# lv_observer.h (/api/public/core/lv_observer_h)
Functions [#functions]
lv_subject_set_int [#lv_subject_set_int]
Set value of an integer Subject and notify Observers.
```c title=" " lineNumbers=1
void lv_subject_set_int(lv_subject_t *subject, int32_t value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `value` | `int32_t` | new value |
lv_subject_set_min_value_int [#lv_subject_set_min_value_int]
Set a minimum value for an integer subject
```c title=" " lineNumbers=1
void lv_subject_set_min_value_int(lv_subject_t *subject, int32_t min_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `min_value` | `int32_t` | the minimum value |
lv_subject_set_max_value_int [#lv_subject_set_max_value_int]
Set a maximum value for an integer subject
```c title=" " lineNumbers=1
void lv_subject_set_max_value_int(lv_subject_t *subject, int32_t max_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `max_value` | `int32_t` | the maximum value |
lv_subject_set_float [#lv_subject_set_float]
Set value of an float Subject and notify Observers.
```c title=" " lineNumbers=1
void lv_subject_set_float(lv_subject_t *subject, float value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `value` | `float` | new value |
lv_subject_set_min_value_float [#lv_subject_set_min_value_float]
Set a minimum value for a float subject
```c title=" " lineNumbers=1
void lv_subject_set_min_value_float(lv_subject_t *subject, float min_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `min_value` | `float` | the minimum value |
lv_subject_set_max_value_float [#lv_subject_set_max_value_float]
Set a maximum value for a float subject
```c title=" " lineNumbers=1
void lv_subject_set_max_value_float(lv_subject_t *subject, float max_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `max_value` | `float` | the maximum value |
lv_subject_set_string_buffer_static [#lv_subject_set_string_buffer_static]
Set the buffers where a string subject will store its value The buffers must out-live the subject
```c title=" " lineNumbers=1
void lv_subject_set_string_buffer_static(lv_subject_t *subject, char *buf, char *prev_buf, size_t size)
```
Parameters
| Name | Type | Description |
| ---------- | -------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------ |
| `subject` | | pointer to Subject |
| `buf` | `char *` | pointer to buffer to store string |
| `prev_buf` | `char *` | pointer to buffer to store previous string. **May** be `NULL`. When NULL the previous string is not tracked. |
| `size` | `size_t` | size of buffer(s) |
A string Subject stores its own copy of the string, not just the pointer.
lv_subject_set_string [#lv_subject_set_string]
Set a Subject's string value and notify Observers if it changed.
```c title=" " lineNumbers=1
void lv_subject_set_string(lv_subject_t *subject, const char *buf)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `buf` | `const char *` | new string |
The Subject stores its own copy of the string, not just the pointer.
lv_subject_set_pointer [#lv_subject_set_pointer]
Set value of a pointer Subject and notify Observers (regardless of whether it changed).
```c title=" " lineNumbers=1
void lv_subject_set_pointer(lv_subject_t *subject, void *ptr)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ---------------------------- |
| `subject` | | pointer to Subject |
| `ptr` | `void *` | new value **May** be `NULL`. |
lv_subject_set_color [#lv_subject_set_color]
Set value of a color Subject and notify Observers if it changed.
```c title=" " lineNumbers=1
void lv_subject_set_color(lv_subject_t *subject, lv_color_t color)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `color` | | new value |
lv_subject_set_group_list_static [#lv_subject_set_group_list_static]
Set the subject list of a group subject
```c title=" " lineNumbers=1
void lv_subject_set_group_list_static(lv_subject_t *subject, lv_subject_t *list[], uint32_t list_len)
```
Parameters
| Name | Type | Description |
| ---------- | -------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------- |
| `subject` | | pointer to group Subject |
| `list` | | list of other Subject addresses; when any of these have values updated, Observers of `group_subject` will be notified. |
| `list_len` | `uint32_t` | number of elements in `list[]` |
lv_subject_set_external_data [#lv_subject_set_external_data]
Attaches external user data to an integer Subject with lifecycle management.
Associates arbitrary user-defined data with an LVGL observer and registers a destructor callback that will be automatically invoked when the observer is deleted. This enables:
* Safe resource cleanup through the destructor mechanism
* Contextual data storage for observer callbacks
* Proper memory management for observer-related resources
```c title=" " lineNumbers=1
void lv_subject_set_external_data(lv_subject_t *subject, void *data, void(*free_cb)(void *data))
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `subject` | | pointer to Subject |
| `data` | `void *` | User-defined data pointer to associate **May** be `NULL`. |
| `free_cb` | `void(*)(void *data)` | Cleanup function called when: **May** be `NULL`.
- Observer is explicitly deleted
- Observed object is deleted
- New data replaces current association NULL indicates no cleanup required |
lv_observer_set_user_data [#lv_observer_set_user_data]
Set Observer's user data.
```c title=" " lineNumbers=1
void lv_observer_set_user_data(lv_observer_t *observer, void *user_data)
```
Parameters
| Name | Type | Description |
| ----------- | ---------------------------------------------------------- | ----------------------------------------------------------- |
| `observer` | | pointer to Observer |
| `user_data` | `void *` | pointer to user-owned data (may be NULL) **May** be `NULL`. |
lv_obj_set_subject_increment_event_min_value [#lv_obj_set_subject_increment_event_min_value]
Set the minimum subject value to set by the event
```c title=" " lineNumbers=1
void lv_obj_set_subject_increment_event_min_value(lv_obj_t *obj, lv_subject_increment_dsc_t *dsc, int32_t min_value)
```
Parameters
| Name | Type | Description |
| ----------- | ------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------ |
| `obj` | | pointer to the Widget to which the event is attached |
| `dsc` | | pointer to the descriptor returned by |
| `min_value` | `int32_t` | the minimum value to set |
lv_obj_set_subject_increment_event_max_value [#lv_obj_set_subject_increment_event_max_value]
Set the maximum subject value to set by the event
```c title=" " lineNumbers=1
void lv_obj_set_subject_increment_event_max_value(lv_obj_t *obj, lv_subject_increment_dsc_t *dsc, int32_t max_value)
```
Parameters
| Name | Type | Description |
| ----------- | ------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------ |
| `obj` | | pointer to the Widget to which the event is attached |
| `dsc` | | pointer to the descriptor returned by |
| `max_value` | `int32_t` | the maximum value to set |
lv_obj_set_subject_increment_event_rollover [#lv_obj_set_subject_increment_event_rollover]
Set what to do when the min/max value is crossed.
```c title=" " lineNumbers=1
void lv_obj_set_subject_increment_event_rollover(lv_obj_t *obj, lv_subject_increment_dsc_t *dsc, bool rollover)
```
Parameters
| Name | Type | Description |
| ---------- | ------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------ |
| `obj` | | pointer to the Widget to which the event is attached |
| `dsc` | | pointer to the descriptor returned by |
| `rollover` | `bool` | false: stop at the min/max value; true: jump to the other end |
the subject also can have min/max values and always the smaller range will be considered
lv_obj_add_subject_set_int_event [#lv_obj_add_subject_set_int_event]
Set the value of an integer subject.
```c title=" " lineNumbers=1
void lv_obj_add_subject_set_int_event(lv_obj_t *obj, lv_subject_t *subject, lv_event_code_t trigger, int32_t value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | -------------------------------------------------- |
| `obj` | | pointer to a widget |
| `subject` | | pointer to a subject to change |
| `trigger` | | the trigger on which the subject should be changed |
| `value` | `int32_t` | the value to set |
lv_obj_add_subject_set_float_event [#lv_obj_add_subject_set_float_event]
Set the value of a float subject.
```c title=" " lineNumbers=1
void lv_obj_add_subject_set_float_event(lv_obj_t *obj, lv_subject_t *subject, lv_event_code_t trigger, float value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | -------------------------------------------------- |
| `obj` | | pointer to a widget |
| `subject` | | pointer to a subject to change |
| `trigger` | | the trigger on which the subject should be changed |
| `value` | `float` | the value to set |
lv_obj_add_subject_set_string_event [#lv_obj_add_subject_set_string_event]
Set the value of a string subject.
```c title=" " lineNumbers=1
void lv_obj_add_subject_set_string_event(lv_obj_t *obj, lv_subject_t *subject, lv_event_code_t trigger, const char *value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | -------------------------------------------------- |
| `obj` | | pointer to a widget |
| `subject` | | pointer to a subject to change |
| `trigger` | | the trigger on which the subject should be changed |
| `value` | `const char *` | the value to set |
lv_subject_get_int [#lv_subject_get_int]
Get current value of an integer Subject.
```c title=" " lineNumbers=1
int32_t lv_subject_get_int(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `int32_t` — current value
lv_subject_get_previous_int [#lv_subject_get_previous_int]
Get previous value of an integer Subject.
```c title=" " lineNumbers=1
int32_t lv_subject_get_previous_int(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `int32_t` — current value
lv_subject_get_float [#lv_subject_get_float]
Get current value of an float Subject.
```c title=" " lineNumbers=1
float lv_subject_get_float(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `float` — current value
lv_subject_get_previous_float [#lv_subject_get_previous_float]
Get previous value of an float Subject.
```c title=" " lineNumbers=1
float lv_subject_get_previous_float(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `float` — current value
lv_subject_get_string [#lv_subject_get_string]
Get current value of a string Subject.
```c title=" " lineNumbers=1
const char * lv_subject_get_string(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `const char *` — pointer to buffer containing current value
lv_subject_get_previous_string [#lv_subject_get_previous_string]
Get previous value of a string Subject.
```c title=" " lineNumbers=1
const char * lv_subject_get_previous_string(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `const char *` — pointer to buffer containing previous value
lv_subject_get_pointer [#lv_subject_get_pointer]
Get current value of a pointer Subject.
```c title=" " lineNumbers=1
const void * lv_subject_get_pointer(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `const void *` — current value
lv_subject_get_previous_pointer [#lv_subject_get_previous_pointer]
Get previous value of a pointer Subject.
```c title=" " lineNumbers=1
const void * lv_subject_get_previous_pointer(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** `const void *` — previous value
lv_subject_get_color [#lv_subject_get_color]
Get current value of a color Subject.
```c title=" " lineNumbers=1
lv_color_t lv_subject_get_color(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** — current value
lv_subject_get_previous_color [#lv_subject_get_previous_color]
Get previous value of a color Subject.
```c title=" " lineNumbers=1
lv_color_t lv_subject_get_previous_color(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
**Returns:** — previous value
lv_subject_get_group_element [#lv_subject_get_group_element]
Get an element from Subject Group's list.
```c title=" " lineNumbers=1
lv_subject_t * lv_subject_get_group_element(lv_subject_t *subject, int32_t index)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ----------------------------- |
| `subject` | | pointer to Group-type Subject |
| `index` | `int32_t` | index of element to get |
**Returns:** — pointer to indexed Subject from list, or NULL if index is out of bounds
lv_observer_get_target [#lv_observer_get_target]
Get target of an Observer.
```c title=" " lineNumbers=1
void * lv_observer_get_target(lv_observer_t *observer)
```
Parameters
| Name | Type | Description |
| ---------- | ---------------------------------------------------------- | ------------------- |
| `observer` | | pointer to Observer |
**Returns:** `void *` — pointer to saved target
lv_observer_get_target_obj [#lv_observer_get_target_obj]
Get target Widget of Observer. This is the same as , except it returns `target` as an `lv_obj_t *`.
```c title=" " lineNumbers=1
lv_obj_t * lv_observer_get_target_obj(lv_observer_t *observer)
```
Parameters
| Name | Type | Description |
| ---------- | ---------------------------------------------------------- | ------------------- |
| `observer` | | pointer to Observer |
**Returns:** — pointer to saved Widget target
lv_observer_get_user_data [#lv_observer_get_user_data]
Get Observer's user data.
```c title=" " lineNumbers=1
void * lv_observer_get_user_data(const lv_observer_t *observer)
```
Parameters
| Name | Type | Description |
| ---------- | ---------------------------------------------------------------- | ------------------- |
| `observer` | | pointer to Observer |
**Returns:** `void *` — void pointer to saved user data
lv_subject_create [#lv_subject_create]
Create a subject of type `type`
```c title=" " lineNumbers=1
lv_subject_t * lv_subject_create(lv_subject_type_t type)
```
Parameters
| Name | Type | Description |
| ------ | ------------------------------------ | ----------------------------------------------------------------- |
| `type` | | the type of the subject. See |
lv_subject_delete [#lv_subject_delete]
Delete a subject
```c title=" " lineNumbers=1
void lv_subject_delete(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ---------------------------------------- |
| `subject` | | the subject to delete **May** be `NULL`. |
lv_subject_init_int [#lv_subject_init_int]
Initialize an integer-type Subject.
\> **Deprecated:** The subject init API is deprecated, use `lv_subject_create` instead
```c title=" " lineNumbers=1
void lv_subject_init_int(lv_subject_t *subject, int32_t value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `value` | `int32_t` | initial value |
lv_subject_init_float [#lv_subject_init_float]
Initialize an float-type Subject.
\> **Deprecated:** The subject init API is deprecated, use `lv_subject_create` instead
```c title=" " lineNumbers=1
void lv_subject_init_float(lv_subject_t *subject, float value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `value` | `float` | initial value |
lv_subject_init_string [#lv_subject_init_string]
Initialize a string-type Subject.
\> **Deprecated:** The subject init API is deprecated, use `lv_subject_create` instead
```c title=" " lineNumbers=1
void lv_subject_init_string(lv_subject_t *subject, char *buf, char *prev_buf, size_t size, const char *value)
```
Parameters
| Name | Type | Description |
| ---------- | -------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------ |
| `subject` | | pointer to Subject |
| `buf` | `char *` | pointer to buffer to store string |
| `prev_buf` | `char *` | pointer to buffer to store previous string. **May** be `NULL`. When NULL the previous string is not tracked. |
| `size` | `size_t` | size of buffer(s) |
| `value` | `const char *` | initial value of string, e.g. "hello" |
A string Subject stores its own copy of the string, not just the pointer.
lv_subject_snprintf [#lv_subject_snprintf]
Format a new string, updating Subject, and notify Observers if it changed.
```c title=" " lineNumbers=1
void lv_subject_snprintf(lv_subject_t *subject, const char *format,...)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `format` | `const char *` | format string |
| `...` | | |
lv_subject_init_pointer [#lv_subject_init_pointer]
Initialize a pointer-type Subject.
\> **Deprecated:** The subject init API is deprecated, use `lv_subject_create` instead
```c title=" " lineNumbers=1
void lv_subject_init_pointer(lv_subject_t *subject, void *value)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | -------------------------------- |
| `subject` | | pointer to Subject |
| `value` | `void *` | initial value **May** be `NULL`. |
lv_subject_init_color [#lv_subject_init_color]
Initialize a color-type Subject.
\> **Deprecated:** The subject init API is deprecated, use `lv_subject_create` instead
```c title=" " lineNumbers=1
void lv_subject_init_color(lv_subject_t *subject, lv_color_t color)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
| `color` | | initial value |
lv_subject_init_group [#lv_subject_init_group]
Initialize a Group-type Subject.
\> **Deprecated:** The subject init API is deprecated, use `lv_subject_create` instead
```c title=" " lineNumbers=1
void lv_subject_init_group(lv_subject_t *group_subject, lv_subject_t *list[], uint32_t list_len)
```
Parameters
| Name | Type | Description |
| --------------- | -------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------- |
| `group_subject` | | pointer to Group-type Subject |
| `list` | | list of other Subject addresses; when any of these have values updated, Observers of `group_subject` will be notified. |
| `list_len` | `uint32_t` | number of elements in `list[]` |
lv_subject_deinit [#lv_subject_deinit]
Remove all Observers from a Subject and free allocated memory, and delete any associated Widget-Binding events. This leaves `subject` "disconnected" from all Observers and all associated Widget events established through Widget Binding.
\> **Deprecated:** The subject init API is deprecated, use `lv_subject_create`/`lv_subject_delete` instead
```c title=" " lineNumbers=1
void lv_subject_deinit(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------------------------- |
| `subject` | | pointer to Subject **May** be `NULL`. |
This can safely be called regardless of whether any Observers added with or bound to a Widget Property with one of the `..._bind_...()` functions.
lv_subject_add_observer [#lv_subject_add_observer]
Add Observer to Subject. When Subject's value changes `observer_cb` will be called.
```c title=" " lineNumbers=1
lv_observer_t * lv_subject_add_observer(lv_subject_t *subject, lv_observer_cb_t observer_cb, void *user_data)
```
Parameters
| Name | Type | Description |
| ------------- | -------------------------------------------------------- | ------------------------------------- |
| `subject` | | pointer to Subject |
| `observer_cb` | | notification callback |
| `user_data` | `void *` | optional user data **May** be `NULL`. |
**Returns:** — pointer to newly-created Observer
lv_subject_add_observer_obj [#lv_subject_add_observer_obj]
Add Observer to Subject for a Widget. When the Widget is deleted, Observer will be unsubscribed from Subject automatically.
```c title=" " lineNumbers=1
lv_observer_t * lv_subject_add_observer_obj(lv_subject_t *subject, lv_observer_cb_t observer_cb, lv_obj_t *obj, void *user_data)
```
Parameters
| Name | Type | Description |
| ------------- | -------------------------------------------------------- | -------------------------------------------------------------------------------------- |
| `subject` | | pointer to Subject |
| `observer_cb` | | notification callback |
| `obj` | | pointer to Widget. **May** be `NULL`. When NULL the Observer is not bound to a Widget. |
| `user_data` | `void *` | optional user data **May** be `NULL`. |
**Returns:** — pointer to newly-created Observer
Do not call on Observers created this way. Only clean up such Observers by either:
* deleting the Widget, or
* calling to gracefully de-couple and remove all Observers.
lv_subject_add_observer_with_target [#lv_subject_add_observer_with_target]
Add an Observer to a Subject and also save a target pointer.
```c title=" " lineNumbers=1
lv_observer_t * lv_subject_add_observer_with_target(lv_subject_t *subject, lv_observer_cb_t observer_cb, void *target, void *user_data)
```
Parameters
| Name | Type | Description |
| ------------- | -------------------------------------------------------- | ------------------------------------- |
| `subject` | | pointer to Subject |
| `observer_cb` | | notification callback |
| `target` | `void *` | any pointer **May** be `NULL`. |
| `user_data` | `void *` | optional user data **May** be `NULL`. |
**Returns:** — pointer to newly-created Observer
lv_observer_delete [#lv_observer_delete]
Remove Observer from its Subject.
```c title=" " lineNumbers=1
void lv_observer_delete(lv_observer_t *observer)
```
Parameters
| Name | Type | Description |
| ---------- | ---------------------------------------------------------- | -------------------------------------- |
| `observer` | | pointer to Observer **May** be `NULL`. |
lv_obj_remove_from_subject [#lv_obj_remove_from_subject]
Remove Observers associated with Widget `obj` from specified `subject` or all Subjects.
```c title=" " lineNumbers=1
void lv_obj_remove_from_subject(lv_obj_t *obj, lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ---------------------------------------------------------------------------------------------------- |
| `obj` | | pointer to Widget whose Observers should be removed |
| `subject` | | Subject to remove Widget from. **May** be `NULL`. When NULL the Widget is removed from all Subjects. |
This function can be used e.g. when a Widget's Subject(s) needs to be replaced by other Subject(s)
lv_subject_notify [#lv_subject_notify]
Notify all Observers of Subject.
```c title=" " lineNumbers=1
void lv_subject_notify(lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | ------------------ |
| `subject` | | pointer to Subject |
lv_obj_add_subject_increment_event [#lv_obj_add_subject_increment_event]
Add an event handler to increment (or decrement) the value of a subject on a trigger.
```c title=" " lineNumbers=1
lv_subject_increment_dsc_t * lv_obj_add_subject_increment_event(lv_obj_t *obj, lv_subject_t *subject, lv_event_code_t trigger, int32_t step)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | --------------------------------------------------------------------------------------------------- |
| `obj` | | pointer to a widget |
| `subject` | | pointer to a subject to change |
| `trigger` | | the trigger on which the subject should be changed |
| `step` | `int32_t` | value to add on trigger if the minimum value is reached, the maximum value will be set on rollover. |
lv_obj_add_subject_toggle_event [#lv_obj_add_subject_toggle_event]
Toggle the value of an integer subject on an event. If it was != 0 it will be 0. If it was 0, it will be 1.
```c title=" " lineNumbers=1
void lv_obj_add_subject_toggle_event(lv_obj_t *obj, lv_subject_t *subject, lv_event_code_t trigger)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | -------------------------------------------------- |
| `obj` | | pointer to a widget |
| `subject` | | pointer to a subject to toggle |
| `trigger` | | the trigger on which the subject should be changed |
lv_obj_bind_bool [#lv_obj_bind_bool]
Bind a boolean value to a Widget: `set_bool_cb` is called with the Subject's value (as a `bool`) on subscribing and whenever it changes. A dedicated per-flag setter such as `lv_obj_set_hidden` can be passed directly.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_bool(lv_obj_t *obj, lv_subject_t *subject, lv_obj_set_bool_t set_bool_cb)
```
Parameters
| Name | Type | Description |
| ------------- | -------------------------------------------------------- | ----------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to an integer Subject |
| `set_bool_cb` | | callback that applies the boolean value to the Widget |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_int [#lv_obj_bind_int]
Bind an integer value to a Widget: `set_int_cb` is called with the Subject's value on subscribing and whenever it changes.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_int(lv_obj_t *obj, lv_subject_t *subject, lv_obj_set_int_t set_int_cb)
```
Parameters
| Name | Type | Description |
| ------------ | -------------------------------------------------------- | ----------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to an integer Subject |
| `set_int_cb` | | callback that applies the integer value to the Widget |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_float [#lv_obj_bind_float]
Bind a float value to a Widget: `set_float_cb` is called with the Subject's value on subscribing and whenever it changes.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_float(lv_obj_t *obj, lv_subject_t *subject, lv_obj_set_float_t set_float_cb)
```
Parameters
| Name | Type | Description |
| -------------- | -------------------------------------------------------- | --------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to a float Subject |
| `set_float_cb` | | callback that applies the float value to the Widget |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_string [#lv_obj_bind_string]
Bind a string value to a Widget: `set_string_cb` is called with the Subject's value on subscribing and whenever it changes.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_string(lv_obj_t *obj, lv_subject_t *subject, lv_obj_set_string_t set_string_cb)
```
Parameters
| Name | Type | Description |
| --------------- | -------------------------------------------------------- | ---------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to a string Subject |
| `set_string_cb` | | callback that applies the string value to the Widget |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_color [#lv_obj_bind_color]
Bind a color value to a Widget: `set_color_cb` is called with the Subject's value on subscribing and whenever it changes.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_color(lv_obj_t *obj, lv_subject_t *subject, lv_obj_set_color_t set_color_cb)
```
Parameters
| Name | Type | Description |
| -------------- | -------------------------------------------------------- | --------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to a color Subject |
| `set_color_cb` | | callback that applies the color value to the Widget |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_pointer [#lv_obj_bind_pointer]
Bind a pointer value to a Widget: `set_pointer_cb` is called with the Subject's value on subscribing and whenever it changes.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_pointer(lv_obj_t *obj, lv_subject_t *subject, lv_obj_set_pointer_t set_pointer_cb)
```
Parameters
| Name | Type | Description |
| ---------------- | -------------------------------------------------------- | ----------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to a pointer Subject |
| `set_pointer_cb` | | callback that applies the pointer value to the Widget |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_flag_if_eq [#lv_obj_bind_flag_if_eq]
Set Widget's flag(s) if an integer Subject's value is equal to a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_flag_if_eq(lv_obj_t *obj, lv_subject_t *subject, lv_obj_flag_t flag, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | --------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `flag` | | flag(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_OBJ_FLAG_HIDDEN`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_flag_if_not_eq [#lv_obj_bind_flag_if_not_eq]
Set Widget's flag(s) if an integer Subject's value is not equal to a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_flag_if_not_eq(lv_obj_t *obj, lv_subject_t *subject, lv_obj_flag_t flag, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | --------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `flag` | | flag(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_OBJ_FLAG_HIDDEN`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_flag_if_gt [#lv_obj_bind_flag_if_gt]
Set Widget's flag(s) if an integer Subject's value is greater than a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_flag_if_gt(lv_obj_t *obj, lv_subject_t *subject, lv_obj_flag_t flag, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | --------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `flag` | | flag(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_OBJ_FLAG_HIDDEN`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_flag_if_ge [#lv_obj_bind_flag_if_ge]
Set Widget's flag(s) if an integer Subject's value is greater than or equal to a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_flag_if_ge(lv_obj_t *obj, lv_subject_t *subject, lv_obj_flag_t flag, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | --------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `flag` | | flag(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_OBJ_FLAG_HIDDEN`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_flag_if_lt [#lv_obj_bind_flag_if_lt]
Set Widget's flag(s) if an integer Subject's value is less than a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_flag_if_lt(lv_obj_t *obj, lv_subject_t *subject, lv_obj_flag_t flag, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | --------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `flag` | | flag(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_OBJ_FLAG_HIDDEN`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_flag_if_le [#lv_obj_bind_flag_if_le]
Set Widget's flag(s) if an integer Subject's value is less than or equal to a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_flag_if_le(lv_obj_t *obj, lv_subject_t *subject, lv_obj_flag_t flag, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | --------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `flag` | | flag(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_OBJ_FLAG_HIDDEN`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_state_if_eq [#lv_obj_bind_state_if_eq]
Set Widget's state(s) if an integer Subject's value is equal to a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_state_if_eq(lv_obj_t *obj, lv_subject_t *subject, lv_state_t state, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | -------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `state` | | state(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_STATE_CHECKED`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_state_if_not_eq [#lv_obj_bind_state_if_not_eq]
Set a Widget's state(s) if an integer Subject's value is not equal to a reference value, clear flag otherwise
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_state_if_not_eq(lv_obj_t *obj, lv_subject_t *subject, lv_state_t state, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | -------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `state` | | state(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_STATE_CHECKED`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_state_if_gt [#lv_obj_bind_state_if_gt]
Set Widget's state(s) if an integer Subject's value is greater than a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_state_if_gt(lv_obj_t *obj, lv_subject_t *subject, lv_state_t state, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | -------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `state` | | state(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_STATE_CHECKED`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_state_if_ge [#lv_obj_bind_state_if_ge]
Set Widget's state(s) if an integer Subject's value is greater than or equal to a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_state_if_ge(lv_obj_t *obj, lv_subject_t *subject, lv_state_t state, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | -------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `state` | | state(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_STATE_CHECKED`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_state_if_lt [#lv_obj_bind_state_if_lt]
Set Widget's state(s) if an integer Subject's value is less than a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_state_if_lt(lv_obj_t *obj, lv_subject_t *subject, lv_state_t state, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | -------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `state` | | state(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_STATE_CHECKED`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_state_if_le [#lv_obj_bind_state_if_le]
Set Widget's state(s) if an integer Subject's value is less than or equal to a reference value, clear flag otherwise.
\> **Deprecated:** Use or instead.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_state_if_le(lv_obj_t *obj, lv_subject_t *subject, lv_state_t state, int32_t ref_value)
```
Parameters
| Name | Type | Description |
| ----------- | -------------------------------------------------------- | -------------------------------------------------------------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to Subject |
| `state` | | state(s) (can be bit-wise OR-ed) to set or clear (e.g. `LV_STATE_CHECKED`) |
| `ref_value` | `int32_t` | reference value to compare Subject's value with |
**Returns:** — pointer to newly-created Observer
lv_obj_bind_checked [#lv_obj_bind_checked]
Set an integer Subject to 1 when a Widget is checked and set it 0 when unchecked, and clear Widget's checked state when Subject's value changes to 0 and set it when non-zero.
```c title=" " lineNumbers=1
lv_observer_t * lv_obj_bind_checked(lv_obj_t *obj, lv_subject_t *subject)
```
Parameters
| Name | Type | Description |
| --------- | -------------------------------------------------------- | -------------------- |
| `obj` | | pointer to Widget |
| `subject` | | pointer to a Subject |
**Returns:** — pointer to newly-created Observer
Ensure Widget's `LV_OBJ_FLAG_CHECKABLE` flag is set.
Enums [#enums]
lv_subject_type_t [#lv_subject_type_t]
Values for lv\_subject\_t's `type` field
| Name | Value | Description |
| ------------------------- | ----- | ------------------------------------- |
| `LV_SUBJECT_TYPE_INVALID` | `0` | indicates Subject not initialized yet |
| `LV_SUBJECT_TYPE_NONE` | `1` | a null value like None or NILt |
| `LV_SUBJECT_TYPE_INT` | `2` | an int32\_t |
| `LV_SUBJECT_TYPE_FLOAT` | `3` | a float, requires `LV_USE_FLOAT 1` |
| `LV_SUBJECT_TYPE_POINTER` | `4` | a void pointer |
| `LV_SUBJECT_TYPE_COLOR` | `5` | an |
| `LV_SUBJECT_TYPE_GROUP` | `6` | an array of Subjects |
| `LV_SUBJECT_TYPE_STRING` | `7` | a char pointer |
* `lv_subject_create` — param `type`
Structs [#structs]
lv_subject_value_t [#lv_subject_value_t]
A common type to handle all the various observable types in the same way
| Member | Type | Description |
| --------- | ----------------------------- | ---------------------------------------------------------------------- |
| `num` | `int32_t` | Integer number (opacity, enums, booleans or "normal" numbers) |
| `pointer` | `const void *` | Constant pointer (string buffer, format string, font, cone text, etc.) |
| `color` | | Color |
| `float_v` | `float` | Floating point value |
\_lv_subject_t [#_lv_subject_t]
The Subject (an observable value)
| Member | Type | Description |
| ---------------------- | ------------------------------------- | ----------------------------------------------------------------------------------- |
| `ext_data` | | |
| `subs_ll` | | Subscribers |
| `value` | | Current value |
| `prev_value` | | Previous value |
| `min_value` | | Minimum value for min. int or float |
| `max_value` | | Maximum value for max. int or float |
| `user_data` | `void *` | Additional parameter, can be used freely by user |
| `type` | `uint32_t` | One of the LV\_SUBJECT\_TYPE\_... values |
| `size` | `uint32_t` | String buffer size or group length |
| `notify_restart_query` | `uint32_t` | If an Observer was deleted during notification, start notifying from the beginning. |
Typedefs [#typedefs]
lv_observer_cb_t [#lv_observer_cb_t]
```c title=" " lineNumbers=1
typedef void(* lv_observer_cb_t) (lv_observer_t *observer, lv_subject_t *subject)
```
Callback called to notify Observer that Subject's value has changed
* `lv_subject_add_observer` — param `observer_cb`
* `lv_subject_add_observer_obj` — param `observer_cb`
* `lv_subject_add_observer_with_target` — param `observer_cb`
lv_obj_set_bool_t [#lv_obj_set_bool_t]
```c title=" " lineNumbers=1
typedef void(* lv_obj_set_bool_t) (lv_obj_t *obj, bool value)
```
Generic callback called to set a boolean value on a Widget.
* `lv_obj_bind_bool` — param `set_bool_cb`
lv_obj_set_int_t [#lv_obj_set_int_t]
```c title=" " lineNumbers=1
typedef void(* lv_obj_set_int_t) (lv_obj_t *obj, int32_t value)
```
Generic callback called to set an int value on a Widget.
* `lv_obj_bind_int` — param `set_int_cb`
lv_obj_set_float_t [#lv_obj_set_float_t]
```c title=" " lineNumbers=1
typedef void(* lv_obj_set_float_t) (lv_obj_t *obj, float value)
```
Generic callback called to set a float value on a Widget.
* `lv_obj_bind_float` — param `set_float_cb`
lv_obj_set_string_t [#lv_obj_set_string_t]
```c title=" " lineNumbers=1
typedef void(* lv_obj_set_string_t) (lv_obj_t *obj, const char *value)
```
Generic callback called to set a string value on a Widget.
* `lv_obj_bind_string` — param `set_string_cb`
lv_obj_set_color_t [#lv_obj_set_color_t]
```c title=" " lineNumbers=1
typedef void(* lv_obj_set_color_t) (lv_obj_t *obj, lv_color_t value)
```
Generic callback called to set a color value on a Widget.
* `lv_obj_bind_color` — param `set_color_cb`
lv_obj_set_pointer_t [#lv_obj_set_pointer_t]
```c title=" " lineNumbers=1
typedef void(* lv_obj_set_pointer_t) (lv_obj_t *obj, const void *value)
```
Generic callback called to set a pointer value on a Widget.
* `lv_obj_bind_pointer` — param `set_pointer_cb`
Dependencies [#dependencies]