# Data Binding (/syntax/data-binding)



Overview [#overview]

With the help of subjects, 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:

| Binding Type           | Condition        | Description                                                                        |
| ---------------------- | ---------------- | ---------------------------------------------------------------------------------- |
| `bind_flag_if_eq`      | Equals           | Set a flag if the subject's value equals the reference value                       |
| `bind_flag_if_not_eq`  | Not equals       | Set a flag if the subject's value does not equal the reference value               |
| `bind_flag_if_gt`      | Greater than     | Set a flag if the subject's value is greater than the reference value              |
| `bind_flag_if_ge`      | Greater or equal | Set a flag if the subject's value is greater than or equal to the reference value  |
| `bind_flag_if_lt`      | Less than        | Set a flag if the subject's value is less than the reference value                 |
| `bind_flag_if_le`      | Less or equal    | Set a flag if the subject's value is less than or equal to the reference value     |
| `bind_state_if_eq`     | Equals           | Set a state if the subject's value equals the reference value                      |
| `bind_state_if_not_eq` | Not equals       | Set a state if the subject's value does not equal the reference value              |
| `bind_state_if_gt`     | Greater than     | Set a state if the subject's value is greater than the reference value             |
| `bind_state_if_ge`     | Greater or equal | Set a state if the subject's value is greater than or equal to the reference value |
| `bind_state_if_lt`     | Less than        | Set a state if the subject's value is less than the reference value                |
| `bind_state_if_le`     | Less or equal    | 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 that change the value of a subject on pressed, release, etc.

Learn more about these in the [events documentation](./events).