` Component

Links and forms sometimes seem completely unrelated. But, in fact, they work in very similar ways.

In plain HTML, there are three ways to navigate to another page:

An <a> element that links to another page: Navigates to the URL in its href attribute with the GET HTTP method.
A <form method="GET">: Navigates to the URL in its action attribute with the GET HTTP method and the form data from its inputs encoded in the URL query string.
A <form method="POST">: Navigates to the URL in its action attribute with the POST HTTP method and the form data from its inputs encoded in the body of the request.

Since we have a client-side router, we can do client-side link navigations without reloading the page, i.e., without a full round-trip to the server and back. It makes sense that we can do client-side form navigations in the same way.

The router provides a <Form> component, which works like the HTML <form> element, but uses client-side navigations instead of full page reloads. <Form/> works with both GET and POST requests. With method="GET", it will navigate to the URL encoded in the form data. With method="POST" it will make a POST request and handle the server’s response.

<Form/> provides the basis for some components like <ActionForm/> and <MultiActionForm/> that we’ll see in later chapters. But it also enables some powerful patterns of its own.

For example, imagine that you want to create a search field that updates search results in real time as the user searches, without a page reload, but that also stores the search in the URL so a user can copy and paste it to share results with someone else.

It turns out that the patterns we’ve learned so far make this easy to implement.

async fn fetch_results() {
	// some async function to fetch our search results
}

#[component]
pub fn FormExample() -> impl IntoView {
    // reactive access to URL query strings
    let query = use_query_map();
	// search stored as ?q=
    let search = move || query().get("q").cloned().unwrap_or_default();
	// a resource driven by the search string
	let search_results = create_resource(search, fetch_results);

	view! {
		<Form method="GET" action="">
			<input type="search" name="q" value=search/>
			<input type="submit"/>
		</Form>
		<Transition fallback=move || ()>
			/* render search results */
		</Transition>
	}
}

Whenever you click Submit, the <Form/> will “navigate” to ?q={search}. But because this navigation is done on the client side, there’s no page flicker or reload. The URL query string changes, which triggers search to update. Because search is the source signal for the search_results resource, this triggers search_results to reload its resource. The <Transition/> continues displaying the current search results until the new ones have loaded. When they are complete, it switches to displaying the new result.

This is a great pattern. The data flow is extremely clear: all data flows from the URL to the resource into the UI. The current state of the application is stored in the URL, which means you can refresh the page or text the link to a friend and it will show exactly what you’re expecting. And once we introduce server rendering, this pattern will prove to be really fault-tolerant, too: because it uses a <form> element and URLs under the hood, it actually works really well without even loading your WASM on the client.

We can actually take it a step further and do something kind of clever:

view! {
	<Form method="GET" action="">
		<input type="search" name="q" value=search
			oninput="this.form.requestSubmit()"
		/>
	</Form>
}

You’ll notice that this version drops the Submit button. Instead, we add an oninput attribute to the input. Note that this is not on:input, which would listen for the input event and run some Rust code. Without the colon, oninput is the plain HTML attribute. So the string is actually a JavaScript string. this.form gives us the form the input is attached to. requestSubmit() fires the submit event on the <form>, which is caught by <Form/> just as if we had clicked a Submit button. Now the form will “navigate” on every keystroke or input to keep the URL (and therefore the search) perfectly in sync with the user’s input as they type.

Live example

Click to open CodeSandbox.

CodeSandbox Source

use leptos::*;
use leptos_router::*;

#[component]
fn App() -> impl IntoView {
    view! {
        <Router>
            <h1><code>"<Form/>"</code></h1>
            <main>
                <Routes>
                    <Route path="" view=FormExample/>
                </Routes>
            </main>
        </Router>
    }
}

#[component]
pub fn FormExample() -> impl IntoView {
    // reactive access to URL query
    let query = use_query_map();
    let name = move || query().get("name").cloned().unwrap_or_default();
    let number = move || query().get("number").cloned().unwrap_or_default();
    let select = move || query().get("select").cloned().unwrap_or_default();

    view! {
        // read out the URL query strings
        <table>
            <tr>
                <td><code>"name"</code></td>
                <td>{name}</td>
            </tr>
            <tr>
                <td><code>"number"</code></td>
                <td>{number}</td>
            </tr>
            <tr>
                <td><code>"select"</code></td>
                <td>{select}</td>
            </tr>
        </table>
        // <Form/> will navigate whenever submitted
        <h2>"Manual Submission"</h2>
        <Form method="GET" action="">
            // input names determine query string key
            <input type="text" name="name" value=name/>
            <input type="number" name="number" value=number/>
            <select name="select">
                // `selected` will set which starts as selected
                <option selected=move || select() == "A">
                    "A"
                </option>
                <option selected=move || select() == "B">
                    "B"
                </option>
                <option selected=move || select() == "C">
                    "C"
                </option>
            </select>
            // submitting should cause a client-side
            // navigation, not a full reload
            <input type="submit"/>
        </Form>
        // This <Form/> uses some JavaScript to submit
        // on every input
        <h2>"Automatic Submission"</h2>
        <Form method="GET" action="">
            <input
                type="text"
                name="name"
                value=name
                // this oninput attribute will cause the
                // form to submit on every input to the field
                oninput="this.form.requestSubmit()"
            />
            <input
                type="number"
                name="number"
                value=number
                oninput="this.form.requestSubmit()"
            />
            <select name="select"
                onchange="this.form.requestSubmit()"
            >
                <option selected=move || select() == "A">
                    "A"
                </option>
                <option selected=move || select() == "B">
                    "B"
                </option>
                <option selected=move || select() == "C">
                    "C"
                </option>
            </select>
            // submitting should cause a client-side
            // navigation, not a full reload
            <input type="submit"/>
        </Form>
    }
}

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

The <Form/> Component

The `<Form/>` Component