Newbie Object Sharing Question
Newbie Object Sharing Question
- Subject: Newbie Object Sharing Question
- From: Brian Slick <email@hidden>
- Date: Fri, 30 Jan 2009 14:46:04 -0500
I'm sure there is a really easy concept somewhere that I'm just not
getting, but well I'm just not getting it. This is for the iPhone,
but I believe it is generic to Cocoa.
Let's say I'm making a task list program, and the primary UI is a
UITabBar. On one tab, I want a table view presenting the list a
certain way, and on another tab I want a table view presenting the
same list in a different way. Let's assume the user can create new
items in either view.
I envision a couple of model objects like so:
MyListItem
MyListItemArray
And a couple of view controllers:
ViewControllerTabA
ViewControllerTabB
I want both view controllers to reference MyListItemArray. So in each
one I do something like:
MyListItemArray *listArray = [[MyListItemArray alloc] init];
I then provide the various table delegate methods, and link those to
listArray. I fire up the program, and see a few items in my list. I
add a couple more items, then tap on the other view. The new items
are not there. Using the log I have verified that the array size is
the same as it started, so the new items simply are not there.
I throw in a few more log comments, trying to figure out exactly what
I have. I see the log messages for the initialization of the list
items, but then I notice when I tap the next tab, I see those same
initialization messages again. It starts to sink in that what I have
done is create a *local* instance of MyListItemArray in each view
controller, and what's local to ViewControllerTabA has nothing to do
with what is local to ViewControllerTabB. I'm getting local copies of
the array, not the source array.
It starts to occur to me that I don't actually want an instance, I
want the real deal. But I have no idea how to make that happen. How
do I grab a hold of MyListItemArray and use it in the table data
source methods without instantiating it?
I read through some documentation about model objects and objects in
general, and stumbled upon the concept of Singletons. Some additional
searching lead me to this blog post:
http://cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html
...which seems to describe exactly what I want. I reconfigured
MyListItemArray as a singleton, and remapped my data source methods
accordingly, and everything seemingly works perfectly. Items added in
one view are displayed in the other, and so on. It works so well that
I have to assume there is a catch.
I can't shake the feeling that this seems more difficult than in ought
to be, and generally when I feel that way there tends to be a single
line of code solution that I haven't found yet. Are singletons really
the only way (that doesn't involve saving to a file, I suppose) to
share a model object across multiple view controllers? I think I'm
missing something really fundamental.
Thanks,
Brian
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