animations.rst

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:

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 :cpplv_obj_set_x(obj, value) or :cpplv_obj_set_width(obj, value)

Create an animation

To create an animation an :cpplv_anim_t variable has to be initialized and configured with lv_anim_set_...() functions.

/* 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_duration(&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_cb(&a, lv_anim_path_ease_in);

/*Set a callback to indicate when the animation is completed.*/
lv_anim_set_completed_cb(&a, completed_cb);

/*Set a callback to indicate when the animation is deleted (idle).*/
lv_anim_set_deleted_cb(&a, deleted_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_duration(&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 :cpplv_obj_set_x and :cpplv_obj_set_y. However, only one animation can exist with a given variable and function pair and :cpplv_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:

• :cpplv_anim_path_linear: linear animation
• :cpplv_anim_path_step: change in one step at the end
• :cpplv_anim_path_ease_in: slow at the beginning
• :cpplv_anim_path_ease_out: slow at the end
• :cpplv_anim_path_ease_in_out: slow at the beginning and end
• :cpplv_anim_path_overshoot: overshoot the end value
• :cpplv_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 :cpplv_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, :cpplv_anim_speed_to_time(20, 0, 100) will yield 5000 milliseconds. For example, in the case of :cpplv_obj_set_x unit is pixels so 20 means 20 px/sec speed.

Delete animations

You can delete an animation with :cpplv_anim_delete(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 :cpplv_anim_start.

Secondly, create an animation timeline object by calling :cpplv_anim_timeline_create.

Thirdly, add animation elements to the animation timeline by calling :cpplv_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 :cpplv_anim_timeline_start(at) to start the animation timeline.

It supports forward and backward playback of the entire animation group, using :cpplv_anim_timeline_set_reverse(at, reverse). Note that if you want to play in reverse from the end of the timeline, you need to call :cpplv_anim_timeline_set_progress(at, LV_ANIM_TIMELINE_PROGRESS_MAX) after adding all animations and before starting to play.

Call :cpplv_anim_timeline_stop(at) to stop the animation timeline.

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

Call :cpplv_anim_timeline_get_playtime(at) function to get the total duration of the entire animation timeline.

Call :cpplv_anim_timeline_get_reverse(at) function to get whether to reverse the animation timeline.

Call :cpplv_anim_timeline_delete(at) function to delete the animation timeline. Note: If you need to delete an object during animation, be sure to delete the anim timeline before deleting the object. Otherwise, the program may crash or behave abnormally.

Examples

API