On Mon, Jun 30, 2008 at 12:38 AM, Hamish Allan <email@hidden> wrote:
> On Mon, Jun 30, 2008 at 5:33 AM, Charles Srstka
> <email@hidden> wrote:
>
>> Wait, what? Okay, that contradicts the understanding that I had come to...
>> so the built-in views establish two-way bindings *without* overriding
>> bind:toObject:withKeyPath:options:? Great, that puts us back to square one!
>
> It would seem so, but in fact I was mistaken about the equivalence
> between "value" and "objectValue" bindings: the binding to objectValue
> is only a one-way binding. I guess that difference would make it safer
> to use a separate namespace.
I am largely bindings clueless, so beware....
NSObject's -bind:... method does two things. First, it sets up an
automatic one-way binding from the model object to the view object.
When the model is changed, the view's key is automatically set to the
new value. Second, it sets up an info dictionary for that binding.
Although Apple's sample code shows overriding -bind:... to store
information about the new binding, it doesn't look like this is
necessary. You can simply use -infoForBinding: to obtain the info
dictionary, extract the bound object and key, and use that information
to update the model object. I'd assume this is what the Apple classes
do, and it seems to me to be a lot simpler than overriding -bind:...
to stash away a bunch of information that's already being stored for
you anyway.
The reason the objectValue binding doesn't work both ways is because
NSControl doesn't implement this kind of thing when its objectValue
changes. You need to implement the view->model changes manually they
don't come for free just because you're KVC compliant, and NSControl
doesn't implement it for objectValue. You get the model->view
direction for free with KVC complaince, which is why it halfway works.
Mike
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