Registers type as extension for the extension point with name extension_point_name.
If type has already been registered as an extension for this extension point, the existing
IOExtension object is returned.
| extension_point_name |
the name of the extension point |
| type |
the Type to register as extension |
| extension_name |
the name for the extension |
| priority |
the priority for the extension |
|
a IOExtension object for Type |