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C H A P T E R 2 3
Endpoint Interface
Using the Endpoint Interface
23-23
Next, the
Cancel
(or
Disconnect
) method returns, and any pending
synchronous request is canceled by throwing an exception that contains error code
­16005. Or, if the cancellation was invoked as the result of a time-out expiration,
then whatever method timed out throws an exception containing error code ­16005.
Other Operations
23
The
Option
method allows you to get and set options apart from the options
parameter to the
Bind
,
Connect
,
Listen
,
Accept
, and
Output
methods.
You can check the state of a connection by calling the
State
method.
Custom communication tools can return special events to the endpoint object
through the
EventHandler
message. This message is sent to the endpoint
whenever an event occurs that is not handled by one of the usual endpoint event
handlers. A custom communication tool and an endpoint can use this mechanism to
pass events from the communication tool up to the endpoint layer.
Error Handling
23
By specifying an
ExceptionHandler
method in your endpoint, you can handle
exception conditions not caught by local
try...onexception
clauses, as well
as exceptions not caught by
CompletionScript
methods.
When you call an endpoint method synchronously, and an error occurs in that
method, the system throws an exception (usually of type
|evt.ex.comm|
). You
can catch these exceptions in your application by using the
try
. . .
onexception
construct. It's a good idea to bracket every endpoint method call
with this exception catching construct.
If an error occurs as a result of an asynchronous request, no exception is thrown,
but the error is returned in the result parameter to the
CompletionScript
method associated with that request. If you did not define a
CompletionScript
method, or if the error is unsolicited, the error is forwarded to your
ExceptionHandler
method. If you did not define an
ExceptionHandler
method, then the communication system throws an exception. This exception is
caught by the operating system, which displays a warning message to the user.
Constants for error codes generated by the Endpoint interface are defined in
"Endpoint Error Code Constants" (Table 20-4 on page 20-4) in Newton
Programmer's Reference
.
When you use the
Option
method (or any method that takes options as a
parameter), not only can the method itself fail, but a failure can occur in processing
each of the individual option requests. If the latter happens, the
result
slot in the
returned option frame is set to one of the option error codes listed in "Option Error
Code Constants" (Table 20-5 on page 20-5) in Newton Programmer's Reference. If
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