On Aug 2, 2009, at 18:07, Gregory Holden wrote:
1) NSArrayController and canRemove, how does it know?
A familiar example to those who ran through Aaron Hillegass'
Learning Cocoa Programming.
I have an NSArrayController, an NSButton, and an NSTableView
- NSArrayController's contentArray is an NSMutableArray in a
MyDocument class.
- NSButton sends the action 'remove:' to the NSArrayController on
click
- The column in NSTableView has a binding to NSArrayController to
display 'arrangedObjects.personName'
The last piece is the magical piece that is driving me nutty. The
NSButton has a binding enabled = NSArrayController's -(BOOL)canEnable.
I can't figure out how the NSArrayController knows when the
NSTableView has a row selected.
Forget about the NSArrayController for a moment. The NSTableView has a
"selectedIndexes" property which (in isolation) represents what rows
are actually selected. It also has a "selectedIndexes" *binding*,
which (when bound to another object) is automatically synchronized to
the other object's idea of what the selection is. (That's what
bindings are for, to synchronize properties between objects.)
Depending on the application, this "other" object might be a
controller (such as a NSWindowController) or the data model itself.
(Imagine, for example, an application that wanted to save the
selection in the document file -- the selection would be an attribute
of the data model. Or imagine, for example, an application with
multiple windows per document, each of which may have its own
selection -- the selection would likely be an attribute of the
NSWindowController.)
Now consider the NSArrayController. It's what's known as an
"mediating" controller -- that is, it links a "coordinating"
controller (such as a NSWindowController, or a NSViewController, or a
NSDocument) to a view object, providing additional services. (For
example, it provides the ability to sort and filter the data model
from the view's perspective.) One of its mediating services is to
synchronize the view's selection with the "coordinating" controller's
idea of what the selection is. So now we have the selection in 3
places: the view, the mediating controller, and the coordinating
controller or data model.
So the short answer to your question is: The NSArrayController knows
what the NSTableView has selected via the table view's
"selectedIndexes" binding, which operates automatically (though not
magically).
2) requires 'insertObject:in<key>AtIndex' for
'removeObjectFrom<key>AtIndex' to work
If I don't implement 'insertObject:in<key>AtIndex' then
'removeObjectFrom<key>AtIndex' is never called. Why? The only
reason I can come up with is possibly a shortcut in Cocoa that if
insert is not implemented then remove probably isn't either.
Which bring me to my next question, how does it know the insert is
implemented?
I overrode:
-(BOOL)respondsToSelector:(SEL)aSelector
but it never queries for the insert. What magic does it work with?
It is merely the definition of "KVC compliance" that an object with an
array property should implement *both* 'insertObject:in<key>AtIndex'
and 'removeObjectFrom<key>AtIndex'. (Actually, there are two
possible "insert" methods, and two possible "remove" methods, one of
each of which must be implemented.) Without them both, the object is
not KVC-compliant, and the lone implemented method is never called.
It's certainly possible that KVC doesn't use "respondsToSelector:" to
determine compliance. The are Objective-C runtime functions it can
call directly to find out the same information. How it finds out is an
implementation detail that we don't really need to know, though it of
course isn't magic either.
(I'd guess it doesn't call "respondsToSelector:" because it wants to
ensure that the "insert" and "remove" methods are both implemented in
the same class, and neither is inherited from a superclass. But that's
pure speculation, and could easily be false. I don't know how or why
it's implemented the way it is.)
HTH
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