Re: File's Owner
Re: File's Owner
- Subject: Re: File's Owner
- From: Andy Lee <email@hidden>
- Date: Fri, 23 May 2008 16:55:13 -0400
On May 23, 2008, at 2:49 PM, Johnny Lundy wrote:
I decided to once again go verbatim through Cocoa Fundamentals. So
far it has been largely "uh-huh, yep", but the File's Owner, my
nemesis, comes up again.
I tried to answer your File's Owner and NSApp questions last week
(copied below). If you can indicate at what point in my explanations
you start getting lost, I can try to explain better.
Again, I'm sensing a very fundamental disconnect, perhaps more
fundamental than you realize. Anybody can recite documentation, so
there's no need to tell us that and no need to recite the
documentation. I think it would be more helpful to tell us at what
*part* of the documentation you start to get lost, and what
specifically is confusing about it, perhaps as relates to your other
programming experience. If the answer is that you draw a complete
blank, then we shouldn't be addressing that issue at all; you need to
step back to a more fundamental level and see if you feel fluent at
that level.
--Andy
Here's my reply from last week:
On May 15, 2008, at 11:11 AM, Andy Lee wrote:
On May 14, 2008, at 10:33 PM, Johnny Lundy wrote:
So it's the application instance. I don't understand what the
application instance is. I can recite the documentation, which says
it is a shared instance of NSApplication, also known as NSApp which
is a global variable, none of which help me.
To me this indicates a very fundamental disconnect, because what you
recited explains NSApp perfectly. I can't tell where the disconnect
is, so I'll try to spell this out quickly. No offense intended here
-- I honestly don't know where the disconnect is.
You are writing a Cocoa application. There is a class called
NSApplication that represents the concept of a Cocoa application.
That's the purpose of a class in an object-oriented language, to
describe a *class* of things conceptually. Classes are usually
named for the thing they describe -- hence "NSApplication".
NSApplication handles the generic things that Cocoa applications do:
managing the main menu, managing the window list, dealing with mouse
and keyboard events, and so on.
Cocoa applications create objects (instances of classes). That's
what they do all day. Objects get created, they send messages to
each other, they get dealloc'ed. Thousands and thousands of
objects, all day long. In your application there is one special
object that is an instance of NSApplication. It represents the
application itself. I repeat: there is an object in your
application that represents the running application itself. There
is only one such object, and it is stored in a global variable
called NSApp. That is what "application instance" means.
Why would someone bind to File's Owner? What specifically is it?
Unlike most other objects in a nib file (First Responder being the
exception), File's Owner is not a specific object. It is a
placeholder for an "owner" object that will be specified when the
nib file is loaded. You edit a nib file knowing that it will be
loaded at some point in your program, and *at that future time* the
owner of the file (hence "File's Owner") will be specified.
The file's owner gives your program access to the objects in the nib
file, through the connections you made to it in the nib.
I think newcomers sometimes assume that when you load a nib you get
some sort of NibObject that you can query to get at the objects in
the nib, perhaps using the name you assigned in the nib file. This
is an understandable assumption, but you don't get any such object.
You need to specify a file's owner for the nib (using the second
argument to +loadNibNamed:owner:), and you need to have constructed
the nib so it connects the file's owner to at least one of its
objects.
--Andy
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| | >File's Owner (From: Johnny Lundy <email@hidden>) |