Apple Newton Utilities User's Guide download pdf (Page 808)

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101

CHAPTER 22

Transport Interface

22-20 Using the Transport Interface

Error Handling 22

The default exception handling method implemented by protoTransport is

HandleThrow, which catches and handles exceptions resulting from any supplied

transport methods such as

SendRequest and ReceiveRequest. You must

provide your own exception handler for any methods that you deﬁne, or you can

pass them to

HandleThrow, as follows:

try begin

... // do something

Throw();

onException |evt.ex| do

:HandleThrow();

end

When handling an exception, HandleThrow ﬁrst calls IgnoreError to give

your transport a chance to screen out benign errors. If

IgnoreError returns

true, HandleThrow returns nil and stops.

Assuming the error is not rejected by

IgnoreError, HandleThrow next checks

to see if an item is currently being processed. If so, it sends your transport the

ItemCompleted message and returns true. Note that ItemCompleted calls

HandleError to display an error alert to the user. If no item is currently being

processed,

HandleThrow sends the HandleError message itself to display an

error alert.

The

HandleError method calls TranslateError to give your transport a

chance to translate an error code into an error message that can be displayed to the

user. If your transport can’t translate the error (for example, because it’s a

system-deﬁned error) you should simply call the inherited

TranslateError

method, which handles system-deﬁned errors.

Power-Off Handling 22

The protoTransport object registers a power-off handler with the system

whenever the transport is not in the idle state. If the system is about to power off,

this power-off handler sends the transport the

PowerOffCheck message.

The default

PowerOffCheck method in protoTransport displays a slip

asking the user to conﬁrm that it is OK to break the connection. Then, when the

power is about to be turned off, the system sends the transport the

CancelRequest message and waits until the transport is idle before turning the

power off.

You can override the default

PowerOffCheck method if you wish.

There is also a power-on handler that sends a

CancelRequest message to the

transport when the system turns on after shutting down unexpectedly while

the transport is active.

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