Author: Eric Soldan
Year: 1990
... discusses a Window Manager anomaly with the handling of the visRgn and the updateRgn between BeginUpdate and EndUpdate calls.
Apple II
Technical Notes
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Developer Technical Support
Apple IIGS
#75: BeginUpdate Anomaly
Written by: Eric Soldan January 1990
This Technical Note discusses a Window Manager anomaly with the handling of
the visRgn and the updateRgn between BeginUpdate and EndUpdate calls.
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If an application calls BeginUpdate, it needs to be fully aware of what is
going on behind the scenes in terms of its visRgn and updateRgn. Typically an
application has TaskMaster handle the update events. TaskMaster calls
BeginUpdate, the application update procedure, then EndUpdate. So any
application that uses TaskMaster to handle updates, whether or not it makes
any BeginUpdate calls directly, needs to be aware of problem described in this
Note.
BeginUpdate is responsible for intersecting the visRgn and the updateRgn and
making the intersection of these two regions the temporary visRgn. (EndUpdate
undoes this effect.) Following are the steps BeginUpdate takes to do this:
1. Localize the updateRgn. (All grafPort regions are local,
therefore the visRgn is local. All window regions are global,
therefore the updateRgn is global. One of them has to change if
they are to be intersected correctly.)
2. Intersect the visRgn and localized updateRgn, then place the
result in the updateRgn.
3. Swap the visRgn and updateRgn handles.
The handle swapping has two effects:
o Makes the intersection region the current visRgn.
o Saves the real visRgn as the updateRgn. (Saving the real
visRgn is necessary because everything has to be restored to
normal by EndUpdate.)
EndUpdate restores things to normal after an update procedure is finished.
When an application calls EndUpdate, it swaps back the handles and sets the
updateRgn to empty.
So What's the Problem?
The problem is that the updateRgn is not a very good place to save the visRgn.
Since InvalRect and InvalRgn modify the updateRgn, if either of these two
calls is made between a BeginUpdate and EndUpdate, they modify the saved
visRgn. When the update is finished, EndUpdate restores the modified visRgn
instead of the original.
The solution to this problem seems simple enough: don't call InvalRect or
InvalRgn between BeginUpdate and EndUpdate. Unfortunately, there are other
calls which can call BeginUpdate, EndUpdate, InvalRect, and InvalRgn, so an
application might inadvertently call one of these routines.
If this situation isn't bad enough already, you could really mess things up by
opening another window between BeginUpdate and EndUpdate calls. Opening a
window at this time may seem like a perfectly normal thing (i.e., to display
an alert); however, opening a window forces the recalculation of the visRgn
for any windows obscured by the new window. If the window being updated has
its visRgn recalculated, the application obviously loses the visRgn that
BeginUpdate created. This doesn't seem too serious since the visRgn is
restored to the entire visible part of the window when the new window is
closed; however, it does mean that the application would have to update the
entire window instead of the original updateRgn.
Unfortunately, the Window Manager also posts update events for the portion of
the window that was obscured, and it does this by changing the updateRgn. Of
course the updateRgn for the window being updated is really the visRgn that is
being "safely" preserved until the EndUpdate call. So, there are some really
good reasons why this can't be done.
Okay, so along with not making calls to InvalRect and InvalRgn between
BeginUpdate and EndUpdate, an application cannot open any other windows
either. Good.
Now to make things even worse.
If you use the QuickDraw Auxiliary function CopyPixels in an update procedure,
you can get yourself in trouble. On ROM 01 machines running System Software
5.0 and later, the QuickDraw Auxiliary tool file has dynamic segments. If a
disk is off-line and an application calls CopyPixels for the first time, the
system prompts the user to insert the system disk, so it can load the dynamic
segment holding CopyPixels. Unfortunately, it does this via a regular window,
which causes all the terrible things already discussed. So if an application
has a CopyPixels call in its update procedure, it must also be called prior to
the update procedure to make sure QuickDraw Auxiliary is loaded before
beginning the update. If necessary, an application can use a dummy call after
starting (and after starting QuickDraw Auxiliary of course), to make sure
QuickDraw Auxiliary is loaded prior to any updates. This dynamic segment is
never unloaded, so it is around any time the update procedure needs it.
There are some other dynamic segments in QuickDraw Auxiliary, so keep a
watchful eye on this nasty problem.
But I Have to Do...
If you absolutely must do some of the things previously discussed, there is a
way to accomplish it. It is not simple, but it can be done.
Assuming that BeginUpdate has been called, and an application is in its update
procedure:
1. Create a new region and copy the visRgn into it. Doing this
allows the application to restore the visRgn to just the area to
be updated that BeginUpdate calculated. This needs to be done for
any other windows which obscure a part the the window being
updated. Again, these are not windows that an application would
open directly. CopyPixels may open a window, since it is a
dynamic segment and may need to get loaded from a disk that is
off-line.
2. Create a new region, then swap its handle with the updateRgn
handle. This protects the real visRgn and lets an application
call InvalRect and InvalRgn at any time if necessary. It also
means the application doesn't need to worry about the system
making these calls either. The updateRgn is also an empty region
after the swap, so any contributions to it constitute a valid
update event that needs to be handled.
3. Do the update part of the update procedure. In this part, if the
application has any calls to CopyPixels, or any other QuickDraw
Auxiliary dynamic segment functions, after the call is completed,
copy the saved visRgn back to the visRgn of the grafPort. The
closing of the dynamic segment alert window recalculates the
visRgn, and copying it undoes this effect. Do not do the same
for the updateRgn. Leave the updateRgn alone. We are
accumulating an actual updateRgn, and the closing of the alert
window for the dynamic segment may have contributed to this
region.
There are two methods for leaving the update procedure. Although the second
method works whether or not an application uses TaskMaster, if an application
does not use TaskMaster, then the first method is simpler.
The procedure without using TaskMaster (i.e., you made the BeginUpdate call,
and you will make the EndUpdate call) is as follows:
A. Dispose of the region created in Step 1. This region was only
needed to restore the partial visRgn that BeginUpdate calculated
after a window was opened.
B. Swap the updateRgn handle with the region handle created in Step 2.
C. Make the EndUpdate call.
D. If the region created in Step 2 is not empty, copy this region
into the updateRgn for the window with CopyRgn. You can't just do
an InvalRgn with it because InvalRgn globalizes the region and the
region is already global. You want to copy the region since this
generates a valid update event. You can use CopyRgn instead of
UnionRgn because the update region is empty.
E. Dispose of the region created in Step 2.
With TaskMaster, things are a little messier. Since TaskMaster makes the
EndUpdate call, you have less control over the situation. It is important to
do the EndUpdate before generating the update event. Posting the update event
has to happen outside the update procedure, since you have to leave the update
procedure for TaskMaster to do the EndUpdate. So it follows that you do Steps
A and B, post an application event to handle the rest externally, and when the
application event is handled, do Steps D and E.
Some consideration was given to posting an application event via the PostEvent
call. Unfortunately, there is a possibility that this application event will
drop out of the queue not handled. When the queue is full, the oldest event
is dropped, and this could occur to application events, which would be very
bad in this case. Due to this possibility, posting an application event
refers to setting a global variable that is checked before the TaskMaster call
in the main event loop. This can be considered equivalent to posting an event
via the PostEvent call.
So, the TaskMaster case would be as follows:
A. Dispose of the region created in Step 1.
B. Swap the updateRgn handle with the region handle created in Step 2.
C. Store the handle of the region created in Step 2 in a global
variable named eventUpdateRgn. Store the current window port in a
global variable named eventWindowPort.
D. Return to TaskMaster, which returns to the main event loop.
E. Immediately after the TaskMaster call in the main event loop,
check the global variable eventUpdateRgn. If it is not NULL then:
a. Copy the region into the updateRgn of the window
eventWindowPort. Using CopyRgn is the easiest way to copy the
region. (Copying the region posts an update event if the event
UpdateRgn is not NULL.
b. Dispose of the region eventUpdateRgn, then set the variable
eventUpdateRgn to NULL, so that this "event" won't be handled
again.
Of course, the simplest way to handle all of this is to avoid situations where
you have to take the steps described above. If things like opening a window
(or allowing the system to open one) and InvalRect and InvalRgn can be avoided
between calls to BeginUpdate and EndUpdate, so can all of this ugliness.
Further Reference
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o Apple IIGS Toolbox Reference, Volume 2