# Event Handlers

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.

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.

Event handlers are similar to regular attributes, but their name usually starts with `on`- 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.

For example, to handle clicks on an element, we can specify an `onclick` handler:

```rust
{{#include ../../../examples/event_click.rs:rsx}}
```

## The `Event` object

Event handlers receive an [`Event`](https://docs.rs/dioxus-core/latest/dioxus_core/struct.Event.html) object containing information about the event. Different types of events contain different types of data. For example, mouse-related events contain [`MouseData`](https://docs.rs/dioxus/latest/dioxus/events/struct.MouseData.html), which tells you things like where the mouse was clicked and what mouse buttons were used.

In the example above, this event data was logged to the terminal:

```
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) } }
```

To learn what the different event types for HTML provide, read the [events module docs](https://docs.rs/dioxus-html/latest/dioxus_html/events/index.html).

### Event propagation

Some events will trigger first on the element the event originated at upward. For example, a click event on a `button` inside a `div` would first trigger the button's event listener and then the div's event listener.

> For more information about event propagation see [the mdn docs on event bubbling](https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Building_blocks/Events#event_bubbling)

If you want to prevent this behavior, you can call `stop_propagation()` on the event:

```rust
{{#include ../../../examples/event_nested.rs:rsx}}
```

## Prevent Default

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.

In some instances, might want to avoid this default behavior. For this, you can add the `prevent_default` 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:

```rust
{{#include ../../../examples/event_prevent_default.rs:prevent_default}}
```

Any event handlers will still be called.

> Normally, in React or JavaScript, you'd call "preventDefault" on the event in the callback. Dioxus does *not* currently support this behavior. Note: this means you cannot conditionally prevent default behavior based on the data in the event.

## Handler Props

Sometimes, you might want to make a component that accepts an event handler. A simple example would be a `FancyButton` component, which accepts an `on_click` handler:

```rust
{{#include ../../../examples/event_handler_prop.rs:component_with_handler}}
```

Then, you can use it like any other handler:

```rust
{{#include ../../../examples/event_handler_prop.rs:usage}}
```

> Note: just like any other attribute, you can name the handlers anything you want! Though they must start with `on`, for the prop to be automatically turned into an `EventHandler` at the call site.
>
> You can also put custom data in the event, rather than e.g. `MouseData`
