Thanks for the responses guys, I think I follow most of it.
My documents for this app are essentially movies (4D images, the 4th dimension
is time). I want this particular slider to set the timepoint for all open
documents, in a 'write only' fashion. Currently there is no UI to set the
timepoints individually, and each document carries internal logic to prevent
setting a timepoint beyond the end of the file.
Currently I have implemented this behaviour by binding the slider to a property
on my document controller, and the setter for this forwards the desired
timepoint onto all open documents. I was hoping to replace this with a binding
just to eliminate some glue code.
I thought it would be simple because I have another slider which is bound to
the 'contrast' property of the selection from the array controller, and this
behaves exactly as I want it to (the selection is controlled via a table view,
and the slider will set the contrast of all selected rows).
I hope the above is clear. Is keeping the property on the document controller
likely the simplest way forward?
Toby
On 21 Jan 2012, at 08:54, Ken Thomases <k...@codeweavers.com> wrote:
> On Jan 21, 2012, at 1:00 AM, Quincey Morris wrote:
>
>> On Jan 19, 2012, at 03:03 , Tobias Wood wrote:
>>
>>> Binding the NSSlider's value to the NSArrayController's
>>> "arrangedObjects.propertyName" causes my program to get a SIGABRT on
>>> opening, with the following uncaught exception:
>>> "Cannot create double from object ( ) of class __NSArray0"
>>>
>>> I assume this is to do with the empty array.
>>
>> No, it's because you can't bind through a key path that contains a
>> collection property ("arrangedObjects" in this case).
>
> Not to get too deep into the weeds, but arrangedObjects is, like selection,
> also a proxy and it does allow binding through it. That's a big part of its
> purpose/value.
>
>> This situation is complicated by the fact that NSArray responds to a
>> selector it doesn't recognize (such as your "propertyName") by returning the
>> result of sending this selector to every element in the array.
>
> Um, not quite. This isn't about not responding to a selector, it's about its
> implementation of -valueForKey:. NSArray implements -valueForKey: by
> returning an array of the results of invoking -valueForKey: with the same key
> argument on each of its elements.
>
>> This array result isn't convertible to the slider's numeric value by any
>> standard KVC mechanism, so you get the exception.
>
> That's the meat of the issue.
>
>
>>> I have tried binding the NSSlider's enable to "arrangedObjects.@count" and
>>> "arrangedObjects.canRemove". Neither fixes the problem. If I bind the value
>>> to "selection.propertyName", the enabling/disabling works as expected (i.e.
>>> the slider is greyed out until the array has at least one entry). Hence I
>>> am confused.
>>
>> There's an important difference between "selection" and "arrangedObjects".
>> The latter is an array, but "selection" is a proxy object that represents
>> either a single object from the controller (*the* selected object), or a
>> special multiple-selection marker, or a special invalid value marker, or a
>> special no-value marker. The special markers are used (for example) for
>> making text fields show "multiple selection" or "no value" messages, and for
>> automatically enabling/disabling controls as you noted.
>
> You correctly diagnosed that the exception was due to the inability to make a
> double from the array. That's the whole of the necessary explanation for the
> symptom. Tobias had simply made an incorrect assumption that it was due to
> the array being empty, but neither that nor the no-selection marker from the
> selection property are relevant.
>
> It is true that the automatic enabling/disabling when bound to
> selection.propertyName is due to the selection's markers and the
> Conditionally Sets Enabled bindings option (which is on by default). But
> that doesn't resolve the confusion about why it was throwing an exception in
> some cases and not others.
>
>
>> Again, there's a complication. Table view *column* bindings are special in
>> that they appear to be able to bind through collection objects, but in fact
>> their key paths are treated as 2 separate parts by the bindings mechanism.
>> The first part allows the column to find the array of values for all table
>> rows, and to choose the correct value per row based on the row index; the
>> second part allows the column to find the actual value to display.
>
> Table columns do have special handling for array-valued bindings. It's that
> they distribute the array values among their rows. Table columns are not
> special in being able to bind through collection properties. That's a
> feature of NSArrayController.
>
> The table column's bindings still have just an observed object (often the
> array controller) and a single key path (often of the form
> "arrangedObject.propertyName"). It's just that, when they receive an array
> from [observedObject valueForKeyPath:observedKeyPath], they know to pick one
> element from that array for each row.
>
>
>> It's not entirely clear what you mean. Are you saying you want to have a
>> single slider in your window, that somehow represents all the array
>> elements? If so, what value would the slider show when the array elements
>> have different values for the relevant property? Is there a table view
>> involved in this somewhere?
>
> Yes, these are the important questions. Tobias, what do you actually intend
> the slider to signify? What should happen when its value is adjusted by the
> user?
>
> Regards,
> Ken
>
_______________________________________________
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
