lvgl/docs/overview/animation.md

```eval_rst
.. include:: /header.rst 
:github_url: |github_link_base|/overview/animation.md
```
# Animations

You can automatically change the value of a variable between a start and an end value using animations.
Animation will happen by periodically calling an "animator" function with the corresponding value parameter.

The *animator* functions have the following prototype:
```c
void func(void * var, lv_anim_var_t value);
```
This prototype is compatible with the majority of the property *set* functions in LVGL. For example `lv_obj_set_x(obj, value)` or `lv_obj_set_width(obj, value)`


## Create an animation
To create an animation an `lv_anim_t` variable has to be initialized and configured with `lv_anim_set_...()` functions.

```c

/* INITIALIZE AN ANIMATION
 *-----------------------*/

lv_anim_t a;
lv_anim_init(&a);

/* MANDATORY SETTINGS
 *------------------*/

/*Set the "animator" function*/
lv_anim_set_exec_cb(&a, (lv_anim_exec_xcb_t) lv_obj_set_x); 

/*Set target of the animation*/
lv_anim_set_var(&a, obj); 

/*Length of the animation [ms]*/
lv_anim_set_time(&a, duration);

/*Set start and end values. E.g. 0, 150*/
lv_anim_set_values(&a, start, end);

/* OPTIONAL SETTINGS
 *------------------*/

/*Time to wait before starting the animation [ms]*/
lv_anim_set_delay(&a, delay);

/*Set path (curve). Default is linear*/
lv_anim_set_path(&a, lv_anim_path_ease_in);

/*Set a callback to indicate when the animation is ready (idle).*/
lv_anim_set_ready_cb(&a, ready_cb);

/*Set a callback to indicate when the animation is started (after delay).*/
lv_anim_set_start_cb(&a, start_cb);

/*When ready, play the animation backward with this duration. Default is 0 (disabled) [ms]*/
lv_anim_set_playback_time(&a, time);

/*Delay before playback. Default is 0 (disabled) [ms]*/
lv_anim_set_playback_delay(&a, delay);

/*Number of repetitions. Default is 1. LV_ANIM_REPEAT_INFINITE for infinite repetition*/
lv_anim_set_repeat_count(&a, cnt);

/*Delay before repeat. Default is 0 (disabled) [ms]*/
lv_anim_set_repeat_delay(&a, delay);

/*true (default): apply the start value immediately, false: apply start value after delay when the anim. really starts. */
lv_anim_set_early_apply(&a, true/false);

/* START THE ANIMATION
 *------------------*/
lv_anim_start(&a);                             /*Start the animation*/
```


You can apply multiple different animations on the same variable at the same time.
For example, animate the x and y coordinates with `lv_obj_set_x` and `lv_obj_set_y`. However, only one animation can exist with a given variable and function pair and `lv_anim_start()` will remove any existing animations for such a pair.

## Animation path

You can control the path of an animation. The most simple case is linear, meaning the current value between *start* and *end* is changed with fixed steps.
A *path* is a function which calculates the next value to set based on the current state of the animation. Currently, there are the following built-in path functions:

- `lv_anim_path_linear` linear animation
- `lv_anim_path_step` change in one step at the end
- `lv_anim_path_ease_in` slow at the beginning
- `lv_anim_path_ease_out` slow at the end
- `lv_anim_path_ease_in_out` slow at the beginning and end
- `lv_anim_path_overshoot` overshoot the end value
- `lv_anim_path_bounce` bounce back a little from the end value (like hitting a wall)


## Speed vs time
By default, you set the animation time directly. But in some cases, setting the animation speed is more practical.

The `lv_anim_speed_to_time(speed, start, end)` function calculates the required time in milliseconds to reach the end value from a start value with the given speed.
The speed is interpreted in _unit/sec_ dimension. For example,  `lv_anim_speed_to_time(20,0,100)` will yield 5000 milliseconds. For example, in the case of `lv_obj_set_x` *unit* is pixels so *20* means *20 px/sec* speed.

## Delete animations

You can delete an animation with `lv_anim_del(var, func)` if you provide the animated variable and its animator function.

## Timeline
A timeline is a collection of multiple animations which makes it easy to create complex composite animations.

Firstly, create an animation element but don’t call `lv_anim_start()`.

Secondly, create an animation timeline object by calling `lv_anim_timeline_create()`.

Thirdly, add animation elements to the animation timeline by calling `lv_anim_timeline_add(at, start_time, &a)`. `start_time` is the start time of the animation on the timeline. Note that `start_time` will override the value of `delay`.

Finally, call `lv_anim_timeline_start(at)` to start the animation timeline.

It supports forward and backward playback of the entire animation group, using `lv_anim_timeline_set_reverse(at, reverse)`.

Call `lv_anim_timeline_stop(at)` to stop the animation timeline.

Call `lv_anim_timeline_set_progress(at, progress)` function to set the state of the object corresponding to the progress of the timeline.

Call `lv_anim_timeline_get_playtime(at)` function to get the total duration of the entire animation timeline.

Call `lv_anim_timeline_get_reverse(at)` function to get whether to reverse the animation timeline.

Call `lv_anim_timeline_del(at)` function to delete the animation timeline.

![](/misc/anim-timeline.png "timeline diagram")

## Examples

```eval_rst

.. include:: ../../examples/anim/index.rst

```
## API

```eval_rst

.. doxygenfile:: lv_anim.h
  :project: lvgl

```