Removes a method or procedure as a handler
for an ABL or .NET class event.
Return type: VOID
Access: PUBLIC
Applies
to: ABL or .NET class events
Syntax
[ publisher : ] event-name:Unsubscribe
( [ subscriber : ] handler-method ) [ NO-ERROR ]
|
[ publisher : ] event-name:Unsubscribe
( [ subscriber-handle , ] handler-procedure ) [ NO-ERROR ]
|
- [
publisher : ]
- The object reference to an ABL or .NET object or the type name
of an ABL or .NET class that can publish the instance or static
event, respectively. If you do not specify a publisher,
it defaults to either THIS-OBJECT when event-name identifies
an instance event defined in the current class instance, or to the
class type name of the current class definition when event-name identifies
a static event defined in the current class hierarchy.
-
event-name
- The name of an ABL or .NET event that publisher can
publish. At compile time, ABL verifies that event-name is
an accessible member of the class referenced by publisher.
The event can be an abstract event.
- [
subscriber : ]
handler-method
- An ABL class-based method that you want to remove as a handler for event-name,
where handler-method is the unquoted name of
the instance or static method. If specified, subscriber can
be either the object reference to an ABL class instance where handler-method is
defined, or the type name of an ABL class where the static handler-method is defined.
If you do not specify a subscriber, it defaults
to THIS-OBJECT when handler-method identifies
an instance method defined in the current class hierarchy, or to the
class type name of the current class definition when handler-method identifies
a static method defined in the current class hierarchy.
Note that
the access mode (PUBLIC, PROTECTED, or PRIVATE) of the handler-method definition
does not matter to the publisher. For example,
you can remove a PRIVATE method of the unsubscribing class instance
as a handler for an event defined and published by yet another class
instance.
The signature of handler-method must
be run-time compatible with the signature defined for event-name.
This includes an overloaded method as long as ABL can identify the
signature for a unique overloading that is compatible with the event
signature.
Thus, ABL raises a compile-time error if handler-method:
- Is not accessible to the context where this Unsubscribe( ) method
executes
- Does not have a run-time compatible signature
- Does not have a unique overloading that is compatible with the
event signature
- [
subscriber-handle , ]
handler-procedure
- A procedure that you want to remove as a handler for event-name,
where handler-procedure is a character expression
that evaluates to the name of an internal procedure. If specified, subscriber-handle is
a handle to a persistent procedure, or other external procedure
on the call stack, where the internal procedure is defined. Otherwise, subscriber-handle defaults
to THIS-PROCEDURE.
At compile-time, ABL checks this Unsubscribe( ) method
call only to verify that any specified subscriber-handle is
a HANDLE data type and that handler-procedure is
specified by a character expression. Otherwise, ABL raises a run-time
error if the procedure context specified by subscriber-handle is
not accessible or it does not define handler-procedure as
an internal procedure.
- NO-ERROR
- Suppresses ABL errors or error messages that would otherwise occur and diverts them to
the ERROR-STATUS system handle. If an error occurs, the
action of the statement is not completed and execution continues with the next
statement. If the statement fails, any persistent side-effects of the statement are
backed out. If the statement includes an expression that contains other executable
elements, like methods, the work performed by these elements may or may not be
completed, depending on the order in which the AVM resolves the expression elements and
the occurrence of the error.
To check for errors after a statement that uses the
NO-ERROR option:
- Check the ERROR-STATUS:ERROR attribute to see if the AVM raised the ERROR
condition.
- Check if the ERROR-STATUS:NUM-MESSAGES attribute is greater than zero to see if
the AVM has generated error messages.
- Use ERROR-STATUS:GET-MESSAGE( message-num ) to retrieve a
particular message, where message-num is 1 for the first
message.
If the statement does not include the NO-ERROR option, you can use a CATCH end
block to handle errors raised by the statement.
Following are some other
important usage notes on the NO-ERROR option:
- NO-ERROR does not suppress errors that raise the STOP or QUIT condition.
- A CATCH statement, which introduces a CATCH end block, is analogous to a NO-ERROR
option in that it also suppresses errors, but it does so for an entire block of
code. It is different in that the error messages are contained in a class-based
error object (generated by the AVM or explicitly thrown), as opposed to the
ERROR-STATUS system handle. Also, if errors raised in the block are not handled by a
compatible CATCH block, ON ERROR phrase, or UNDO statement, then the error is not
suppressed, but handled with the default error processing for that block type.
- When a statement contains the NO-ERROR option and resides in a block with a CATCH
end block, the NO-ERROR option takes precedence over the CATCH block. That is, an
error raised on the statement with the NO-ERROR option will not be handled by a
compatible CATCH end block. The error is redirected to the ERROR-STATUS system
handle as normal.
- If an error object is thrown to a statement that includes the NO-ERROR option, the
information and messages in the error object are used to set the ERROR-STATUS system
handle. This interoperability feature is important for those integrating code that
uses the traditional NO-ERROR technique with the newer, structured error handling
that features error objects and CATCH end blocks.
- To access more comprehensive error information for a .NET exception, use a CATCH
end block instead of the NO-ERROR option. For more information on handling .NET
exceptions, see the sections on .NET error handling in OpenEdge Development:
GUI for .NET Programming.
The Unsubscribe( ) method
cancels an event subscription by removing the specified method or
internal procedure from the list of handlers for the specified event-name.
If you do not specify a publisher, you can only
invoke Unsubscribe( ) in an ABL class
definition and on an ABL or .NET event that the ABL class defines
or inherits. When you cancel an event handler subscription, ABL
does not raise either a run-time error if the specified event handler
is not currently on the subscription list for the specified event.
