# fsVarsRegisterVarsStatusUpdateHandler<no value>
The **fsVarsRegisterVarsStatusUpdateHandler** function can be used to register a handler which will be called when the states of variables are updated. The function will be called when `LVar` are reset or when `LVar`, `OVar` and `ZVar` are removed.

 

##### Syntax

``` wasm
FsVarError fsVarsRegisterVarsStatusUpdateHandler(
    FsVarsStatusUpdateHandler handler,
    void* pUserParam,
    bool bUseObjIdSpecialValue
);
```

 

##### Parameters

<table><thead><tr class="header"><th>Parameters</th><th>Description</th></tr></thead><tbody><tr class="odd"><td><code>handler</code></td><td>The pointer to the function that will be called when the variable state is updated.</td></tr><tr class="even"><td><code>pUserParam</code></td><td>A pointer to the user defined parameter returned when the handler is called.</td></tr><tr class="odd"><td><code>bUseObjIdSpecialValue</code></td><td><p>When set to <em>true</em>, the value of the object ID given by the handler is transformed to use special values (<code>FS_OBJECT_ID_USER_AIRCRAFT</code> for the user aircraft for example) instead of the actual object ID.</p><p>Default: <code>true</code></p></td></tr></tbody></table>

 

##### Return Values

The function returns `FsVarError`, where 0 means there is no error (in which case the handler will be called each time the simulation needs to notify a change in the variables).

 

The handler

Syntax:

``` wasm
typedef void (*FsVarsStatusUpdateHandler)(
    FsVarTypeFlags varsUpdated,
    FsVarUpdateOp operation,
    FsObjectId objectId,
    void* pUserParam
);
```

Parameters:

{{< table-wrapper >}}

| Parameters    | Description                                                                                                                                                                                                                                                                                                                                         |
|---------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `varsUpdated` | This flag contains all the variable types affected by the operation .                                                                                                                                                                                                                                                                               |
| `operation`   | Enum describing the operation affecting the variables.                                                                                                                                                                                                                                                                                              |
| `objectId`    | ID of the object affected by the operation, when relevent (if not `FS_OBJECT_ID_NONE` is sent). If the handler has been registered with the boolean `bUseObjectIdSpecialValue` set to `true`, the ID is transformed to correspond to the special value of the object (for example the ID of the user aircraft will be `FS_OBJECT_ID_USER_AIRCRAFT`) |
| `pUserParam`  | Pointer given when the handler was registered and given back when the handler is called.                                                                                                                                                                                                                                                            |

{{< /table-wrapper >}}


##### 

##### Example

``` wasm
void handler(FsVarsTypeFlags varsUpdated, FsVarUpdateOp op, FsObjectId objId, void* pUserParam)
{
    // Process variables status update
}
extern "C" MSFS_CALLBACK void module_init()
{
    FsVarError err = fsVarsRegisterVarsStatusUpdateHandler(handler, pUserData, true);
    if (err != FS_VAR_ERROR_NONE)
    {
        // Error
    }
}
```