lvgl/docs/overview/task.md

.. include:: /header.rst 
:github_url: |github_link_base|/overview/task.md

Tasks

LVGL has a built-in task system. You can register a function to have it be called periodically. The tasks are handled and called in lv_task_handler(), which needs to be called periodically every few milliseconds. See Porting for more information.

The tasks are non-preemptive, which means a task cannot interrupt another task. Therefore, you can call any LVGL related function in a task.

Create a task

To create a new task, use lv_task_create(task_cb, period_ms, LV_TASK_PRIO_OFF/LOWEST/LOW/MID/HIGH/HIGHEST, user_data). It will create an lv_task_t * variable, which can be used later to modify the parameters of the task. lv_task_create_basic() can also be used. It allows you to create a new task without specifying any parameters.

A task callback should have void (*lv_task_cb_t)(lv_task_t *); prototype.

For example:

void my_task(lv_task_t * task)
{
  /*Use the user_data*/
  uint32_t * user_data = task->user_data;
  printf("my_task called with user data: %d\n", *user_data);

  /*Do something with LVGL*/
  if(something_happened) {
    something_happened = false;
    lv_btn_create(lv_scr_act(), NULL);
  }
}

...

static uint32_t user_data = 10;
lv_task_t * task = lv_task_create(my_task, 500, LV_TASK_PRIO_MID, &user_data);

Ready and Reset

lv_task_ready(task) makes the task run on the next call of lv_task_handler().

lv_task_reset(task) resets the period of a task. It will be called again after the defined period of milliseconds has elapsed.

Set parameters

You can modify some parameters of the tasks later:

• lv_task_set_cb(task, new_cb)
• lv_task_set_period(task, new_period)
• lv_task_set_prio(task, new_priority)

One-shot tasks

You can make a task to run only once by callinglv_task_once(task). The task will automatically be deleted after being called for the first time.

Measure idle time

You can get the idle percentage time lv_task_handler with lv_task_get_idle(). Note that, it doesn't measure the idle time of the overall system, only lv_task_handler. It can be misleading if you use an operating system and call lv_task_handler in an task, as it won't actually measure the time the OS spends in an idle thread.

Asynchronous calls

In some cases, you can't do an action immediately. For example, you can't delete an object right now because something else is still using it or you don't want to block the execution now. For these cases, you can use the lv_async_call(my_function, data_p) to make my_function be called on the next call of lv_task_handler. data_p will be passed to function when it's called. Note that, only the pointer of the data is saved so you need to ensure that the variable will be "alive" while the function is called. You can use static, global or dynamically allocated data.

For example:

void my_screen_clean_up(void * scr)
{
  /*Free some resources related to `scr`*/

  /*Finally delete the screen*/
  lv_obj_del(scr);  
}

...

/*Do somethings with the object on the current screen*/

/*Delete screen on next call of `lv_task_handler`. So not now.*/
lv_async_call(my_screen_clean_up, lv_scr_act());

/*The screen is still valid so you can do other things with it*/

If you just want to delete an object, and don't need to clean anything up in my_screen_cleanup, you could just use lv_obj_del_async, which will delete the object on the next call to lv_task_handler.

API

.. doxygenfile:: lv_task.h
  :project: lvgl

.. doxygenfile:: lv_async.h
  :project: lvgl