macOS "singleton" for each document

[MrFusion]

Is it possible to have maximum one instance of a custom class for each NSDocument? If so, how would you guarantee such a thing?

 

[chown33]

Subclass NSDocument. Give it a read-only property that returns the custom "only-one-per-NSDocument" class. In the getter for the property, only make an instance of the custom class when the ivar supporting it is null. Manage ownership to match the NSDocument instance lifetime.

http://developer.apple.com/library/...ptual/Documents/Tasks/SubclassNSDocument.html

If you can't subclass NSDocument, then it's harder. You have to correlate each NSDocument instance with a single instance of your custom class. An NSMutableDictionary can do the correlation, but you won't be able to use an NSDocument instance as the key, because a copy will be stored as the key. You can use the object-address of the NSDocument to make an NSNumber, and use that as the key. Then you're basically using the object's pointer as a key.

There's probably an NSSomethingOrOther collection class that does this mapping between any-pointer-as-key and any-other-pointer-as-value, but I'm not recalling what the class name is. It would behoove you to read the Collections Programming Guide. It would behoove me to do the same, but I have less vested interest in doing so right now.

 

[Sydde]

If you can't subclass NSDocument, then it's harder.

"can't subclass NSDocument" makes no sense. NSDocument is an abstract class that merely provides the basic conveniences you need to handle a document-based application. On it's own, it has no useful functionality, you have to subclass it to be able to do anything with it.

 

[MrFusion]

Subclass NSDocument. Give it a read-only property that returns the custom "only-one-per-NSDocument" class. In the getter for the property, only make an instance of the custom class when the ivar supporting it is null. Manage ownership to match the NSDocument instance lifetime.

http://developer.apple.com/library/...ptual/Documents/Tasks/SubclassNSDocument.html

If you can't subclass NSDocument, then it's harder. You have to correlate each NSDocument instance with a single instance of your custom class. An NSMutableDictionary can do the correlation, but you won't be able to use an NSDocument instance as the key, because a copy will be stored as the key. You can use the object-address of the NSDocument to make an NSNumber, and use that as the key. Then you're basically using the object's pointer as a key.

There's probably an NSSomethingOrOther collection class that does this mapping between any-pointer-as-key and any-other-pointer-as-value, but I'm not recalling what the class name is. It would behoove you to read the Collections Programming Guide. It would behoove me to do the same, but I have less vested interest in doing so right now.

Yes, that works provided that I only use "MyDocument" to retrieve an "only-one-per-NSDocument" instance. It doesn't prevent me (or someone else) from using the init method of the "only-one-per-NSDocument" and getting a second instance.
I will have a look at the Collections Programming Guide.

 

[Sydde]

Yes, that works provided that I only use "MyDocument" to retrieve an "only-one-per-NSDocument" instance. It doesn't prevent me (or someone else) from using the init method of the "only-one-per-NSDocument" and getting a second instance.
I will have a look at the Collections Programming Guide.

I think it would be as simple as this:

// in the implementation for your custom singleton class

- (id)init {
     [COLOR="Blue"]MyDocument[/COLOR]   *thisDocument = [[NSDocumentController sharedDocumentController] currentDocument];
     if ( ![thisDocument isKindOfClass:[COLOR="blue"]MyDocument[/COLOR]] ) {
          // safety check, just in case another kind of document might exist within your application
          // probably not needed
          [self release];
          self = nil;
     } else {
          if ( [thisDocument [COLOR="blue"]mySingletonClass[/COLOR]] != nil ) {
               // the object already exists, return the existing one
               [self release];
               self = [thisDocument [COLOR="blue"]mySingletonClass[/COLOR]];
          } else {
               // initialize the new instance
               self = [super init];

                /* the init code for you class goes here */

          }
     }
     return self;
}

(substituting your class names, of course)

Alternately (perhaps more effectively) you could write a class method that uses a similar technique, but creates and inits the singleton if it does not yet exist. In that case, the method should include an (id)sender parameter: if the sender is a view type object, the class method would use [[[sender window] windowController] document] instead of currentDocument to handle the possibility that the currentDocument is not what you want (e.g., a drag dropped on a window that is not the top one).

 

[MrFusion]

I think it would be as simple as this:

// in the implementation for your custom singleton class

- (id)init {
     [COLOR="Blue"]MyDocument[/COLOR]   *thisDocument = [[NSDocumentController sharedDocumentController] currentDocument];
     if ( ![thisDocument isKindOfClass:[COLOR="blue"]MyDocument[/COLOR]] ) {
          // safety check, just in case another kind of document might exist within your application
          // probably not needed
          [self release];
          self = nil;
     } else {
          if ( [thisDocument [COLOR="blue"]mySingletonClass[/COLOR]] != nil ) {
               // the object already exists, return the existing one
               [self release];
               self = [thisDocument [COLOR="blue"]mySingletonClass[/COLOR]];
          } else {
               // initialize the new instance
               self = [super init];

                /* the init code for you class goes here */

          }
     }
     return self;
}

(substituting your class names, of course)

Alternately (perhaps more effectively) you could write a class method that uses a similar technique, but creates and inits the singleton if it does not yet exist. In that case, the method should include an (id)sender parameter: if the sender is a view type object, the class method would use [[[sender window] windowController] document] instead of currentDocument to handle the possibility that the currentDocument is not what you want (e.g., a drag dropped on a window that is not the top one).

Brilliant! I will play around with these suggestions and see what fits best.