere in the component props. This is because props are built into a struct, so all generic types must be used somewhere in the struct. This is often easily accomplished using an optional PhantomData prop. You can then specify a generic in the view using the syntax for expressing types: <Component<T>/> (not with the turbofish-style <Component::<T>/>).

#[component]
fn SizeOf<T: Sized>(#[prop(optional)] _ty: PhantomData<T>) -> impl IntoView {
    std::mem::size_of::<T>()
}

#[component]
pub fn App() -> impl IntoView {
    view! {
        <SizeOf<usize>/>
        <SizeOf<String>/>
    }
}

Note that there are some limitations. For example, our view macro parser can’t handle nested generics like <SizeOf<Vec<T>>/>.

into Props

There’s one more way we could implement this, and it would be to use #[prop(into)]. This attribute automatically calls .into() on the values you pass as props, which allows you to easily pass props with different values.

In this case, it’s helpful to know about the Signal type. Signal is an enumerated type that represents any kind of readable reactive signal. It can be useful when defining APIs for components you’ll want to reuse while passing different sorts of signals. The MaybeSignal type is useful when you want to be able to take either a static or reactive value.

#[component]
fn ProgressBar(
    #[prop(default = 100)]
    max: u16,
    #[prop(into)]
    progress: Signal<i32>
) -> impl IntoView
{
    view! {
        <progress
            max=max
            value=progress
        />
    }
}

#[component]
fn App() -> impl IntoView {
    let (count, set_count) = create_signal(0);
    let double_count = move || count() * 2;

    view! {
        <button on:click=move |_| { set_count.update(|n| *n += 1); }>
            "Click me"
        </button>
        // .into() converts `ReadSignal` to `Signal`
        <ProgressBar progress=count/>
        // use `Signal::derive()` to wrap a derived signal
        <ProgressBar progress=Signal::derive(double_count)/>
    }
}

Optional Generic Props

Note that you can’t specify optional generic props for a component. Let’s see what would happen if you try:

#[component]
fn ProgressBar<F: Fn() -> i32 + 'static>(
    #[prop(optional)] progress: Option<F>,
) -> impl IntoView {
    progress.map(|progress| {
        view! {
            <progress
                max=100
                value=progress
            />
        }
    })
}

#[component]
pub fn App() -> impl IntoView {
    view! {
        <ProgressBar/>
    }
}

Rust helpfully gives the error

xx |         <ProgressBar/>
   |          ^^^^^^^^^^^ cannot infer type of the type parameter `F` declared on the function `ProgressBar`
   |
help: consider specifying the generic argument
   |
xx |         <ProgressBar::<F>/>
   |                     +++++

You can specify generics on components with a <ProgressBar<F>/> syntax (no turbofish in the view macro). Specifying the correct type here is not possible; closures and functions in general are unnameable types. The compiler can display them with a shorthand, but you can’t specify them.

However, you can get around this by providing a concrete type using Box<dyn _> or &dyn _:

#[component]
fn ProgressBar(
    #[prop(optional)] progress: Option<Box<dyn Fn() -> i32>>,
) -> impl IntoView {
    progress.map(|progress| {
        view! {
            <progress
                max=100
                value=progress
            />
        }
    })
}

#[component]
pub fn App() -> impl IntoView {
    view! {
        <ProgressBar/>
    }
}

Because the Rust compiler now knows the concrete type of the prop, and therefore its size in memory even in the None case, this compiles fine.

In this particular case, &dyn Fn() -> i32 will cause lifetime issues, but in other cases, it may be a possibility.

Documenting Components

This is one of the least essential but most important sections of this book. It’s not strictly necessary to document your components and their props. It may be very important, depending on the size of your team and your app. But it’s very easy, and bears immediate fruit.

To document a component and its props, you can simply add doc comments on the component function, and each one of the props:

/// Shows progress toward a goal.
#[component]
fn ProgressBar(
    /// The maximum value of the progress bar.
    #[prop(default = 100)]
    max: u16,
    /// How much progress should be displayed.
    #[prop(into)]
    progress: Signal<i32>,
) -> impl IntoView {
    /* ... */
}

That’s all you need to do. These behave like ordinary Rust doc comments, except that you can document individual component props, which can’t be done with Rust function arguments.

This will automatically generate documentation for your component, its Props type, and each of the fields used to add props. It can be a little hard to understand how powerful this is until you hover over the component name or props and see the power of the #[component] macro combined with rust-analyzer here.

Advanced Topic: #[component(transparent)]

All Leptos components return -> impl IntoView. Some, though, need to return some data directly without any additional wrapping. These can be marked with #[component(transparent)], in which case they return exactly the value they return, without the rendering system transforming them in any way.

This is mostly used in two situations:

  1. Creating wrappers around <Suspense/> or <Transition/>, which return a transparent suspense structure to integrate with SSR and hydration properly.
  2. Refactoring <Route/> definitions for leptos_router out into separate components, because <Route/> is a transparent component that returns a RouteDefinition struct rather than a view.

In general, you should not need to use transparent components unless you are creating custom wrapping components that fall into one of these two categories.

Click to open CodeSandbox.

CodeSandbox Source 
use leptos::*;

// Composing different components together is how we build
// user interfaces. Here, we'll define a reusable <ProgressBar/>.
// You'll see how doc comments can be used to document components
// and their properties.

/// Shows progress toward a goal.
#[component]
fn ProgressBar(
    // Marks this as an optional prop. It will default to the default
    // value of its type, i.e., 0.
    #[prop(default = 100)]
    /// The maximum value of the progress bar.
    max: u16,
    // Will run `.into()` on the value passed into the prop.
    #[prop(into)]
    // `Signal<T>` is a wrapper for several reactive types.
    // It can be helpful in component APIs like this, where we
    // might want to take any kind of reactive value
    /// How much progress should be displayed.
    progress: Signal<i32>,
) -> impl IntoView {
    view! {
        <progress
            max={max}
            value=progress
        />
        <br/>
    }
}

#[component]
fn App() -> impl IntoView {
    let (count, set_count) = create_signal(0);

    let double_count = move || count() * 2;

    view! {
        <button
            on:click=move |_| {
                set_count.update(|n| *n += 1);
            }
        >
            "Click me"
        </button>
        <br/>
        // If you have this open in CodeSandbox or an editor with
        // rust-analyzer support, try hovering over `ProgressBar`,
        // `max`, or `progress` to see the docs we defined above
        <ProgressBar max=50 progress=count/>
        // Let's use the default max value on this one
        // the default is 100, so it should move half as fast
        <ProgressBar progress=count/>
        // Signal::derive creates a Signal wrapper from our derived signal
        // using double_count means it should move twice as fast
        <ProgressBar max=50 progress=Signal::derive(double_count)/>
    }
}

fn main() {
    leptos::mount_to_body(App)
}