Apple Newton Utilities User's Guide download pdf (Page 851)

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CHAPTER 23

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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Apple Newton Utilities User's Guide Page 851

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