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Input device interface

Types of input devices

To register an input device an lv_indev_drv_t variable has to be initialized. Be sure to register at least one display before you register any input devices.

/*Register at least one display before you register any input devices*/
lv_disp_drv_register(&disp_drv);

static lv_indev_drv_t indev_drv;
lv_indev_drv_init(&indev_drv);      /*Basic initialization*/
indev_drv.type =...                 /*See below.*/
indev_drv.read_cb =...              /*See below.*/
/*Register the driver in LVGL and save the created input device object*/
lv_indev_t * my_indev = lv_indev_drv_register(&indev_drv);

The type member can be:

  • LV_INDEV_TYPE_POINTER touchpad or mouse
  • LV_INDEV_TYPE_KEYPAD keyboard or keypad
  • LV_INDEV_TYPE_ENCODER encoder with left/right turn and push options
  • LV_INDEV_TYPE_BUTTON external buttons virtually pressing the screen

read_cb is a function pointer which will be called periodically to report the current state of an input device.

Visit Input devices to learn more about input devices in general.

Touchpad, mouse or any pointer

Input devices that can click points on the screen belong to this category.

indev_drv.type = LV_INDEV_TYPE_POINTER;
indev_drv.read_cb = my_input_read;

...

void my_input_read(lv_indev_drv_t * drv, lv_indev_data_t*data)
{
  if(touchpad_pressed) {
    data->point.x = touchpad_x;
    data->point.y = touchpad_y;
    data->state = LV_INDEV_STATE_PRESSED;
  } else {
    data->state = LV_INDEV_STATE_RELEASED;
  }
}

To set a mouse cursor use lv_indev_set_cursor(my_indev, &img_cursor). (my_indev is the return value of lv_indev_drv_register)

Keypad or keyboard

Full keyboards with all the letters or simple keypads with a few navigation buttons belong here.

To use a keyboard/keypad:

  • Register a read_cb function with LV_INDEV_TYPE_KEYPAD type.
  • An object group has to be created: lv_group_t * g = lv_group_create() and objects have to be added to it with lv_group_add_obj(g, obj)
  • The created group has to be assigned to an input device: lv_indev_set_group(my_indev, g) (my_indev is the return value of lv_indev_drv_register)
  • Use LV_KEY_... to navigate among the objects in the group. See lv_core/lv_group.h for the available keys.
indev_drv.type = LV_INDEV_TYPE_KEYPAD;
indev_drv.read_cb = keyboard_read;

...

void keyboard_read(lv_indev_drv_t * drv, lv_indev_data_t*data){
  data->key = last_key();            /*Get the last pressed or released key*/

  if(key_pressed()) data->state = LV_INDEV_STATE_PRESSED;
  else data->state = LV_INDEV_STATE_RELEASED;
}

Encoder

With an encoder you can do the following:

  1. Press its button
  2. Long-press its button
  3. Turn left
  4. Turn right

In short, the Encoder input devices work like this:

  • By turning the encoder you can focus on the next/previous object.
  • When you press the encoder on a simple object (like a button), it will be clicked.
  • If you press the encoder on a complex object (like a list, message box, etc.) the object will go to edit mode whereby you can navigate inside the object by turning the encoder.
  • To leave edit mode, long press the button.

To use an Encoder (similarly to the Keypads) the objects should be added to groups.

indev_drv.type = LV_INDEV_TYPE_ENCODER;
indev_drv.read_cb = encoder_read;

...

void encoder_read(lv_indev_drv_t * drv, lv_indev_data_t*data){
  data->enc_diff = enc_get_new_moves();

  if(enc_pressed()) data->state = LV_INDEV_STATE_PRESSED;
  else data->state = LV_INDEV_STATE_RELEASED;
}

Using buttons with Encoder logic

In addition to standard encoder behavior, you can also utilize its logic to navigate(focus) and edit widgets using buttons. This is especially handy if you have only few buttons available, or you want to use other buttons in addition to encoder wheel.

You need to have 3 buttons available:

  • LV_KEY_ENTER will simulate press or pushing of the encoder button
  • LV_KEY_LEFT will simulate turning encoder left
  • LV_KEY_RIGHT will simulate turning encoder right
  • other keys will be passed to the focused widget

If you hold the keys it will simulate an encoder advance with period specified in indev_drv.long_press_repeat_time.

indev_drv.type = LV_INDEV_TYPE_ENCODER;
indev_drv.read_cb = encoder_with_keys_read;

...

void encoder_with_keys_read(lv_indev_drv_t * drv, lv_indev_data_t*data){
  data->key = last_key();            /*Get the last pressed or released key*/
                                     /* use LV_KEY_ENTER for encoder press */
  if(key_pressed()) data->state = LV_INDEV_STATE_PRESSED;
  else {
      data->state = LV_INDEV_STATE_RELEASED;
      /* Optionally you can also use enc_diff, if you have encoder*/
      data->enc_diff = enc_get_new_moves();
  }
}

Button

Buttons mean external "hardware" buttons next to the screen which are assigned to specific coordinates of the screen. If a button is pressed it will simulate the pressing on the assigned coordinate. (Similarly to a touchpad)

To assign buttons to coordinates use lv_indev_set_button_points(my_indev, points_array). points_array should look like const lv_point_t points_array[] = { {12,30},{60,90}, ...}

indev_drv.type = LV_INDEV_TYPE_BUTTON;
indev_drv.read_cb = button_read;

...

void button_read(lv_indev_drv_t * drv, lv_indev_data_t*data){
    static uint32_t last_btn = 0;   /*Store the last pressed button*/
    int btn_pr = my_btn_read();     /*Get the ID (0,1,2...) of the pressed button*/
    if(btn_pr >= 0) {               /*Is there a button press? (E.g. -1 indicated no button was pressed)*/
       last_btn = btn_pr;           /*Save the ID of the pressed button*/
       data->state = LV_INDEV_STATE_PRESSED;  /*Set the pressed state*/
    } else {
       data->state = LV_INDEV_STATE_RELEASED; /*Set the released state*/
    }

    data->btn = last_btn;            /*Save the last button*/
}

Other features

Parameters

The default value of the following parameters can be changed in lv_indev_drv_t:

  • scroll_limit Number of pixels to slide before actually scrolling the object.
  • scroll_throw Scroll throw (momentum) slow-down in [%]. Greater value means faster slow-down.
  • long_press_time Press time to send LV_EVENT_LONG_PRESSED (in milliseconds)
  • long_press_repeat_time Interval of sending LV_EVENT_LONG_PRESSED_REPEAT (in milliseconds)
  • read_timer pointer to the lv_timer which reads the input device. Its parameters can be changed by lv_timer_...() functions. LV_INDEV_DEF_READ_PERIOD in lv_conf.h sets the default read period.

Feedback

Besides read_cb a feedback_cb callback can be also specified in lv_indev_drv_t. feedback_cb is called when any type of event is sent by the input devices (independently of its type). This allows generating feedback for the user, e.g. to play a sound on LV_EVENT_CLICKED.

Associating with a display

Every input device is associated with a display. By default, a new input device is added to the last display created or explicitly selected (using lv_disp_set_default()). The associated display is stored and can be changed in disp field of the driver.

Buffered reading

By default, LVGL calls read_cb periodically. Because of this intermittent polling there is a chance that some user gestures are missed.

To solve this you can write an event driven driver for your input device that buffers measured data. In read_cb you can report the buffered data instead of directly reading the input device. Setting the data->continue_reading flag will tell LVGL there is more data to read and it should call read_cb again.

Further reading

API


.. doxygenfile:: lv_hal_indev.h
  :project: lvgl