NSOutlineView assertion failure on redraw .. ?

[Michael Hanna]

I have an NSTreeController bound to a data model:

        [m_rulesDMTreeController bind:@"contentArray"
                             toObject:rulesDM
                          withKeyPath:@"rules"
                              options:nil];

and an NSTableColumn(inside an nsoutlineview) bound to the tree
controller.arrangedObjects.Name keypath. Infrequently, but not always,
I will get an assertion error:

2008-07-02 14:13:19.387 Submit[390:10b] *** Assertion failure in
-[NSOutlineView _expandItemEntry:expandChildren:startLevel:](),
/SourceCache/AppKit/AppKit-949.33/TableView.subproj/NSOutlineView.m:1003
2008-07-02 14:13:19.388 Submit[390:10b] Invalid parameter not
satisfying: itemEntry->children == NULL

This occurred when I ran this code:

[cut]
            //  make the cocoa bindings system be aware of the change
with -mutableArrayValueForKey

            NSMutableArray* sourceChildren = [sourceParent
mutableArrayValueForKey:@"children"];
            NSMutableArray* destinationChildren = [destinationParent
mutableArrayValueForKey:@"children"];

            //  re-parent the group we're moving and do the move operation

            [movedGroup setParentGuid:destinationParentGuid];

            [self willChangeValueForKey:@"rules"];

            [destinationChildren addObject:movedGroup];
            [sourceChildren removeObject:movedGroup];

            //  if the source parent has zero children, delete it from
m_elements

            if( [sourceChildren count] == 0 )
            {
                [self deleteRule:sourceParent allRuleGroups:[self ruleGroups]];
            }

            [self didChangeValueForKey:@"rules"];
[cut]

I have an observer watching the "rules" data model and the assertion
error occurs before the data model gets changed.

This doesn't happen that frequently, but enough to be a concern. Any
suggestions why the outlineview is re-populating before the data model
changes(apparently)?

Failing any reasonable workaround, is there a way I can detect when
this assertion occurs so I can attempt a 'fallback' mechanism?

Michael
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