Input Device (lv_indev)

Creating an Input Device

To create an input device on the Default Display:

/* Create and set up at least one display before you register any input devices. */
lv_indev_t * indev = lv_indev_create();        /* Create input device connected to Default Display. */
lv_indev_set_type(indev, LV_INDEV_TYPE_...);   /* Touch pad is a pointer-like device. */
lv_indev_set_read_cb(indev, my_input_read);    /* Set driver function. */

If you have multiple displays, you will need to ensure the Default Display is set to the display your input device is "connected to" before making the above calls.

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

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

Touchpad, Touch-Screen, Mouse or Any Pointer

Input devices that can click points on the display belong to the POINTER category. Here is an example of a simple input-device Read Callback function:

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

Pointer input devices (like a mouse) can have a cursor.

...
lv_indev_t * mouse_indev = lv_indev_create();
...
LV_IMAGE_DECLARE(mouse_cursor_icon);                          /* Declare the image source. */
lv_obj_t * cursor_obj = lv_image_create(lv_screen_active());  /* Create image Widget for cursor. */
lv_image_set_src(cursor_obj, &mouse_cursor_icon);             /* Set image source. */
lv_indev_set_cursor(mouse_indev, cursor_obj);                 /* Connect image to Input Device. */

Note that the cursor object should have lv_obj_remove_flag(cursor_obj, LV_OBJ_FLAG_CLICKABLE). For images, clicking is disabled by default.

Gestures

Pointer input devices can detect basic gestures. By default, most Widgets send gestures to their parents so they can be detected on the Screen Widget in the form of an LV_EVENT_GESTURE event. For example:

void my_event(lv_event_t * e)
{
    lv_obj_t * screen = lv_event_get_current_target(e);
    lv_dir_t dir = lv_indev_get_gesture_dir(lv_indev_active());
    switch(dir) {
        case LV_DIR_LEFT:
            ...
            break;
        case LV_DIR_RIGHT:
            ...
            break;
        case LV_DIR_TOP:
            ...
            break;
        case LV_DIR_BOTTOM:
            ...
            break;
    }
}

...

lv_obj_add_event_cb(screen1, my_event, LV_EVENT_GESTURE, NULL);

To prevent passing the gesture event to the parent from a Widget, use lv_obj_remove_flag(widget, LV_OBJ_FLAG_GESTURE_BUBBLE).

Note that, gestures are not triggered if a Widget is being scrolled.

If you did some action on a gesture you can call lv_indev_wait_release(lv_indev_active()) in the event handler to prevent LVGL sending further input-device-related events.

Crown Behavior

A "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 the LV_EVENT_ROTARY event 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_sensitivity element (1/256 unit), and
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. The value of the Widget will be incremented by that value or the Widget will be scrolled that amount of pixels.

Multi-touch gestures

LVGL has the ability to recognize multi-touch gestures, when a gesture is detected a LV_EVENT_GESTURE is passed to the object on which the gesture occurred. Currently, only the pinch gesture is supported more gesture types will be implemented soon.

To enable the multi-touch gesture recognition set the LV_USE_GESTURE_RECOGNITION option in the lv_conf.h file.

Touch event collection

The driver or application collects touch events until the indev read callback is called. It is the responsibility of the driver to call the gesture recognition function of the appropriate type. For example to recognise pinch gestures call lv_indev_gesture_detect_pinch.

After calling the gesture detection function, it's necessary to call the lv_indev_set_gesture_data function to set the gesture_data and gesture_type fields of the structure lv_indev_data_t

/* The recognizer keeps the state of the gesture */
static lv_indev_gesture_recognizer_t recognizer;

/* An array that stores the collected touch events */
static lv_indev_touch_data_t touches[10];

/* A counter that needs to be incremented each time a touch event is recieved */
static uint8_t touch_cnt;

static void touch_read_callback(lv_indev_t * drv, lv_indev_data_t * data)
{

     lv_indev_touch_data_t * touch;
     uint8_t i;


     touch = &touches[0];
     lv_indev_gesture_detect_pinch(recognizer, &touches[0],
                                   touch_cnt);

     touch_cnt = 0;

     /* Set the gesture information, before returning to LVGL */
     lv_indev_set_gesture_data(data, recognizer);

}

A touch event is represented by the lv_indev_touch_data_t structure, the fields being 1:1 compatible with events emitted by the libinput library

Handling touch events

Touch events are handled like any other event. First, setup a listener for the LV_EVENT_GESTURE event type by defining and setting the callback function.

The state or scale of the pinch gesture can be retrieved by calling the lv_event_get_pinch_scale and lv_indev_get_gesture_state from within the callback.

An example of such an application is available in the source tree examples/others/gestures/lv_example_gestures.c

Keypad or Keyboard

Full keyboards with all the letters or simple keypads with a few navigation buttons belong in the keypad category.

You can fully control the user interface without a touchpad or mouse by using a keypad or encoder. It works similar to the TAB key on the PC to select an element in an application or web page.

To use a keyboard or keypad:

Register a Read Callback function for your device and set its type to LV_INDEV_TYPE_KEYPAD.
Create a Widget Group (lv_group_t * g = lv_group_create()) and add Widgets to it with lv_group_add_obj(g, widget).
Assign the group to an input device: lv_indev_set_group(indev, g).
Use LV_KEY_... to navigate among the Widgets 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

A common example of an encoder is a device with a turning knob that tells the hosting device when the knob is being turned, and in which direction.

With an encoder your application can receive events from the following:

press of its button,
oong-press of its button,
turn left, and
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 Widget 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 (similar to the Keypads) the Widgets should be added to a group.

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;
}

Widget Groups

When input focus needs to be managed among a set of Widgets (e.g. to capture user input from a keypad or encoder), that set of Widgets is placed in a group which thereafter manages how input focus moves from Widget to Widget.

In each group there is exactly one object with focus which receives the pressed keys or the encoder actions. For example, if a Text Area has focus and you press some letter on a keyboard, the keys will be sent and inserted into the text area. Similarly, if a Slider has focus and you press the left or right arrows, the slider's value will be changed.

You need to associate an input device with a group. An input device can send key events to only one group but a group can receive data from more than one input device.

To create a group use lv_group_t * g = lv_group_create() and to add a Widget to the group use lv_group_add_obj(g, widget).

Once a Widget has been added to a group, you can find out what group it is in using lv_obj_get_group(widget).

To find out if a Widget in a group has focus, call lv_obj_is_focused(widget). If the Widget is not part of a group, this function will return false.

To associate a group with an input device use lv_indev_set_group(indev, g).

Keys

There are some predefined keys which have special meaning:

LV_KEY_NEXT: Move focus to next object
LV_KEY_PREV: Move focus to previous object
LV_KEY_ENTER: Triggers LV_EVENT_PRESSED, LV_EVENT_CLICKED, or LV_EVENT_LONG_PRESSED etc. events
LV_KEY_UP: Increase value or move up
LV_KEY_DOWN: Decrease value or move down
LV_KEY_RIGHT: Increase value or move to the right
LV_KEY_LEFT: Decrease value or move to the left
LV_KEY_ESC: Close or exit (e.g. close a Drop-Down List)
LV_KEY_DEL: Delete (e.g. a character on the right in a Text Area)
LV_KEY_BACKSPACE: Delete (e.g. a character on the left in a Text Area)
LV_KEY_HOME: Go to the beginning/top (e.g. in a Text Area)
LV_KEY_END: Go to the end (e.g. in a Text Area)

The most important special keys in your read_cb() function are:

You should translate some of your keys to these special keys to support navigation in a group and interact with selected Widgets.

Usually, it's enough to use only LV_KEY_LEFT and LV_KEY_RIGHT because most Widgets can be fully controlled with them.

With an encoder you should use only LV_KEY_LEFT, LV_KEY_RIGHT, and LV_KEY_ENTER.

Edit and Navigate Mode

Since a keypad has plenty of keys, it's easy to navigate between Widgets and edit them using the keypad. But encoders have a limited number of "keys" and hence it is difficult to navigate using the default options. Navigate and Edit modes are used to avoid this problem with encoders.

In Navigate mode, an encoder's LV_KEY_LEFT or LV_KEY_RIGHT is translated to LV_KEY_NEXT or LV_KEY_PREV. Therefore, the next or previous object will be selected by turning the encoder. Pressing LV_KEY_ENTER will change to Edit mode.

In Edit mode, LV_KEY_NEXT and LV_KEY_PREV is usually used to modify an object. Depending on the Widget's type, a short or long press of LV_KEY_ENTER changes back to Navigate mode. Usually, a Widget which cannot be pressed (like a Slider) leaves Edit mode upon a short click. But with Widgets where a short click has meaning (e.g. Button), a long press is required.

Default Group

Interactive widgets (such as Buttons, Checkboxes, Sliders, etc.) can be automatically added to a default group. Just create a group with lv_group_t * g = lv_group_create() and set the default group with lv_group_set_default(g)

Don't forget to assign one or more input devices to the default group with lv_indev_set_group(my_indev, g).

Styling

When a Widget receives focus either by clicking it via touchpad or by navigating to it with an encoder or keypad, it goes to the LV_STATE_FOCUSED state. Hence, focused styles will be applied to it.

If a Widget switches to edit mode it enters the LV_STATE_FOCUSED | LV_STATE_EDITED states so any style properties assigned to these states will be shown.

See Styles for more details.

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 an 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();
  }
}

Hardware Button

A Hardware Button here is an external button (switch) typically next to the screen which is assigned to specific coordinates of the screen. If a button is pressed it will simulate the pressing on the assigned coordinate, similar to a touchpad.

To assign Hardware 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:

points_array cannot be allowed to 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 Widget
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 calling 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 input devices (independently of their type). This allows generating feedback for the user, e.g. to play a sound on LV_EVENT_CLICKED.

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 an Input Device 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 a file descriptor (fd).

You can do this by:

/* 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);

Note

lv_indev_read(), lv_timer_handler() and _lv_display_refr_timer() cannot run at the same time.

Note

For devices in event-driven mode, data->continue_reading is ignored.

API