Thanks a lot for the clarifications about the role of the NSArrayController. I
somehow got confused about the whole binding thing, its not so muddy now. Now,
the example I gave was a bad simplification of the real thing as the
"totalPrice" is more a "proposedTotalPrice". It's based on a calculation but
can/will be changed by the user.
So, what I need is a way to calculate a property, but let the user change it if
he wants. For example, having 3 values "a", "b" and "c" (a + b = c). The user
enters a value for "a" or "b", then "c" is calculated. If the user changes "c",
it now lose its dependance on "a" and "b". This would be easy if the 3 values
were in an NSTextField instead of a tableview as I could add an action to the
first 2 fields and none to the third.
Is this can be done using bindings or should I stop fighting the framework and
and use a datasource?
Thanks,
Andre Masse
On 28/07/2011, at 22:13 , Quincey Morris wrote:
> On Jul 28, 2011, at 15:48, Andre Masse wrote:
>
>> For example, lets say I have a tableview with 3 columns: quantity, unit
>> price and total. I want to calculate total using (quantity * unit price).
>> Pretty simple using a datasource but I've no idea how to do that with
>> bindings (using an NSArrayController).
>
> Well, that's the problem right there. You seem to think bindings have
> something to do with array controllers. They don't. (Bindings and array
> controllers are often used together, of course, both on the "content" side of
> the array controller and on the "arrangedObjects" side, but that's just a
> case of two things being used to complement each other.)
>
> When working with bindings, it helps to be very diligent in thinking in terms
> of properties.
>
> If you're supplying a table view's cells via bindings, then you must have a
> model property for *each* column. In your case, your data model is (or
> includes) an array property (say, "invoiceItems"), whose elements are objects
> (of class, say, "InvoiceItem") with properties "quantity", "unitPrice" and
> "totalPrice".
>
> If the total is always calculated from the other two properties, it is
> typically a "derived" property (one whose value is derived on the fly, rather
> than actually stored), but aside from that implementation detail, there's no
> difference in how you use the properties to populate your table.
>
> In code terms, the short answer to your question is:
>
>> + (NSSet) keyPathsForValuesAffectingTotalPrice {
>> return [NSSet setWithObjects: @"quantity", @"unitPrice", nil];
>> }
>>
>> - (NSInteger) totalPrice { // or whatever the correct data type is
>> return self.quantity * self.unitPrice;
>> }
>
> The second method provides the derived property value. The first method
> provides KVO compliance.
>
> So how does the array controller enter this picture at all? It's a glue
> object (aka "mediating controller") whose function is to sort and filter the
> model array property. It has no role in sourcing actual data to your table***.
>
>
>
> *** It's certainly possible to *give* it a role, but I would argue that this
> is a terrible idea.
>
>
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