Tasks

LittlevGL 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 LittlevGL 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 LittlevGL*/
  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

Typedefs

typedef void (*lv_task_cb_t)(struct _lv_task_t *)

Tasks execute this type type of functions.

typedef uint8_t lv_task_prio_t
typedef struct _lv_task_t lv_task_t

Descriptor of a lv_task

Enums

enum [anonymous]

Possible priorities for lv_tasks

Values:

LV_TASK_PRIO_OFF = 0
LV_TASK_PRIO_LOWEST
LV_TASK_PRIO_LOW
LV_TASK_PRIO_MID
LV_TASK_PRIO_HIGH
LV_TASK_PRIO_HIGHEST
_LV_TASK_PRIO_NUM

Functions

void lv_task_core_init(void)

Init the lv_task module

lv_task_t *lv_task_create_basic(void)

Create an “empty” task. It needs to initialzed with at least lv_task_set_cb and lv_task_set_period

Return

pointer to the craeted task

lv_task_t *lv_task_create(lv_task_cb_t task_xcb, uint32_t period, lv_task_prio_t prio, void *user_data)

Create a new lv_task

Return

pointer to the new task

Parameters
  • task_xcb: a callback which is the task itself. It will be called periodically. (the ‘x’ in the argument name indicates that its not a fully generic function because it not follows the func_name(object, callback, ...) convention)

  • period: call period in ms unit

  • prio: priority of the task (LV_TASK_PRIO_OFF means the task is stopped)

  • user_data: custom parameter

void lv_task_del(lv_task_t *task)

Delete a lv_task

Parameters
  • task: pointer to task_cb created by task

void lv_task_set_cb(lv_task_t *task, lv_task_cb_t task_cb)

Set the callback the task (the function to call periodically)

Parameters
  • task: pointer to a task

  • task_cb: the function to call periodically

void lv_task_set_prio(lv_task_t *task, lv_task_prio_t prio)

Set new priority for a lv_task

Parameters
  • task: pointer to a lv_task

  • prio: the new priority

void lv_task_set_period(lv_task_t *task, uint32_t period)

Set new period for a lv_task

Parameters
  • task: pointer to a lv_task

  • period: the new period

void lv_task_ready(lv_task_t *task)

Make a lv_task ready. It will not wait its period.

Parameters
  • task: pointer to a lv_task.

void lv_task_once(lv_task_t *task)

Delete the lv_task after one call

Parameters
  • task: pointer to a lv_task.

void lv_task_reset(lv_task_t *task)

Reset a lv_task. It will be called the previously set period milliseconds later.

Parameters
  • task: pointer to a lv_task.

void lv_task_enable(bool en)

Enable or disable the whole lv_task handling

Parameters
  • en: true: lv_task handling is running, false: lv_task handling is suspended

uint8_t lv_task_get_idle(void)

Get idle percentage

Return

the lv_task idle in percentage

struct _lv_task_t
#include <lv_task.h>

Descriptor of a lv_task

Public Members

uint32_t period

How often the task should run

uint32_t last_run

Last time the task ran

lv_task_cb_t task_cb

Task function

void *user_data

Custom user data

uint8_t prio

Task priority

uint8_t once

1: one shot task