If a signal handler is connected to `generate_value` a signal is emitted when the file module accesses a
value using gegl_metadata_get_value.
The signal handler must generate a value of the type specified in the pspec argument. The signal handler's return value indicates the success of the operation.
If no handler is connected the mapped metadata value is accessed normally,
| pspec |
A ParamSpec declaring the metadata value |
| value |
An initialised Value. |
|
true if a value is generated successfully. |