On Dec 25, 2011, at 14:05 , Luc Van Bogaert wrote:
> I'm adding a custom user setting to a NSPrintPanel by adding an "accessory
> view controller" for a view that contains just one control (a checkbox) and
> which is loaded from a nib file. The checkbox's value has a binding to the
> view controller's "representedObject.printSettings.checkboxvalue" keypath.
> (The representedObject is a NSPrintInfo object).
>
> I have been able to verify that this binding works correctly.
>
> One thing I haven't gotten to work yet, is to automatically update the
> "preview" in the print panel when I chance the state of the checkbox. The
> preview updates itself correctly when I change one of the "built-in"
> settings, and it even correctly represents my own custom setting. But, how
> can I trigger an automatic update when I just change my own custom setting?
>
> I assume I should implement the "keyPathsForValuesAffectingPreview" protocol
> method in my accessory view controller, which I did, but so far without
> success. Here's my implementation:
>
> - (NSSet *)keyPathsForValuesAffectingPreview
> {
> return [NSSet setWithObjects:
> [NSString
> stringWithFormat:@"representedObject.printSettings.checkboxvalue"],
> nil];
> }
The immediate problem is that binding through a mutable dictionary like this
(printSettings) isn't KVO compliant. Properties with values that are mutable
objects are generally a problem for KVC/KVO, since it's really designed around
the idea that property values are immutable objects like NSString and NSNumber.
The secondary problem is that you're trying, simultaneously, to use 2 different
ways to get the accessory panel to update automatically, and both ways aren't
working. If your binding was working, then 'keyPathsForValuesAffectingPreview'
wouldn't be necessary. If you had a working
'keyPathsForValuesAffectingPreview', then I suspect you'd find that the
checkbox still wouldn't change -- not because it wasn't updating, but because
the change wouldn't be propagated.
I think the simplest approach is to add a "checkboxvalue" property to the view
controller, bind the checkbox to File's Owner.checkboxvalue, and then make this
property KVO-compliant:
1. The getter can just return
self.representedObject.printSettings.checkboxvalue.
2. The setter can just change
self.representedObject.printSettings.checkboxvalue.
3. The view controller needs to keep track of the current print settings object
by observing its own "representedObject.printSettings" keypath.
4. Whenever the print settings object changes, the view controller needs to
observe the "checkboxvalue" key of the new object and stop observing the same
key of the old object.
5. Whenever the observed "checkboxvalue" changes, transfer the new value to
self.checkboxvalue. Note that this starts a messaging loop between the 2
"checkboxvalue" properties, but the KVO machinery prevents this from being an
infinite loop. You can short circuit the remaining minor duplication of effort,
if you wish, by having your setter check the dictionary property value to avoid
re-setting it to the existing value.
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