Overview

Get familiar with the data binding concept of LVGL.

Concept

The lv_observer module is an implementation of the Subject-Observer pattern used to realize data binding in LVGL.

A Subject is a value being "observed", usually a global variable. It can store an integer, string, color, or other basic type.

An Observer is attached (subscribed) to a Subject and gets notified whenever the Subject changes.

This pattern separates the data from the places that consume it. Imagine a Subject that stores the temperature of a room: the sensor sets the value and doesn't need to know who uses it. Maybe a Label Widget is subscribed to update its text, maybe a popup is shown when the temperature is too high, maybe an I/O pin is set to 1 to start heating when it drops too low. Each subscriber only has to know that the Subject exists; it can subscribe to it and unsubscribe when it is deleted or no longer interested.

Quick Reference

This section shows the basics briefly. Read the other pages in this section to learn all the details.

The following Subject types are supported:

• int
• float (requires LV_USE_FLOAT)
• string
• color
• pointer
• group

You can create a subject with lv_subject_create:

lv_subject_t * subject_2 = lv_subject_create(LV_SUBJECT_TYPE_INT);

Set a Subject's value like below. This also notifies all subscribed Observers when the value changes.

lv_subject_set_int(subject_1, 20);

You can subscribe with an Observer callback:

void my_observer(lv_observer_t * observer, lv_subject_t * subject)
{
    printf("New value: %d\n", lv_subject_get_int(subject));
}

lv_subject_add_observer(subject_1, my_observer, NULL);   /*3rd arg is optional user data*/

When you subscribe, the callback is called immediately to pick up the current value.

You can also subscribe together with a Widget. This way the Observer is removed automatically when the Widget is deleted. This is convenient, because you don't need to check whether the Widget still exists. While it exists it gets updated; once deleted it unsubscribes itself.

lv_subject_add_observer_obj(subject_1, my_observer, label_1, NULL);

Some Widgets also offer an API to easily create one- or two-way bindings. For example:

/*Update the label when the Subject changes, using a format string*/
lv_label_bind_text(label_1, subject_1, "%d °C");

/*Keep the Subject and the selected drop-down option in sync (two-way)*/
lv_dropdown_bind_value(dropdown_1, subject_2);

When a Subject is no longer needed it can be deleted:

lv_subject_delete(subject_2);

Subjects that are still alive when lv_deinit() is called are deleted automatically.

That's it in short. Continue reading to learn more about Subjects and Observers.