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                        <h1 id="event-handlers"><a class="header" href="#event-handlers">Event Handlers</a></h1>
<p>Event handlers are used to respond to user actions. For example, an event handler could be triggered when the user clicks, scrolls, moves the mouse, or types a character.</p>
<p>Event handlers are attached to elements. For example, we usually don't care about all the clicks that happen within an app, only those on a particular button.</p>
<p>Event handlers are similar to regular attributes, but their name usually starts with <code>on</code>- and they accept closures as values. The closure will be called whenever the event it listens for is triggered and will be passed that event.</p>
<p>For example, to handle clicks on an element, we can specify an <code>onclick</code> handler:</p>
<pre><pre class="playground"><code class="language-rust edition2018">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>cx.render(rsx! {
    button {
        onclick: move |event| println!(&quot;Clicked! Event: {event:?}&quot;),
        &quot;click me!&quot;
    }
})
<span class="boring">}
</span></code></pre></pre>
<h2 id="the-event-object"><a class="header" href="#the-event-object">The <code>Event</code> object</a></h2>
<p>Event handlers receive an <a href="https://docs.rs/dioxus-core/latest/dioxus_core/struct.Event.html"><code>Event</code></a> object containing information about the event. Different types of events contain different types of data. For example, mouse-related events contain <a href="https://docs.rs/dioxus/latest/dioxus/events/struct.MouseData.html"><code>MouseData</code></a>, which tells you things like where the mouse was clicked and what mouse buttons were used.</p>
<p>In the example above, this event data was logged to the terminal:</p>
<pre><code>Clicked! Event: UiEvent { bubble_state: Cell { value: true }, data: MouseData { coordinates: Coordinates { screen: (242.0, 256.0), client: (26.0, 17.0), element: (16.0, 7.0), page: (26.0, 17.0) }, modifiers: (empty), held_buttons: EnumSet(), trigger_button: Some(Primary) } }
Clicked! Event: UiEvent { bubble_state: Cell { value: true }, data: MouseData { coordinates: Coordinates { screen: (242.0, 256.0), client: (26.0, 17.0), element: (16.0, 7.0), page: (26.0, 17.0) }, modifiers: (empty), held_buttons: EnumSet(), trigger_button: Some(Primary) } }
</code></pre>
<p>To learn what the different event types for HTML provide, read the <a href="https://docs.rs/dioxus-html/latest/dioxus_html/events/index.html">events module docs</a>.</p>
<h3 id="event-propagation"><a class="header" href="#event-propagation">Event propagation</a></h3>
<p>Some events will trigger first on the element the event originated at upward. For example, a click event on a <code>button</code> inside a <code>div</code> would first trigger the button's event listener and then the div's event listener.</p>
<blockquote>
<p>For more information about event propigation see <a href="https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Building_blocks/Events#event_bubbling">the mdn docs on event bubling</a></p>
</blockquote>
<p>If you want to prevent this behavior, you can call <code>stop_propagation()</code> on the event:</p>
<pre><pre class="playground"><code class="language-rust edition2018">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>cx.render(rsx! {
    div {
        onclick: move |_event| {},
        &quot;outer&quot;,
        button {
            onclick: move |event| {
                // now, outer won't be triggered
                event.stop_propagation();
            },
            &quot;inner&quot;
        }
    }
})
<span class="boring">}
</span></code></pre></pre>
<h2 id="prevent-default"><a class="header" href="#prevent-default">Prevent Default</a></h2>
<p>Some events have a default behavior. For keyboard events, this might be entering the typed character. For mouse events, this might be selecting some text.</p>
<p>In some instances, might want to avoid this default behavior. For this, you can add the <code>prevent_default</code> attribute with the name of the handler whose default behavior you want to stop. This attribute can be used for multiple handlers using their name separated by spaces:</p>
<pre><pre class="playground"><code class="language-rust edition2018">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>cx.render(rsx! {
    input {
        prevent_default: &quot;oninput onclick&quot;,
    }
})
<span class="boring">}
</span></code></pre></pre>
<p>Any event handlers will still be called.</p>
<blockquote>
<p>Normally, in React or JavaScript, you'd call &quot;preventDefault&quot; on the event in the callback. Dioxus does <em>not</em> currently support this behavior. Note: this means you cannot conditionally prevent default behavior based on the data in the event.</p>
</blockquote>
<h2 id="handler-props"><a class="header" href="#handler-props">Handler Props</a></h2>
<p>Sometimes, you might want to make a component that accepts an event handler. A simple example would be a <code>FancyButton</code> component, which accepts an <code>on_click</code> handler:</p>
<pre><pre class="playground"><code class="language-rust edition2018">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[derive(Props)]
pub struct FancyButtonProps&lt;'a&gt; {
    on_click: EventHandler&lt;'a, MouseEvent&gt;,
}

pub fn FancyButton&lt;'a&gt;(cx: Scope&lt;'a, FancyButtonProps&lt;'a&gt;&gt;) -&gt; Element&lt;'a&gt; {
    cx.render(rsx!(button {
        class: &quot;fancy-button&quot;,
        onclick: move |evt| cx.props.on_click.call(evt),
        &quot;click me pls.&quot;
    }))
}
<span class="boring">}
</span></code></pre></pre>
<p>Then, you can use it like any other handler:</p>
<pre><pre class="playground"><code class="language-rust edition2018">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>    cx.render(rsx! {
        FancyButton {
            on_click: move |event| println!(&quot;Clicked! {event:?}&quot;)
        }
    })
<span class="boring">}
</span></code></pre></pre>
<blockquote>
<p>Note: just like any other attribute, you can name the handlers anything you want! Though they must start with <code>on</code>, for the prop to be automatically turned into an <code>EventHandler</code> at the call site.</p>
<p>You can also put custom data in the event, rather than e.g. <code>MouseData</code></p>
</blockquote>

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