testbed/Textualize__textual/docs/guide/animation.md

# Animation

Ths chapter discusses how to use Textual's animation system to create visual effects such as movement, blending, and fading.


## Animating styles

Textual's animator can change an attribute from one value to another in fixed increments over a period of time. You can apply animations to [styles](styles.md) such `offset` to move widgets around the screen, and `opacity` to create fading effects.

Apps and widgets both have an [animate][textual.app.App.animate] method which will animate properties on those objects. Additionally, `styles` objects have an identical `animate` method which will animate styles.

Let's look at an example of how we can animate the opacity of a widget to make it fade out.
The following example app contains a single `Static` widget which is immediately animated to an opacity of `0.0` (making it invisible) over a duration of two seconds.

```python hl_lines="14"
--8<-- "docs/examples/guide/animator/animation01.py"
```

The animator updates the value of the `opacity` attribute on the `styles` object in small increments over two seconds. Here's what the output will look like after each half a second.


=== "After 0s"

    ```{.textual path="docs/examples/guide/animator/animation01_static.py"}
    ```

=== "After 0.5s"

    ```{.textual path="docs/examples/guide/animator/animation01.py" press="wait:500"}
    ```


=== "After 1s"

    ```{.textual path="docs/examples/guide/animator/animation01.py" press="wait:1000"}
    ```

=== "After 1.5s"

    ```{.textual path="docs/examples/guide/animator/animation01.py" press="wait:1500"}
    ```

=== "After 2s"

    ```{.textual path="docs/examples/guide/animator/animation01.py" press="wait:2000"}
    ```

## Duration and Speed

When requesting an animation you can specify a *duration* or *speed*.
The duration is how long the animation should take in seconds. The speed is how many units a value should change in one second.
For instance, if you animate a value at 0 to 10 with a speed of 2, it will complete in 5 seconds.

## Easing functions

The easing function determines the journey a value takes on its way to the target value.
It could move at a constant pace, or it might start off slow then accelerate towards its final value.
Textual supports a number of [easing functions](https://easings.net/).

<div class="excalidraw">
--8<-- "docs/images/animation/animation.excalidraw.svg"
</div>


Run the following from the command prompt to preview them.

```bash
textual easing
```

You can specify which easing method to use via the `easing` parameter on the `animate` method. The default easing method is `"in_out_cubic"` which accelerates and then decelerates to produce a pleasing organic motion.

!!! note

    The `textual easing` preview requires the `dev` extras to be installed (using `pip install textual[dev]`).


## Completion callbacks

You can pass an callable to the animator via the `on_complete` parameter. Textual will run the callable when the animation has completed. 

## Delaying animations

You can delay the start of an animation with the `delay` parameter of the `animate` method.
This parameter accepts a `float` value representing the number of seconds to delay the animation by.
For example, `self.box.styles.animate("opacity", value=0.0, duration=2.0, delay=5.0)` delays the start of the animation by five seconds,
meaning the animation will start after 5 seconds and complete 2 seconds after that.