Apple Newton Utilities Guía de usuario descargar pdf (Pagina 625)

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101

CHAPTER 17

Additional System Services

Using Additional System Services 17-9

Avoiding Undo-Related “Bad Package” Errors 17

The AddUndoAction method saves the current context (self) so that later it can

send the “redo” message (the argument to the

methodName parameter of the

AddUndoAction method) to the appropriate object. As a result, it is possible that

references to your application can stay in the system inside the Undo mechanism’s

data structures after the application has been removed. These references can cause

-10401 (bad package) errors when the user taps Undo after ejecting the card on

which the application resides. Using the

EnsureInternal function on all

parameters passed to the AddUndoAction function does not remedy this problem.

You can use the

ClearUndoStacks function to clean up dangling references

self that are left behind by the AddUndoAction method. The

ClearUndoStacks function is generally called from the ViewQuitScript

method of the application base view. You can call this function elsewhere as

necessary but you must do so very cautiously to avoid damaging other

applications’ undo actions.

Note

The ClearUndoStacks function deletes all pending undo

actions—including those posted by other applications. Use this

function cautiously.

◆

Using Idler Objects 17

This section describes how to install an idler object for a speciﬁed view. An idler

object sends

ViewIdleScript messages to the view periodically.

Note that an idler object cannot guarantee its

ViewIdleScript message to be

sent at precisely the time you specify. The

ViewIdleScript message cannot be

sent until an executing method returns; thus the idler’s message may be delayed if a

method is executing when the interval expires.

Using an idler object is straightforward; you need to

■ Send the SetUpIdle message to the view that is to receive the

ViewIdleScript message. This message takes a single parameter: the

number of milliseconds to wait before sending the ﬁrst

ViewIdleScript

message.

■ Implement the ViewIdleScript method that is to be executed. This method

should return the number of milliseconds before the next

ViewIdleScript

message is sent.

■ Call SetUpIdle again, this time passing in the value 0, or return nil from

your

ViewIdleScript. This removes the idler object.

The idler object is removed in any case when the view is closed.

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