I have a table bound to an array controller containing Foo objects and wanted
to enable a "Perform Foo" button whenever there is a non-nil selection. Not
trusting those pesky proxy objects you get from an array controller's
-selection, I wrote a -selectedFoo method which will return either the first
selected Foo or nil if there is no selection, and bound my button's 'enabled'
binding to it via NSIsNotNil value transformer.
Now, NSArrayController has five methods which give the selection in some form,
and they are all documented to be "observable using key-value observing". So
to make this work I implemented a +keyPathsForValuesAffectingSelectedFoo which
returned the path to one of these keys, but when that didn't work I put in all
five.
But I never could get it to work. By logging, I determined the problem: None
of these five observers are firing the -selectedFoo getter when the array and
table controller's selection changes. When I added, as a test, some other
object's key path to keyPathsForValuesAffectingKey, and changed its value, the
-selectedAgent method was invoked and updated the enabled state of my button as
desired. So the problem seems to be that "observeable using key-value
observing" doesn't mean what I think it means. Where am I going wrong?
I solved the problem by eliminating all this code and binding my button's
'enabled' binding instead to the array controller's 'selection' and, to my
surprise, it worked. My surprise is because, according to superclass
NSObjectController documentation, 'selection' returns NSNoSelectionMarker when
there is no selection, not nil which is what my NSIsNotNil value transformer
would expect.
So, what should work does not work and what should not work does work.
Thanks,
Jerry Krinock
// fooArrayController is an IBOutlet
+ (NSSet*)keyPathsForValuesAffectingSelectedFoo {
return [NSSet setWithObjects:
// Shotgun approach. Try 'em all!!
@"fooArrayController.selectedObjects",
@"fooArrayController.selectionIndex",
@"fooArrayController.selection",
@"fooArrayController.selectionIndexes",
@"fooArrayController.selectedObjects",
nil] ;
}
- (Foo*)selectedFoo {
NSArray* selectedFoos = [fooArrayController selectedObjects] ;
if ([selectedFoos count] > 0) {
return [selectedFoos objectAtIndex:0] ;
}
return nil ;
}
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