Input device interface

Types of input devices

To create an input device use

/*Register at least one display before you register any input devices*/
lv_indev_t * indev = lv_indev_create();
lv_indev_set_type(indev, LV_INDEV_TYPE_...);   /*See below.*/
lv_indev_set_read_cb(indev, read_cb);  /*See below.*/

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.

lv_indev_set_type(indev, LV_INDEV_TYPE_POINTER);
...

void my_input_read(lv_indev_t * indev, 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;
  }
}

Mouse cursor

To set a mouse cursor use lv_indev_set_cursor(indev, &img_cursor).

Crown behavior

The "Crown" is a rotary device typically found on smart watches.

When the user clicks somewhere and after that turns the rotary the last clicked widget will be either scrolled or it's value will be incremented/decremented (e.g. in case of a slider).

As this behavior is tightly related to the last clicked widget, the crown support is an extension of the pointer input device. Just set data->diff to the number of turned steps and LVGL will automatically send LV_EVENT_ROTARY or scroll the widget based on the editable flag in the widget's class. Non-editable widgets are scrolled and for editable widgets the event is sent.

To get the steps in an event callback use int32_t diff = lv_event_get_rotary_diff(e)()

The rotary sensitivity can be adjusted on 2 levels:

In the input device by the indev->rotary_sensitvity element (1/256 unit)
By the rotary_sensitivity style property in the widget (1/256 unit)

The final diff is calculated like this:

``diff_final = diff_in * (indev_sensitivity / 256) + (widget_sensitivity / 256); ``

For example, if both the indev and widget sensitivity is set to 128 (0.5), the input diff. will be multiplied by 0.25 (divided by 4). The value of the widget will be incremented by this value or the widget will be scrolled this amount of pixels.

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 and use 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(indev, g)
Use LV_KEY_... to navigate among the objects in the group. See lv_core/lv_group.h for the available keys.

lv_indev_set_type(indev, LV_INDEV_TYPE_KEYPAD);

...

void keyboard_read(lv_indev_t * indev, 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:

Press its button
Long-press its button
Turn left
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.

lv_indev_set_type(indev, LV_INDEV_TYPE_ENCODER);

...

void encoder_read(lv_indev_t * indev, 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;
}

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.

lv_indev_set_type(indev, LV_INDEV_TYPE_ENCODER);

...

void encoder_with_keys_read(lv_indev_t * indev, 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}, ...}

important:: The points_array can't go out of scope. Either declare it as a global variable or as a static variable inside a function.`

lv_indev_set_type(indev, LV_INDEV_TYPE_BUTTON);

...

void button_read(lv_indev_t * indev, 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_id = last_btn;         /*Save the last button*/
}

When the button_read callback in the example above changes the data->btn_id to 0 a press/release action at the first index of the points_array will be performed ({12,30}).

Other features

Parameters

The default value of the following parameters can be changed in lv_indev_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_DEF_REFR_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_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_display_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.

Switching the input device to event-driven mode

Normally the input event is read every LV_DEF_REFR_PERIOD milliseconds (set in lv_conf.h). However, in some cases, you might need more control over when to read the input device. For example, you might need to read it by polling file descriptor (fd).

You can do this in the following way:

/*Update the input device's running mode to LV_INDEV_MODE_EVENT*/
lv_indev_set_mode(indev, LV_INDEV_MODE_EVENT);

...

/*Call this anywhere you want to read the input device*/
lv_indev_read(indev);