Re: Newbie Object Sharing Question
On Jan 30, 2009, at 12:46PM, Brian Slick wrote: It starts to occur to me that I don't actually want an instance, I want the real deal. On Jan 31, 2009, at 9:31AM, Brian Slick wrote: It may be a nonsensical statement here (good to know), but in the 3D CAD world that I'm used to, which I have been leveraging in my attempts to understand OOP, there is a distinction between instances and the source item. A working knowledge of 3D CAD programs does not translate directly into the learning of OOP concepts. In a 3D CAD assembly you could have a master object and multiple instances of that object. In OOP you have classes, instances, and pointers. Not exactly the same thing. I would recommend you forget about 3D CAD for a while and concentrate on the fundamentals of C and Objective-C. Best of luck. Richard ___ 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: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Re: Newbie Object Sharing Question
On Jan 31, 2009, at 10:31 AM, Brian Slick wrote:
And I believe that I do understand the difference between objects
and pointers (I could be mistaken). The part I'm missing is what
this needs to look like in code. I'm afraid I need some handholding
here. If I knew what I needed to do in order for there to be only a
single data object referenced from multiple places, I wouldn't have
asked the question in the first place.
OK, so I recommend that you read a good introductory book on the C
language, followed by Apple's documentation on Objective-C (and maybe
a third-party book on that, too), then the Cocoa Fundamentals Guide.
Well, at the very simplest, here's a trivial example:
NSString* foo = [[NSString alloc] initWithUTF8String:"Here is some
text"];
NSString* bar = foo;
Now, we have two pointers, foo and bar, which both refer to the same
object, a string whose content is "Here is some text". The first
statement allocated and initialized the string object, and assigned
the pointer foo to refer to it. The second line assigned the pointer
bar to refer to the same object. The assignment copied the pointer,
but did not copy the object.
Let's suppose we create a custom class declared like so:
@interface ThingAMaBob : NSObject
{
NSString* incantation;
}
-(id)initWithMagicalIncantation:(NSString*)anIncantation;
@end
and defined like so:
@implementation ThingAMaBob
-(id)initWithMagicalIncantation:(NSString*)anIncantation
{
if (self = [super init])
{
incantation = [anIncantation retain];
}
return self;
}
-(void)dealloc
{
[incantation release];
[super dealloc];
}
@end
We might have code in some other part of our program which goes like
this:
NSString* magic = @"Abracadabra!";
ThingAMaBob* thing1 = [[ThingAMaBob alloc]
initWithMagicalIncantation:magic];
Now, magic contains a pointer to a string object. We're creating a
new ThingAMaBob instance and we've passed a reference to that same
string object into the initializer. Within the initializer there's
the parameter anIncantation which is a pointer which receives that
passed-in reference. We invoke the -retain message on that string
object, which returns a reference to that same string object back, and
we assign the instance variable "incantation" to point to it. So, now
we have three pointers (magic, anIncantation, incantation) which all
refer to the same object.
After the initializer returns, there are just two because
anIncantation is no longer in scope. But it's still the case that
"magic" and the "incantation" instance variable of our new ThingAMaBob
refer to the same string object.
If we then proceed to do:
ThingAMaBob* thing2 = [[ThingAMaBob alloc]
initWithMagicalIncantation:magic];
then much the same thing happens. After that line executes, the
string object is referred to by "magic", the "incantation" instance
variable of the ThingAMaBob instance pointed to by thing1, and the
"incantation" instance variable of the ThingAMaBob instance pointed to
by thing2. So, thing1 and thing2 are sharing a reference to a single
string object (here I used a bit of shorthand; I referred to the
objects pointed to by the two pointers by just mentioning the names of
the pointers).
A singleton implementation is _one_ solution to the issue you're
having, but it's not necessary, and not even recommended for
something as simple as this.
Can you expand upon why not? Based on the blog post I linked, it
sounds like this is exactly the situation in which to do this.
Well, in your original post you already pointed out that it seemed
much more complicated than necessary. Since you can achieve what you
need by simply passing references to an object around, creating a
singleton class is overkill.
It is sometimes appropriate to create a singleton, but it can be
somewhat limiting. If you start out writing a program using a
singleton class, and later realize you need more than one instance of
the class, you will discover that you need to rewrite a bunch of code.
One stumbling block is that you have two view controllers, but
evidently no central application controller. View controllers
should have logic and state specific to a view and that view's
relationship to the model. An application controller manages your
application overall. It manages the other controllers, including
your view controllers. It has the primary responsibility for
managing the application-global (as opposed to, for example,
document-specific) model, and for providing access to that model to
other parts of the program.
Well, I just used the Tab Bar Application template, and other than
the AppDelegate that doesn't contain very much, there is no provided
application controller. The need to create one was not immediately
obvious to me, as none of the examples in the book I'm reading
Re: Newbie Object Sharing Question
On Jan 30, 2009, at 9:47 PM, Ken Thomases wrote: On Jan 30, 2009, at 2:46 PM, Brian Slick wrote: It starts to occur to me that I don't actually want an instance, I want the real deal. That statement is nonsensical. There is no "real deal". An instance is real. It's not some pale reflection of something else. You do want an instance. I suspect that perhaps you don't understand the difference between an object and a pointer to an object. The reason I suspect that is because the simple solution to your dilemma, the thing you're not seeing, is that you want a single instance referenced from multiple places using multiple pointers. It may be a nonsensical statement here (good to know), but in the 3D CAD world that I'm used to, which I have been leveraging in my attempts to understand OOP, there is a distinction between instances and the source item. And I believe that I do understand the difference between objects and pointers (I could be mistaken). The part I'm missing is what this needs to look like in code. I'm afraid I need some handholding here. If I knew what I needed to do in order for there to be only a single data object referenced from multiple places, I wouldn't have asked the question in the first place. 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. Yes, I think your are missing something fundamental -- the ability to share references to an object simply by assigning multiple pointers to point to the same thing. Yes, exactly. How do I do that? A singleton implementation is _one_ solution to the issue you're having, but it's not necessary, and not even recommended for something as simple as this. Can you expand upon why not? Based on the blog post I linked, it sounds like this is exactly the situation in which to do this. One stumbling block is that you have two view controllers, but evidently no central application controller. View controllers should have logic and state specific to a view and that view's relationship to the model. An application controller manages your application overall. It manages the other controllers, including your view controllers. It has the primary responsibility for managing the application-global (as opposed to, for example, document-specific) model, and for providing access to that model to other parts of the program. Well, I just used the Tab Bar Application template, and other than the AppDelegate that doesn't contain very much, there is no provided application controller. The need to create one was not immediately obvious to me, as none of the examples in the book I'm reading through have dealt with accessing the same data set in different tabs. I also do not yet see how this solves my problem. I need to see some code. From my view controllers, I don't believe that I know how to send messages to the AppDelegate (say, for the table data source methods) without creating an instance, which I believe brings me back around to the same problem I started with. If your application has one central list of items, then it would be the application controller's job to create and load that model. It would hold references to the model objects in instance variables. It would provide access to those model objects to other parts of the program. One way would be to pass the references in to the view controllers when those view controllers are initialized. Another way would be for the application controller to expose those references through properties and have the view controllers reference those properties -- the view controllers would need a reference to the application controller to do that. Again, they could get such a reference by being passed it, or if your application controller is your application delegate, they can obtain the reference using [NSApp delegate]. Regards, Ken Thank you for the explanation, but I'm afraid I still have no idea what I specifically need to do, or even what help topic to go search on for addit
Re: Newbie Object Sharing Question
Sorry I misunderstood the question then :) Just a newbie trying to lend a hand where I can. Thanks for correcting me. Joseph Crawford On Jan 30, 2009, at 9:47 PM, Ken Thomases wrote: On Jan 30, 2009, at 1:51 PM, Joseph Crawford wrote: Why don't you create a base controller that those 2 controllers subclass, that way they would inherit the functionality of reading those properties, also they would inherit the methods for getting, setting, etc. BaseController -- Controller 1 -- Controller 2 in Base controller define those properties and any methods for interacting with them in Controller 1 and Controller 2 have your Class specific stuff there, because they both inherit from BaseController you would get those properties. This is completely irrelevant to the original question. Having a common base class does not make two objects share state. On Jan 30, 2009, at 2:46 PM, Brian Slick wrote: I want both view controllers to reference MyListItemArray. You want both view controllers to reference _an instance_ of MyListItemArray. MyListItemArray is a class. So in each one I do something like: MyListItemArray *listArray = [[MyListItemArray alloc] init]; As you seem to have figured out, this is creating (allocating and initializing) a new instance of the class. This newly-created object is separate and distinct from other objects in your program, even ones created by execution of this same line of code at other times. [...] 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. As near as I can tell, there is no "source" array, nor is there any copying going on. You have explicitly created two separate arrays from the get-go. You have then filled them out separately to have similar initial contents. It starts to occur to me that I don't actually want an instance, I want the real deal. That statement is nonsensical. There is no "real deal". An instance is real. It's not some pale reflection of something else. You do want an instance. I suspect that perhaps you don't understand the difference between an object and a pointer to an object. The reason I suspect that is because the simple solution to your dilemma, the thing you're not seeing, is that you want a single instance referenced from multiple places using multiple pointers. 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. Yes, I think your are missing something fundamental -- the ability to share references to an object simply by assigning multiple pointers to point to the same thing. A singleton implementation is _one_ solution to the issue you're having, but it's not necessary, and not even recommended for something as simple as this. One stumbling block is that you have two view controllers, but evidently no central application controller. View controllers should have logic and state specific to a view and that view's relationship to the model. An application controller manages your application overall. It manages the other controllers, including your view controllers. It has the primary responsibility for managing the application-global (as opposed to, for example, document-specific) model, and for providing access to that model to other parts of the program. Since you were concentrating on the view controllers, you thought you had to create the instance of MyListItemArray in each view controller. It follows that you got one such instance for each view controller. You should have a single application controller object (an instance of some custom class). This application controller is often also the application delegate. It is also often instantiated in the main nib, where the "delegate" outlet of the main application object is connected to it. If your application has one central li
Re: Newbie Object Sharing Question
On Jan 30, 2009, at 1:51 PM, Joseph Crawford wrote: Why don't you create a base controller that those 2 controllers subclass, that way they would inherit the functionality of reading those properties, also they would inherit the methods for getting, setting, etc. BaseController -- Controller 1 -- Controller 2 in Base controller define those properties and any methods for interacting with them in Controller 1 and Controller 2 have your Class specific stuff there, because they both inherit from BaseController you would get those properties. This is completely irrelevant to the original question. Having a common base class does not make two objects share state. On Jan 30, 2009, at 2:46 PM, Brian Slick wrote: I want both view controllers to reference MyListItemArray. You want both view controllers to reference _an instance_ of MyListItemArray. MyListItemArray is a class. So in each one I do something like: MyListItemArray *listArray = [[MyListItemArray alloc] init]; As you seem to have figured out, this is creating (allocating and initializing) a new instance of the class. This newly-created object is separate and distinct from other objects in your program, even ones created by execution of this same line of code at other times. [...] 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. As near as I can tell, there is no "source" array, nor is there any copying going on. You have explicitly created two separate arrays from the get-go. You have then filled them out separately to have similar initial contents. It starts to occur to me that I don't actually want an instance, I want the real deal. That statement is nonsensical. There is no "real deal". An instance is real. It's not some pale reflection of something else. You do want an instance. I suspect that perhaps you don't understand the difference between an object and a pointer to an object. The reason I suspect that is because the simple solution to your dilemma, the thing you're not seeing, is that you want a single instance referenced from multiple places using multiple pointers. 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. Yes, I think your are missing something fundamental -- the ability to share references to an object simply by assigning multiple pointers to point to the same thing. A singleton implementation is _one_ solution to the issue you're having, but it's not necessary, and not even recommended for something as simple as this. One stumbling block is that you have two view controllers, but evidently no central application controller. View controllers should have logic and state specific to a view and that view's relationship to the model. An application controller manages your application overall. It manages the other controllers, including your view controllers. It has the primary responsibility for managing the application-global (as opposed to, for example, document-specific) model, and for providing access to that model to other parts of the program. Since you were concentrating on the view controllers, you thought you had to create the instance of MyListItemArray in each view controller. It follows that you got one such instance for each view controller. You should have a single application controller object (an instance of some custom class). This application controller is often also the application delegate. It is also often instantiated in the main nib, where the "delegate" outlet of the main application object is connected to it. If your application has one central list of items, then it would be the application controller's job to create and load that model. It would hold references to the model objects in instance variables. It would provide access
Re: Newbie Object Sharing Question
Why don't you create a base controller that those 2 controllers subclass, that way they would inherit the functionality of reading those properties, also they would inherit the methods for getting, setting, etc. BaseController -- Controller 1 -- Controller 2 in Base controller define those properties and any methods for interacting with them in Controller 1 and Controller 2 have your Class specific stuff there, because they both inherit from BaseController you would get those properties. Joseph Crawford On Jan 30, 2009, at 2:46 PM, Brian Slick wrote: 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 ___ 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: http://lists.apple.com/mailman/options/cocoa-dev/codebowl%40gmail.com This email sent to codeb...@gmail.com ___ 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: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Newbie Object Sharing Question
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 ___ 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: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
