# Data binding (Subjects) (/xml/features/subjects)



Overview [#overview]

With the help of [Subjects](/main-modules/observer/observer), the interface of the UI can be created.

A subject is a global data item whose value can be set either from the application or the UI, and whose value
can be bound to widget properties.

For example, a `room1_temperature` subject's value can be set in the application when the temperature
is measured, and can be bound to a label like this:

```xml title="xml" lineNumbers=1
<lv_label bind_text="room1_temperature"/>
```

Defining subjects [#defining-subjects]

Subjects can be created in `globals.xml` like this:

```xml title="xml" lineNumbers=1
<globals>
    <subjects>
        <int name="battery_power" value="32"/>
        <string name="user_name" value="John"/>
    </subjects>
</globals>
```

As the example shows, a subject consists of a type, name, and initial value.
Currently, only integer and string types are supported.

Simple binding [#simple-binding]

Some widgets (e.g., label, slider) support binding the subject's value directly to the widget.
These bindings use attributes that start with `bind_*` and reference a subject.

```xml title="xml" lineNumbers=1
<lv_slider bind_value="some_subject"/>
<lv_label bind_text="some_subject"/>
```

Once a binding is created, if the subject's value changes (e.g., by adjusting the slider),
all bound widgets will be updated automatically.

Complex binding [#complex-binding]

In more complex cases—when a binding requires multiple parameters—the binding can be added as a child element of a widget.
This allows binding multiple subjects with different parameters. For example:

```xml title="xml" lineNumbers=1
<lv_label text="Hello world">
    <lv_obj-bind_flag_if_eq subject="subject1" flag="hidden" ref_value="10"/>
    <lv_obj-bind_flag_if_gt subject="subject1" flag="clickable" ref_value="20"/>
</lv_label>
```

Explanation of complex bindings:

* `<lv_obj-bind_flag_if_eq>` — Set a flag if the subject's value **equals** the reference value.
* `<lv_obj-bind_flag_if_not_eq>` — Set a flag if the subject's value **does not equal** the reference value.
* `<lv_obj-bind_flag_if_gt>` — Set a flag if the subject's value is **greater than** the reference value.
* `<lv_obj-bind_flag_if_ge>` — Set a flag if the subject's value is **greater than or equal to** the reference value.
* `<lv_obj-bind_flag_if_lt>` — Set a flag if the subject's value is **less than** the reference value.
* `<lv_obj-bind_flag_if_le>` — Set a flag if the subject's value is **less than or equal to** the reference value.
* `<lv_obj-bind_state_if_eq>` — Set a state if the subject's value **equals** the reference value.
* `<lv_obj-bind_state_if_not_eq>` — Set a state if the subject's value **does not equal** the reference value.
* `<lv_obj-bind_state_if_gt>` — Set a state if the subject's value is **greater than** the reference value.
* `<lv_obj-bind_state_if_ge>` — Set a state if the subject's value is **greater than or equal to** the reference value.
* `<lv_obj-bind_state_if_lt>` — Set a state if the subject's value is **less than** the reference value.
* `<lv_obj-bind_state_if_le>` — Set a state if the subject's value is **less than or equal to** the reference value.

Note: The `lv_obj-` prefix can be omitted. For example, you can simply write `<bind_state_if_gt>` instead.

Subject Related Events [#subject-related-events]

Besides binding properties to subjects, it's also possible to add events the change the value
of a subject on pressed, release, etc.

Learn more about these in [Set Subject Value](/xml/ui_elements/events) and
[Increment Subject Value](/xml/ui_elements/events).