Additional System Services

Using Additional System Services

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

to

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.

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.