# Overview (/main-modules/data_binding/overview)



Concept [#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 [#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 <ApiLink name="lv_subject_create" />:

```c title=" " lineNumbers=1
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.

```c title=" " lineNumbers=1
lv_subject_set_int(subject_1, 20);
```

You can subscribe with an Observer callback:

```c title=" " lineNumbers=1
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.

```c title=" " lineNumbers=1
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:

```c title=" " lineNumbers=1
/*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:

```c title=" " lineNumbers=1
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](/main-modules/data_binding/subjects) and
[Observers](/main-modules/data_binding/observers).