SUBSCRIBE statement

Creates a subscription to an ABL named event.

Note: ABL named events are completely different from the key function, mouse, widget, and direct manipulation events described in the "Handle-based Object Events Reference" section. They are also different from the class events described in the "Class Events Reference" section.

Syntax


SUBSCRIBE [ PROCEDURE `subscriber-handle` ] [ TO ] `event-name` { IN `publisher-handle` \| ANYWHERE } [ RUN-PROCEDURE `local-internal-procedure` ] [ NO-ERROR ]

PROCEDURE subscriber-handle

A procedure or handle representing the subscriber.

The PROCEDURE option lets one procedure create a subscription on behalf of another. For example, if you want procedure A to create a subscription on behalf of procedure B, set subscriber-handle to the procedure handle of B.

If the PROCEDURE option does not appear, the AVM creates a subscription on behalf of THIS-PROCEDURE, the procedure that contains the SUBSCRIBE statement.

TO event-name

A quoted string or a character expression representing the name of the event.

IN publisher-handle

Subscribes to the named events published by publisher-handle.

If publisher-handle is not a valid procedure or widget handle at the time the SUBSCRIBE statement executes, the AVM reports a run-time error unless you specify the NO-ERROR option.

ANYWHERE

Subscribes to named events published within the ABL session, regardless of the publisher.

RUN-PROCEDURE local-internal-procedure

A quoted string or character expression representing the name of an internal procedure that resides within the subscribing program. The AVM runs local-internal-procedure when the named event occurs.

If the RUN-PROCEDURE option does not appear, when the named event occurs, the AVM runs an internal procedure with the same name as the named event.

Note: The RUN-PROCEDURE option lets you create a subscription when the event name and the procedure name do not match, or when you must subscribe to two different events that have the same name.

When the named event occurs, the AVM RUNs each subscriber’s local internal procedure, passing the parameters, if any, The order in which the AVM notifies subscribers is undefined. The AVM always performs this RUN with an implicit NO-ERROR, and logs errors to the ERROR-STATUS system handle.

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 done 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 done, depending on the order the AVM resolves the expression elements and the occurrence of the error.

For the SUSCRIBE statement with NO-ERROR, this option tells the AVM not to report a run-time error if publisher-handle or subscriber-handle is not a valid procedure handle, or if the AVM cannot evaluate an event-name expression.

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 generated error messages. ABL handle methods used in a block without a CATCH end block treat errors as warnings and do not raise ERROR, do not set the ERROR-STATUS:ERROR attribute, but do add messages to the ERROR-STATUS system handle. Therefore, this test is the better test for code using handle methods without CATCH end blocks. ABL handle methods used in a block with a CATCH end block raise ERROR and add messages to the error object generated by the AVM. In this case, the AVM does not update the ERROR-STATUS system handle.

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.

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, then the information and messages in the error object will be 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.

Example

For an example, see the reference entry for the PUBLISH statement.

Notes

Within the local internal procedure, you can get a handle to the publisher of the named event by using the SOURCE-PROCEDURE system handle. For more information on the SOURCE-PROCEDURE system handle, see the reference entry.

If the AVM detects a redundant SUBSCRIBE statement—that is, a SUBSCRIBE statement with the same event name, and either the same publisher handle or the same ANYWHERE option—the AVM does not report an error.

If event-name is a string containing spaces or is otherwise not a standard ABL name, use one of the following techniques:

Use the RUN-PROCEDURE option to assign the local internal procedure a more conventional name.

When you define local-internal-procedure, put its name in quotes, as in the following example:


PROCEDURE "spaced event":