Observer

Overview

The lv_observer module implements a standard Observer pattern.

It consists of:

• subjects: each containing a value

• observers: attached to subjects to be notified on value change

A typical use case looks like this:

//It's a global variable
lv_subject_t my_subject;

/*-------
 * main.c
 *-------*/

extern lv_subject_t my_subject;

void main(void)
{
    //Initialize the subject as integer with the default value of 10
    lv_subject_init_int(&my_subject, 10);

    some_module_init();
}

/*--------------
 * some_module.c
 *--------------*/

extern lv_subject_t some_subject;

//Will be called when the related subject's value changes
static void some_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    int32_t v = lv_subject_get_int(subject);
    do_something(v);
}

void some_module_init(void)
{
    //Subscribe to a subject
    lv_subject_add_observer(&some_subject, some_observer_cb, NULL);
}

/*--------------
 * some_system.c
 *--------------*/

extern lv_subject_t some_subject;

void some_event(void)
{
    //Set the subject's value to 30. It will notify `some_observer_cb`
    lv_subject_set_int(&some_subject, 30);
}

Subject

Subject initialization

Subjects have to be static or global lv_subject_t type variables.

To initialize a subject use lv_subject_init_<type>(&subject, params, init_value). The following initializations exist for types:

• Integer void lv_subject_init_int(lv_subject_t * subject, int32_t value)

• String void lv_subject_init_string(lv_subject_t * subject, char * buf, char * prev_buf, size_t size, const char * value)

• Pointer void lv_subject_init_pointer(lv_subject_t * subject, void * value)

• Color void lv_subject_init_color(lv_subject_t * subject, lv_color_t color)

• Group void lv_subject_init_group(lv_subject_t * subject, lv_subject_t * list[], uint32_t list_len)

Set subject value

The following functions can be used to set a subject's value:

• Integer void lv_subject_set_int(lv_subject_t * subject, int32_t value)

• String void lv_subject_copy_string(lv_subject_t * subject, char * buf)

• Pointer void lv_subject_set_pointer(lv_subject_t * subject, void * ptr)

• Color void lv_subject_set_color(lv_subject_t * subject, lv_color_t color)

Get subject's value

The following functions can be used to get a subject's value:

• Integer int32_t lv_subject_get_int(lv_subject_t * subject)

• String const char * lv_subject_get_string(lv_subject_t * subject)

• Pointer const void * lv_subject_get_pointer(lv_subject_t * subject)

• Color lv_color_t lv_subject_get_color(lv_subject_t * subject)

Get subject's previous value

The following functions can be used to get a subject's previous value:

• Integer int32_t lv_subject_get_previous_int(lv_subject_t * subject)

• String const char * lv_subject_get_previous_string(lv_subject_t * subject)

• Pointer const void * lv_subject_get_previous_pointer(lv_subject_t * subject)

• Color lv_color_t lv_subject_get_previous_color(lv_subject_t * subject)

Observer

Subscribe to a subject

To subscribe to a subject the following function can be used:

lv_observer_t * observer = lv_subject_add_observer(&some_subject, some_observer_cb, user_data);

Where the observer callback should look like this:

static void some_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    ...
}

It's also possible to save a target widget when subscribing to a subject. In this case when widget is deleted, it will automatically unsubscribe from the subject.

In the observer callback lv_observer_get_target(observer) can be used to get the saved widget.

lv_observer_t * observer = lv_subject_add_observer_obj(&some_subject, some_observer_cb, obj, user_data);

In more generic case any pointer can be saved a target:

lv_observer_t * observer = lv_subject_add_observer_with_target(&some_subject, some_observer_cb, some_pointer, user_data);

Unsubscribe from a subject

lv_observer_remove(observer)

To unsubscribe from a subject with all widgets you can use:

lv_subject_remove_obj(subject, obj)

Subject groups

There are cases when a subject changes and the value of some other subjects are also required by the observer. As a practical example imagine an instrument which measures either voltage or current. To display the measured value on a label 3 things are required:

1. What do we measure (current or voltage)?

2. What is the measured value?

3. What is the range or unit (mV, V, mA, A)?

When any of these 3 parameters changes the label needs to be updated, and it needs to know all 3 parameters to compose its text.

To handle this you can create an array from some existing subjects and pass this array as a parameter when you initialize a subject with group type.

You can add observers to subject groups in the regular way. The trick is that when any element of the group is notified the subject group will be notified too.

The above Voltage/Current measurement example looks like this in the practice:

lv_obj_t * label = lv_label_create(lv_screen_active());

lv_subject_t subject_mode;  //Voltage or Current
lv_subject_t subject_value; //Measured value
lv_subject_t subject_unit;  //The unit
lv_subject_t subject_all;   //It will be the subject group
lv_subject_t * subject_list[3] = {&subject_mode, &subject_value, &subject_unit};  //The elements of the group

lv_subject_init_int(&subject_mode, 0); //Let's say 0 is Voltage, 1 is Current
lv_subject_init_int(&subject_value, 0);
lv_subject_init_pointer(&subject_unit, "V");
lv_subject_init_group(&subject_all, subject_list, 3);

lv_subject_add_observer_obj(&subject_all, all_observer_cb, label, NULL);

...

static void all_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    lv_obj_t * label = lv_observer_get_target(observer);
    lv_subject_t * subject_mode = lv_subject_get_group_element(subject, 0);
    lv_subject_t * subject_value = lv_subject_get_group_element(subject, 1);
    lv_subject_t * subject_unit = lv_subject_get_group_element(subject, 2);

    int32_t mode = lv_subject_get_int(subject_mode);
    int32_t value = lv_subject_get_int(subject_value);
    const char * unit = lv_subject_get_pointer(subject_unit);

    lv_label_set_text_fmt(label, "%s: %d %s", mode ? "Current" : "Voltage", value, unit);
}

Widget binding

Base object

Set an object flag if an integer subject's value is equal to a reference value, clear the flag otherwise

observer = lv_obj_bind_flag_if_eq(obj, &subject, LV_OBJ_FLAG_*, ref_value);

Set an object flag if an integer subject's value is not equal to a reference value, clear the flag otherwise

observer = lv_obj_bind_flag_if_not_eq(obj, &subject, LV_OBJ_FLAG_*, ref_value);

Set an object state if an integer subject's value is equal to a reference value, clear the flag otherwise

observer = lv_obj_bind_state_if_eq(obj, &subject, LV_STATE_*, ref_value);

Set an object state if an integer subject's value is not equal to a reference value, clear the flag otherwise

observer = lv_obj_bind_state_if_not_eq(obj, &subject, LV_STATE_*, ref_value);

Button

Set an integer subject to 1 when a button is checked and set it 0 when unchecked.

observer = lv_button_bind_checked(obj, &subject);

Label

Bind an integer, string, or pointer (pointing to a string) subject to a label. An optional format string can be added with 1 format specifier (e.g. "%d °C") If the format string is NULL the value will be used directly. In this case on string and pointer type subjects can be used.

observer = lv_label_bind_text(obj, &subject, format_string);

Arc

Bind an integer subject to an arc's value.

observer = lv_arc_bind_value(obj, &subject);

Slider

Bind an integer subject to a slider's value

observer = lv_slider_bind_value(obj, &subject);

Roller

Bind an integer subject to a roller's value

observer = lv_roller_bind_value(obj, &subject);

Drop-down

Bind an integer subject to a drop-down's value

observer = lv_dropdown_bind_value(obj, &subject);

Example

Bind a slider's value to a label

C code  

 View on GitHub

#include "../../lv_examples.h"
#if LV_USE_OBSERVER && LV_USE_SLIDER && LV_USE_LABEL && LV_BUILD_EXAMPLES

static lv_subject_t temperature_subject;

/**
 * A slider sends a message on value change and a label display's that value
 */
void lv_example_observer_1(void)
{
    lv_subject_init_int(&temperature_subject, 28);

    /*Create a slider in the center of the display*/
    lv_obj_t * slider = lv_slider_create(lv_screen_active());
    lv_obj_center(slider);
    lv_slider_bind_value(slider, &temperature_subject);

    /*Create a label below the slider*/
    lv_obj_t * label = lv_label_create(lv_screen_active());
    lv_obj_align(label, LV_ALIGN_CENTER, 0, 30);
    lv_label_bind_text(label, &temperature_subject, "%d °C");
}

#endif

---

Handling login and its states

C code  

 View on GitHub

#include "../../lv_examples.h"
#if LV_USE_OBSERVER && LV_USE_SLIDER && LV_USE_LABEL && LV_BUILD_EXAMPLES

/*This the only interface between the UI and the application*/
static lv_subject_t engine_subject;

static void app_init(void);
static void ui_init(void);

/**
 * Simple PIN login screen to start an engine.
 * The only interface between the UI and the application is a single "subject".
 */
void lv_example_observer_2(void)
{
    lv_subject_init_int(&engine_subject, 0);
    app_init();
    ui_init();
}

/*--------------------------------------------------
 * APPLICATION
 *
 * This part contains a demo application logic.
 * It doesn't know anything about the internals of the UI
 * and uses any the `engine_subject` as an interface.
 * -------------------------------------------------*/
static void engine_state_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    LV_UNUSED(observer);

    int32_t v = lv_subject_get_int(subject);
    LV_UNUSED(v);
    /*In a real application set/clear a pin here*/
    LV_LOG_USER("Engine state: %" LV_PRId32, v);
}

static void app_init(void)
{
    lv_subject_add_observer(&engine_subject, engine_state_observer_cb, NULL);
}

/*--------------------------------------------------
 * USER INTERFACE
 *
 * This part contains only UI related code and data.
 * In a project it would a separate file and the
 * application couldn't see its internals
 * -------------------------------------------------*/

typedef enum {
    LOGGED_OUT,
    LOGGED_IN,
    AUTH_FAILED,
} auth_state_t;

static lv_subject_t auth_state_subject;

static void textarea_event_cb(lv_event_t * e)
{
    lv_obj_t * ta = lv_event_get_target(e);
    if(strcmp(lv_textarea_get_text(ta), "hello") == 0) {
        lv_subject_set_int(&auth_state_subject, LOGGED_IN);
    }
    else {
        lv_subject_set_int(&auth_state_subject, AUTH_FAILED);
    }
}

static void info_label_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    lv_obj_t * label = lv_observer_get_target(observer);
    switch(lv_subject_get_int(subject)) {
        case LOGGED_IN:
            lv_label_set_text(label, "Login successful");
            break;
        case LOGGED_OUT:
            lv_label_set_text(label, "Logged out");
            break;
        case AUTH_FAILED:
            lv_label_set_text(label, "Login failed");
            break;
    }
}

static void log_out_click_event_cb(lv_event_t * e)
{
    LV_UNUSED(e);
    lv_subject_set_int(&auth_state_subject, LOGGED_OUT);
}

static void ui_init(void)
{
    lv_subject_init_int(&auth_state_subject, LOGGED_OUT);

    /*Create a slider in the center of the display*/
    lv_obj_t * ta = lv_textarea_create(lv_screen_active());
    lv_obj_set_pos(ta, 10, 10);
    lv_obj_set_width(ta, 200);
    lv_textarea_set_one_line(ta, true);
    lv_textarea_set_password_mode(ta, true);
    lv_textarea_set_placeholder_text(ta, "The password is: hello");
    lv_obj_add_event_cb(ta, textarea_event_cb, LV_EVENT_READY, NULL);
    lv_obj_bind_state_if_eq(ta, &auth_state_subject, LV_STATE_DISABLED, LOGGED_IN);

    lv_obj_t * kb = lv_keyboard_create(lv_screen_active());
    lv_keyboard_set_textarea(kb, ta);

    lv_obj_t * btn;
    lv_obj_t * label;

    /*Create a log out button which will be active only when logged in*/
    btn = lv_button_create(lv_screen_active());
    lv_obj_set_pos(btn, 220, 10);
    lv_obj_add_event_cb(btn, log_out_click_event_cb, LV_EVENT_CLICKED, NULL);
    lv_obj_bind_state_if_not_eq(btn, &auth_state_subject, LV_STATE_DISABLED, LOGGED_IN);

    label = lv_label_create(btn);
    lv_label_set_text(label, "LOG OUT");

    /*Create a label to show info*/
    label = lv_label_create(lv_screen_active());
    lv_obj_set_pos(label, 10, 60);
    lv_subject_add_observer_obj(&auth_state_subject, info_label_observer_cb, label, NULL);

    /*Create button which will be active only when logged in*/
    btn = lv_button_create(lv_screen_active());
    lv_obj_set_pos(btn, 10, 80);
    lv_obj_add_flag(btn, LV_OBJ_FLAG_CHECKABLE);
    lv_obj_bind_state_if_not_eq(btn, &auth_state_subject, LV_STATE_DISABLED, LOGGED_IN);
    lv_button_bind_checked(btn, &engine_subject);
    label = lv_label_create(btn);
    lv_label_set_text(label, "START ENGINE");
}

#endif

---

Set time with 12/24 mode and AM/PM

C code  

 View on GitHub

#include "../../lv_examples.h"
#if LV_USE_OBSERVER && LV_USE_SLIDER && LV_USE_LABEL && LV_USE_ROLLER && LV_USE_DROPDOWN && LV_FONT_MONTSERRAT_30 && LV_BUILD_EXAMPLES

static lv_subject_t hour_subject;
static lv_subject_t minute_subject;
static lv_subject_t format_subject;
static lv_subject_t am_pm_subject;
static lv_subject_t time_subject;
static lv_subject_t * time_group_array_subject[] = {&hour_subject, &minute_subject, &format_subject, &am_pm_subject};
const char * hour12_options = "01\n02\n03\n04\n05\n06\n07\n08\n09\n10\n11\n12";
const char * hour24_options =
    "00\n01\n02\n03\n04\n05\n06\n07\n08\n09\n10\n11\n12\n13\n14\n15\n16\n17\n18\n19\n20\n21\n22\n23";
const char * minute_options =
    "00\n01\n02\n03\n04\n05\n06\n07\n08\n09\n10\n11\n12\n13\n14\n15\n16\n17\n18\n19\n20\n21\n22\n23\n24\n25\n26\n27\n28\n29\n30\n31\n32\n33\n34\n35\n36\n37\n38\n39\n40\n41\n42\n43\n44\n45\n46\n47\n48\n49\n50\n51\n52\n53\n54\n55\n56\n57\n58\n59";

static void set_btn_clicked_event_cb(lv_event_t * e);
static void close_clicked_event_cb(lv_event_t * e);
static void hour_roller_options_update(lv_observer_t * observer, lv_subject_t * subject);
static void time_observer_cb(lv_observer_t * observer, lv_subject_t * subject);

typedef enum {
    TIME_FORMAT_12,
    TIME_FORMAT_24,
} time_format_t;

typedef enum {
    TIME_AM,
    TIME_PM,
} time_am_pm_t;

/**
 * Show how to handle a complex time setting with hour, minute, 12/24 hour mode, and AM/PM switch
 * In a real application the time can be displayed on multiple screens and it's not trivial
 * how and where to store the current values and how to get them.
 * In this example the widgets to set the time are create/deleted dynamically,
 * yet they always know what the current values are by using subjects.
 */
void lv_example_observer_3(void)
{
    /*Initialize the subjects.
     *The UI will update these and read the current values from here,
     *however the application can update these values at any time and
     *the widgets will be updated automatically. */
    lv_subject_init_int(&hour_subject, 7);
    lv_subject_init_int(&minute_subject, 45);
    lv_subject_init_int(&format_subject, TIME_FORMAT_12);
    lv_subject_init_int(&am_pm_subject, TIME_AM);
    lv_subject_init_group(&time_subject, time_group_array_subject, 4);

    /*Create the UI*/
    lv_obj_t * time_label = lv_label_create(lv_screen_active());
    lv_obj_set_style_text_font(time_label, &lv_font_montserrat_30, 0);
    lv_subject_add_observer_obj(&time_subject, time_observer_cb, time_label, NULL);
    lv_obj_set_pos(time_label, 24, 24);

    lv_obj_t * set_btn = lv_button_create(lv_screen_active());
    lv_obj_set_pos(set_btn, 180, 24);
    lv_obj_add_event_cb(set_btn, set_btn_clicked_event_cb, LV_EVENT_CLICKED, NULL);

    lv_obj_t * set_label = lv_label_create(set_btn);
    lv_label_set_text(set_label, "Set");

    /*Update some subjects to see if the UI is updated as well*/
    lv_subject_set_int(&hour_subject, 9);
    lv_subject_set_int(&minute_subject, 30);
    lv_subject_set_int(&am_pm_subject, TIME_PM);
}

static void set_btn_clicked_event_cb(lv_event_t * e)
{
    lv_obj_t * set_btn = lv_event_get_target(e);
    lv_obj_add_state(set_btn, LV_STATE_DISABLED);

    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont, lv_pct(100), LV_SIZE_CONTENT);
    lv_obj_align(cont, LV_ALIGN_BOTTOM_MID, 0, 0);

    lv_obj_t * hour_roller = lv_roller_create(cont);
    lv_obj_add_flag(hour_roller, LV_OBJ_FLAG_FLEX_IN_NEW_TRACK);
    lv_subject_add_observer_obj(&format_subject, hour_roller_options_update, hour_roller, NULL);
    lv_roller_bind_value(hour_roller, &hour_subject);
    lv_obj_set_pos(hour_roller, 0, 0);

    lv_obj_t * min_roller = lv_roller_create(cont);
    lv_roller_set_options(min_roller, minute_options, LV_ROLLER_MODE_NORMAL);
    lv_roller_bind_value(min_roller, &minute_subject);
    lv_obj_set_pos(min_roller, 64, 0);

    lv_obj_t * format_dropdown = lv_dropdown_create(cont);
    lv_dropdown_set_options(format_dropdown, "12\n24");
    lv_dropdown_bind_value(format_dropdown, &format_subject);
    lv_obj_set_pos(format_dropdown, 128, 0);
    lv_obj_set_width(format_dropdown, 80);

    lv_obj_t * am_pm_dropdown = lv_dropdown_create(cont);
    lv_dropdown_set_options(am_pm_dropdown, "am\npm");
    lv_dropdown_bind_value(am_pm_dropdown, &am_pm_subject);
    lv_obj_bind_state_if_eq(am_pm_dropdown, &format_subject, LV_STATE_DISABLED, TIME_FORMAT_24);
    lv_obj_set_pos(am_pm_dropdown, 128, 48);
    lv_obj_set_width(am_pm_dropdown, 80);

    lv_obj_t * close_btn = lv_button_create(cont);
    lv_obj_align(close_btn, LV_ALIGN_TOP_RIGHT, 0, 0);
    /*Pass the set_btn as user_data to make it non-disabled on close*/
    lv_obj_add_event_cb(close_btn, close_clicked_event_cb, LV_EVENT_CLICKED, set_btn);

    lv_obj_t * close_label = lv_label_create(close_btn);
    lv_label_set_text(close_label, LV_SYMBOL_CLOSE);
}

static void close_clicked_event_cb(lv_event_t * e)
{
    lv_obj_t * set_btn = lv_event_get_user_data(e);
    lv_obj_t * close_btn = lv_event_get_target(e);
    lv_obj_t * cont = lv_obj_get_parent(close_btn);
    lv_obj_remove_state(set_btn, LV_STATE_DISABLED);
    lv_obj_delete(cont);
}

/*Watch all related subject to display the current time correctly*/
static void time_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    int32_t hour = lv_subject_get_int(lv_subject_get_group_element(subject, 0));
    int32_t minute = lv_subject_get_int(lv_subject_get_group_element(subject, 1));
    int32_t format = lv_subject_get_int(lv_subject_get_group_element(subject, 2));
    int32_t am_pm = lv_subject_get_int(lv_subject_get_group_element(subject, 3));

    lv_obj_t * label = lv_observer_get_target(observer);

    if(format == TIME_FORMAT_24) {
        lv_label_set_text_fmt(label, "%d:%02d", hour, minute);
    }
    else {
        lv_label_set_text_fmt(label, "%d:%02d %s", hour + 1, minute, am_pm == TIME_AM ? "am" : "pm");
    }
}

/*Change the hour options on format change*/
static void hour_roller_options_update(lv_observer_t * observer, lv_subject_t * subject)
{
    lv_obj_t * roller = lv_observer_get_target(observer);
    int32_t prev_selected = lv_roller_get_selected(roller);
    int32_t v = lv_subject_get_int(subject);
    if(v == TIME_FORMAT_12) {
        prev_selected--;
        if(prev_selected > 12) prev_selected -= 12;
        lv_roller_set_options(roller, hour12_options, LV_ROLLER_MODE_NORMAL);
    }
    else {
        prev_selected++;
        lv_roller_set_options(roller, hour24_options, LV_ROLLER_MODE_NORMAL);
    }

    lv_roller_set_selected(roller, prev_selected, LV_ANIM_OFF);
    lv_obj_send_event(roller, LV_EVENT_VALUE_CHANGED, NULL);
}

#endif

---

Custom tab view with state management

C code  

 View on GitHub

#include "../../lv_examples.h"
#if LV_USE_OBSERVER && LV_USE_SLIDER && LV_USE_LABEL && LV_USE_ROLLER && LV_USE_DROPDOWN && LV_FONT_MONTSERRAT_30 && LV_BUILD_EXAMPLES

static void cont_observer_cb(lv_observer_t * observer, lv_subject_t * subject);
static void btn_create(lv_obj_t * parent, const char * text);
static void btn_click_event_cb(lv_event_t * e);
static void btn_observer_cb(lv_observer_t * observer, lv_subject_t * subject);
static void indicator_observer_cb(lv_observer_t * observer, lv_subject_t * subject);

static lv_subject_t current_tab_subject;
static lv_subject_t slider_subject[4];
static lv_subject_t dropdown_subject[3];
static lv_subject_t roller_subject[2];

void lv_example_observer_4(void)
{
    lv_subject_init_int(&current_tab_subject, 0);
    lv_subject_init_int(&slider_subject[0], 0);
    lv_subject_init_int(&slider_subject[1], 0);
    lv_subject_init_int(&slider_subject[2], 0);
    lv_subject_init_int(&slider_subject[3], 0);
    lv_subject_init_int(&dropdown_subject[0], 0);
    lv_subject_init_int(&dropdown_subject[1], 0);
    lv_subject_init_int(&dropdown_subject[2], 0);
    lv_subject_init_int(&roller_subject[0], 0);
    lv_subject_init_int(&roller_subject[1], 0);

    lv_obj_t * main_cont = lv_obj_create(lv_screen_active());
    lv_obj_remove_style_all(main_cont);
    lv_obj_set_size(main_cont, lv_pct(100), lv_pct(100));
    lv_obj_set_style_pad_all(main_cont, 0, 0);
    lv_obj_set_flex_flow(main_cont, LV_FLEX_FLOW_COLUMN);

    lv_obj_t * cont = lv_obj_create(main_cont);
    lv_obj_remove_style_all(cont);
    lv_obj_set_flex_grow(cont, 1);
    lv_obj_set_style_pad_all(cont, 8, 0);
    lv_obj_set_width(cont, lv_pct(100));
    lv_subject_add_observer_obj(&current_tab_subject, cont_observer_cb, cont, NULL);
    lv_obj_set_scroll_dir(cont, LV_DIR_VER);

    lv_obj_t * footer = lv_obj_create(main_cont);
    lv_obj_remove_style_all(footer);
    lv_obj_set_style_pad_column(footer, 8, 0);
    lv_obj_set_style_pad_all(footer, 8, 0);
    lv_obj_set_flex_flow(footer, LV_FLEX_FLOW_ROW);
    lv_obj_set_size(footer, lv_pct(100), 60);
    lv_obj_align(footer, LV_ALIGN_BOTTOM_MID, 0, 0);

    btn_create(footer, "First");
    btn_create(footer, "Second");
    btn_create(footer, "Third");

    lv_obj_t * indicator = lv_obj_create(footer);
    lv_obj_remove_style(indicator, NULL, 0);
    lv_obj_set_style_bg_opa(indicator, LV_OPA_40, 0);
    lv_subject_add_observer_obj(&current_tab_subject, indicator_observer_cb, indicator, NULL);
    lv_obj_set_height(indicator, 10);
    lv_obj_align(indicator, LV_ALIGN_BOTTOM_LEFT, 0, 0);
    lv_obj_add_flag(indicator, LV_OBJ_FLAG_IGNORE_LAYOUT);

    /*Be sure the indicator has the correct size*/
    lv_obj_update_layout(indicator);
    lv_subject_notify(&current_tab_subject);
}

static int32_t anim_get_x_cb(lv_anim_t * a)
{
    return lv_obj_get_x_aligned(a->var);
}

static void anim_set_x_cb(void * obj, int32_t v)
{
    lv_obj_set_x(obj, v);
}

static void cont_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    int32_t prev_v = lv_subject_get_previous_int(subject);
    int32_t cur_v = lv_subject_get_int(subject);
    lv_obj_t * cont = lv_observer_get_target(observer);

    /*Animate out the previous content*/
    lv_anim_t a;
    lv_anim_init(&a);
    lv_anim_set_duration(&a, 300);
    lv_anim_set_path_cb(&a, lv_anim_path_ease_in_out);
    lv_anim_set_exec_cb(&a, anim_set_x_cb);
    lv_anim_set_get_value_cb(&a, anim_get_x_cb);
    lv_anim_set_completed_cb(&a, lv_obj_delete_anim_completed_cb);

    uint32_t i;
    uint32_t delay = 0;
    uint32_t child_cnt_prev = lv_obj_get_child_count(cont);
    for(i = 0; i < child_cnt_prev; i++) {
        lv_obj_t * child = lv_obj_get_child(cont, i);
        lv_anim_set_var(&a, child);
        if(prev_v < cur_v) {
            lv_anim_set_values(&a, 0, -20);
        }
        else {
            lv_anim_set_values(&a, 0, 20);
        }
        lv_anim_set_delay(&a, delay);
        lv_anim_start(&a);
        lv_obj_fade_out(child, 200, delay);
        delay += 50;
    }

    /*Create the widgets according to the current value*/
    if(cur_v == 0) {
        for(i = 0; i < 4; i++) {
            lv_obj_t * slider = lv_slider_create(cont);
            lv_slider_bind_value(slider, &slider_subject[i]);
            lv_obj_align(slider, LV_ALIGN_TOP_MID, 0, 10 + i * 30);
        }
    }
    if(cur_v == 1) {
        for(i = 0; i < 3; i++) {
            lv_obj_t * dropdown = lv_dropdown_create(cont);
            lv_dropdown_bind_value(dropdown, &dropdown_subject[i]);
            lv_obj_align(dropdown, LV_ALIGN_TOP_MID, 0, i * 50);
        }
    }
    if(cur_v == 2) {
        for(i = 0; i < 2; i++) {
            lv_obj_t * roller = lv_roller_create(cont);
            lv_roller_bind_value(roller, &roller_subject[i]);
            lv_obj_align(roller, LV_ALIGN_CENTER, - 80 + i * 160, 0);
        }
    }

    /*Animate in the new widgets*/
    lv_anim_set_completed_cb(&a, NULL);
    for(i = child_cnt_prev; i < lv_obj_get_child_count(cont); i++) {
        lv_obj_t * child = lv_obj_get_child(cont, i);
        lv_anim_set_var(&a, child);
        if(prev_v < cur_v) {
            lv_anim_set_values(&a, 20, 0);
        }
        else {
            lv_anim_set_values(&a, -20, -0);
        }
        lv_anim_set_delay(&a, delay);
        lv_anim_start(&a);
        lv_obj_fade_in(child, 200, delay);
        delay += 50;
    }

}

static void btn_create(lv_obj_t * parent, const char * text)
{
    lv_obj_t * btn = lv_button_create(parent);
    lv_obj_set_flex_grow(btn, 1);
    lv_obj_set_height(btn, lv_pct(100));
    lv_obj_set_style_radius(btn, 0, 0);
    lv_subject_add_observer_obj(&current_tab_subject, btn_observer_cb, btn, NULL);
    lv_obj_add_event_cb(btn, btn_click_event_cb, LV_EVENT_CLICKED, NULL);

    lv_obj_t * label = lv_label_create(btn);
    lv_label_set_text(label, text);
    lv_obj_center(label);
}

static void btn_click_event_cb(lv_event_t * e)
{
    lv_obj_t * btn = lv_event_get_target(e);
    uint32_t idx = lv_obj_get_index(btn);
    lv_subject_set_int(&current_tab_subject, idx);
}

static void btn_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    int32_t prev_v = lv_subject_get_previous_int(subject);
    int32_t cur_v = lv_subject_get_int(subject);

    lv_obj_t * btn = lv_observer_get_target(observer);
    int32_t idx = (int32_t)lv_obj_get_index(btn);

    if(idx == prev_v) lv_obj_remove_state(btn, LV_STATE_CHECKED);
    if(idx == cur_v) lv_obj_add_state(btn, LV_STATE_CHECKED);
}

static void indicator_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    int32_t cur_v = lv_subject_get_int(subject);
    lv_obj_t * indicator = lv_observer_get_target(observer);

    lv_obj_t * footer = lv_obj_get_parent(indicator);
    lv_obj_t * btn_act = lv_obj_get_child(footer, cur_v);
    lv_obj_set_width(indicator, lv_obj_get_width(btn_act));

    lv_anim_t a;
    lv_anim_init(&a);
    lv_anim_set_exec_cb(&a, anim_set_x_cb);
    lv_anim_set_duration(&a, 300);
    lv_anim_set_path_cb(&a, lv_anim_path_ease_in_out);
    lv_anim_set_var(&a, indicator);
    lv_anim_set_values(&a, lv_obj_get_x(indicator), lv_obj_get_x(btn_act));
    lv_anim_start(&a);

}

#endif

---

Firmware update process

C code  

 View on GitHub

#include "../../lv_examples.h"
#if LV_USE_OBSERVER && LV_USE_ARC && LV_USE_LABEL && LV_USE_BUTTON && LV_USE_SPINNER && LV_BUILD_EXAMPLES

typedef enum {
    FW_UPDATE_STATE_IDLE,
    FW_UPDATE_STATE_CONNECTING,
    FW_UPDATE_STATE_CONNECTED,
    FW_UPDATE_STATE_DOWNLOADING,
    FW_UPDATE_STATE_CANCEL,
    FW_UPDATE_STATE_READY,
} fw_update_state_t;

static void fw_upload_manager_observer_cb(lv_observer_t * observer, lv_subject_t * subject);
static void fw_update_btn_clicked_event_cb(lv_event_t * e);
static void fw_update_close_event_cb(lv_event_t * e);
static void fw_update_win_observer_cb(lv_observer_t * observer, lv_subject_t * subject);

static lv_subject_t fw_download_percent_subject;
static lv_subject_t fw_update_status_subject;

/**
 * Show how to handle a complete firmware update process with observers.
 * Normally it's hard to implement a firmware update process because in some cases
 *   - the App needs to was for the UI (wait for a button press)
 *   - the UI needs to wait for the App (connecting or downloading)
 * With observers these complex mechanisms can be implemented a simple and clean way.
 */
void lv_example_observer_5(void)
{
    lv_subject_init_int(&fw_download_percent_subject, 0);
    lv_subject_init_int(&fw_update_status_subject, FW_UPDATE_STATE_IDLE);

    lv_subject_add_observer(&fw_update_status_subject, fw_upload_manager_observer_cb, NULL);

    /*Create start FW update button*/
    lv_obj_t * btn = lv_btn_create(lv_screen_active());
    lv_obj_add_event_cb(btn, fw_update_btn_clicked_event_cb, LV_EVENT_CLICKED, NULL);
    lv_obj_center(btn);
    lv_obj_t * label = lv_label_create(btn);
    lv_label_set_text(label, "Firmware update");
}

static void fw_update_btn_clicked_event_cb(lv_event_t * e)
{
    LV_UNUSED(e);
    lv_obj_t * win = lv_win_create(lv_screen_active());
    lv_obj_set_size(win, lv_pct(90), lv_pct(90));
    lv_obj_set_height(lv_win_get_header(win), 40);
    lv_obj_set_style_radius(win, 8, 0);
    lv_obj_set_style_shadow_width(win, 24, 0);
    lv_obj_set_style_shadow_offset_x(win, 2, 0);
    lv_obj_set_style_shadow_offset_y(win, 3, 0);
    lv_obj_set_style_shadow_color(win, lv_color_hex3(0x888), 0);
    lv_win_add_title(win, "Firmware update");
    lv_obj_t * btn = lv_win_add_button(win, LV_SYMBOL_CLOSE, 40);
    lv_obj_add_event_cb(btn, fw_update_close_event_cb, LV_EVENT_CLICKED, NULL);
    lv_obj_center(win);

    lv_subject_set_int(&fw_update_status_subject, FW_UPDATE_STATE_IDLE);
    lv_subject_add_observer_obj(&fw_update_status_subject, fw_update_win_observer_cb, win, NULL);
}

static void fw_update_close_event_cb(lv_event_t * e)
{
    LV_UNUSED(e);
    lv_subject_set_int(&fw_update_status_subject, FW_UPDATE_STATE_CANCEL);
}

static void restart_btn_click_event_cb(lv_event_t * e)
{
    lv_obj_t * win = lv_event_get_user_data(e);
    lv_obj_delete(win);
    lv_subject_set_int(&fw_update_status_subject, FW_UPDATE_STATE_IDLE);
}

static void fw_update_win_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    lv_obj_t * win = lv_observer_get_target(observer);
    lv_obj_t * cont = lv_win_get_content(win);
    fw_update_state_t status = lv_subject_get_int(&fw_update_status_subject);
    if(status == FW_UPDATE_STATE_IDLE) {
        lv_obj_clean(cont);
        lv_obj_t * spinner = lv_spinner_create(cont);
        lv_obj_center(spinner);
        lv_obj_set_size(spinner, 130, 130);

        lv_obj_t * label = lv_label_create(cont);
        lv_label_set_text(label, "Connecting");
        lv_obj_center(label);

        lv_subject_set_int(subject, FW_UPDATE_STATE_CONNECTING);
    }
    else if(status == FW_UPDATE_STATE_DOWNLOADING) {
        lv_obj_clean(cont);
        lv_obj_t * arc = lv_arc_create(cont);
        lv_arc_bind_value(arc, &fw_download_percent_subject);
        lv_obj_center(arc);
        lv_obj_set_size(arc, 130, 130);
        lv_obj_remove_flag(arc, LV_OBJ_FLAG_CLICKABLE);

        lv_obj_t * label = lv_label_create(cont);
        lv_label_bind_text(label, &fw_download_percent_subject, "%d %%");
        lv_obj_center(label);
    }
    else if(status == FW_UPDATE_STATE_READY) {
        lv_obj_clean(cont);
        lv_obj_t * label = lv_label_create(cont);
        lv_label_set_text(label, "Firmware update is ready");
        lv_obj_align(label, LV_ALIGN_CENTER, 0, -20);

        lv_obj_t * btn = lv_button_create(cont);
        lv_obj_align(btn, LV_ALIGN_CENTER, 0, 20);
        lv_obj_add_event_cb(btn, restart_btn_click_event_cb, LV_EVENT_CLICKED, win);

        label = lv_label_create(btn);
        lv_label_set_text(label, "Restart");
    }
    else if(status == FW_UPDATE_STATE_CANCEL) {
        lv_obj_delete(win);
    }
}

static void connect_timer_cb(lv_timer_t * t)
{
    if(lv_subject_get_int(&fw_update_status_subject) != FW_UPDATE_STATE_CANCEL) {
        lv_subject_set_int(&fw_update_status_subject, FW_UPDATE_STATE_CONNECTED);
    }
    lv_timer_delete(t);
}

static void download_timer_cb(lv_timer_t * t)
{
    if(lv_subject_get_int(&fw_update_status_subject) == FW_UPDATE_STATE_CANCEL) {
        lv_timer_delete(t);
        return;
    }

    int32_t v = lv_subject_get_int(&fw_download_percent_subject);
    if(v < 100) {
        lv_subject_set_int(&fw_download_percent_subject, v + 1);
    }
    else {
        lv_subject_set_int(&fw_update_status_subject, FW_UPDATE_STATE_READY);
        lv_timer_delete(t);
    }
}

/**
 * Emulate connection and FW dowlading by timers
 */
static void fw_upload_manager_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    LV_UNUSED(subject);
    LV_UNUSED(observer);

    fw_update_state_t state = lv_subject_get_int(&fw_update_status_subject);
    if(state == FW_UPDATE_STATE_CONNECTING) {
        lv_timer_create(connect_timer_cb, 2000, NULL);
    }
    else if(state == FW_UPDATE_STATE_CONNECTED) {
        lv_subject_set_int(&fw_download_percent_subject, 0);
        lv_subject_set_int(&fw_update_status_subject, FW_UPDATE_STATE_DOWNLOADING);
        lv_timer_create(download_timer_cb, 50, NULL);
    }
}

#endif

---

Modular style update on theme change

C code  

 View on GitHub

#include "../../lv_examples.h"
#if LV_USE_OBSERVER && LV_USE_SLIDER && LV_USE_LABEL && LV_BUILD_EXAMPLES

typedef enum {
    THEME_MODE_LIGHT,
    THEME_MODE_DARK,
} theme_mode_t;

static lv_obj_t * my_panel_create(lv_obj_t * parent);
static lv_obj_t * my_button_create(lv_obj_t * parent, const char * text, lv_event_cb_t event_cb);
static void switch_theme_event_cb(lv_event_t * e);

static lv_subject_t theme_subject;

/**
 * Change between light and dark mode
 */
void lv_example_observer_6(void)
{
    lv_subject_init_int(&theme_subject, THEME_MODE_DARK);

    lv_obj_t * panel1 = my_panel_create(lv_screen_active());
    lv_obj_set_flex_flow(panel1, LV_FLEX_FLOW_COLUMN);
    lv_obj_set_flex_align(panel1, LV_FLEX_ALIGN_START, LV_FLEX_ALIGN_CENTER, LV_FLEX_ALIGN_CENTER);
    lv_obj_set_size(panel1, lv_pct(90), lv_pct(90));
    lv_obj_center(panel1);

    my_button_create(panel1, "Button 1", switch_theme_event_cb);
    my_button_create(panel1, "Button 2", switch_theme_event_cb);
    my_button_create(panel1, "Button 3", switch_theme_event_cb);
    my_button_create(panel1, "Button 4", switch_theme_event_cb);
    my_button_create(panel1, "Button 5", switch_theme_event_cb);
    my_button_create(panel1, "Button 6", switch_theme_event_cb);
    my_button_create(panel1, "Button 7", switch_theme_event_cb);
    my_button_create(panel1, "Button 8", switch_theme_event_cb);
    my_button_create(panel1, "Button 9", switch_theme_event_cb);
    my_button_create(panel1, "Button 10", switch_theme_event_cb);
}

static void switch_theme_event_cb(lv_event_t * e)
{
    LV_UNUSED(e);
    if(lv_subject_get_int(&theme_subject) == THEME_MODE_LIGHT) lv_subject_set_int(&theme_subject, THEME_MODE_DARK);
    else lv_subject_set_int(&theme_subject, THEME_MODE_LIGHT);
}

/*-----------------------------------------
 * my_panel.c
 *
 * It would be a separate file with its own
 * local types, data, and functions
 *------------------------------------------*/

typedef struct {
    lv_style_t style_main;
    lv_style_t style_scrollbar;
} my_panel_styles_t;

static void my_panel_style_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    LV_UNUSED(subject);
    LV_UNUSED(observer);

    theme_mode_t m = lv_subject_get_int(&theme_subject);
    my_panel_styles_t * styles = lv_observer_get_target(observer);
    if(m == THEME_MODE_LIGHT) {
        lv_style_set_bg_color(&styles->style_main, lv_color_hex3(0xfff));
        lv_style_set_shadow_color(&styles->style_main, lv_color_hex3(0x888));
        lv_style_set_text_color(&styles->style_main, lv_color_hex3(0x222));
        lv_style_set_bg_color(&styles->style_scrollbar, lv_color_hex3(0x888));
    }
    if(m == THEME_MODE_DARK) {
        lv_style_set_bg_color(&styles->style_main, lv_color_hex(0x040038));
        lv_style_set_shadow_color(&styles->style_main, lv_color_hex3(0xaaa));
        lv_style_set_text_color(&styles->style_main, lv_color_hex3(0xeee));
        lv_style_set_bg_color(&styles->style_scrollbar, lv_color_hex3(0xaaa));
    }

    lv_obj_report_style_change(&styles->style_main);
    lv_obj_report_style_change(&styles->style_scrollbar);
}

static lv_obj_t * my_panel_create(lv_obj_t * parent)
{
    static bool inited = false;
    static my_panel_styles_t styles;
    if(!inited) {
        inited = true;

        lv_style_init(&styles.style_main);
        lv_style_set_radius(&styles.style_main, 12);
        lv_style_set_bg_opa(&styles.style_main, LV_OPA_COVER);
        lv_style_set_shadow_width(&styles.style_main, 24);
        lv_style_set_shadow_offset_x(&styles.style_main, 4);
        lv_style_set_shadow_offset_y(&styles.style_main, 6);
        lv_style_set_pad_all(&styles.style_main, 12);
        lv_style_set_pad_gap(&styles.style_main, 16);

        lv_style_init(&styles.style_scrollbar);
        lv_style_set_width(&styles.style_scrollbar, 4);
        lv_style_set_radius(&styles.style_scrollbar, 2);
        lv_style_set_pad_right(&styles.style_scrollbar, 8);
        lv_style_set_pad_ver(&styles.style_scrollbar, 8);
        lv_style_set_bg_opa(&styles.style_scrollbar, LV_OPA_50);

        lv_subject_add_observer_with_target(&theme_subject, my_panel_style_observer_cb, &styles, NULL);
    }

    lv_obj_t * panel = lv_obj_create(parent);
    lv_obj_remove_style_all(panel);
    lv_obj_add_style(panel, &styles.style_main, 0);
    lv_obj_add_style(panel, &styles.style_scrollbar, LV_PART_SCROLLBAR);

    return panel;
}

/*-----------------------------------------
 * my_button.c
 *
 * It would be a separate file with its own
 * local types, data, and functions
 *------------------------------------------*/

typedef struct {
    lv_style_t style_main;
    lv_style_t style_pressed;
} my_button_styles_t;

static void my_button_style_observer_cb(lv_observer_t * observer, lv_subject_t * subject)
{
    LV_UNUSED(subject);
    LV_UNUSED(observer);

    theme_mode_t m = lv_subject_get_int(&theme_subject);
    my_button_styles_t * styles = lv_observer_get_target(observer);
    if(m == THEME_MODE_LIGHT) {
        lv_style_set_bg_color(&styles->style_main, lv_color_hex(0x3379de));
        lv_style_set_bg_grad_color(&styles->style_main, lv_color_hex(0xd249a5));
        lv_style_set_shadow_color(&styles->style_main, lv_color_hex(0x3379de));
        lv_style_set_text_color(&styles->style_main, lv_color_hex3(0xfff));

        lv_style_set_color_filter_opa(&styles->style_pressed, LV_OPA_70);
    }
    if(m == THEME_MODE_DARK) {
        lv_style_set_bg_color(&styles->style_main, lv_color_hex(0xde1382));
        lv_style_set_bg_grad_color(&styles->style_main, lv_color_hex(0x4b0c72));
        lv_style_set_shadow_color(&styles->style_main, lv_color_hex(0x4b0c72));
        lv_style_set_text_color(&styles->style_main, lv_color_hex3(0xfff));

        lv_style_set_color_filter_opa(&styles->style_pressed, LV_OPA_30);
    }

    lv_obj_report_style_change(&styles->style_main);
    lv_obj_report_style_change(&styles->style_pressed);
}

static lv_obj_t * my_button_create(lv_obj_t * parent, const char * text, lv_event_cb_t event_cb)
{
    static bool inited = false;
    static my_button_styles_t styles;
    if(!inited) {
        inited = true;

        lv_style_init(&styles.style_main);
        lv_style_set_radius(&styles.style_main, LV_RADIUS_CIRCLE);
        lv_style_set_bg_opa(&styles.style_main, LV_OPA_COVER);
        lv_style_set_bg_grad_dir(&styles.style_main, LV_GRAD_DIR_HOR);
        lv_style_set_shadow_width(&styles.style_main, 24);
        lv_style_set_shadow_offset_y(&styles.style_main, 6);
        lv_style_set_pad_hor(&styles.style_main, 32);
        lv_style_set_pad_ver(&styles.style_main, 12);

        lv_style_init(&styles.style_pressed);
        lv_style_set_color_filter_dsc(&styles.style_pressed, &lv_color_filter_shade);
        lv_subject_add_observer_with_target(&theme_subject, my_button_style_observer_cb, &styles, NULL);
    }

    lv_obj_t * btn = lv_btn_create(parent);
    lv_obj_remove_style_all(btn);
    lv_obj_add_style(btn, &styles.style_main, 0);
    lv_obj_add_style(btn, &styles.style_pressed, LV_STATE_PRESSED);
    lv_obj_add_event_cb(btn, event_cb, LV_EVENT_CLICKED, NULL);

    lv_obj_t * label = lv_label_create(btn);
    lv_label_set_text(label, text);

    return btn;
}

#endif

---

API

lv_observer.h