objective c - Class Cluster as a Singleton? -

sorry length of post; meant document journey problem.

i have question shared object in cocoa app needs change time time , how best store it's accessible few different places. bear me.

class implementation

the shared object implemented class cluster (i.e., https://stackoverflow.com/a/2459385/327179) looks following (note document merely class name; not indicative of actual class does):

in document.h:

typedef enum {     documenttypea,     documenttypeb } documenttype;  @interface document : nsobject {} - (document *) initwithdocumenttype:(nsuinteger)documenttype; - (void) methoda; - (void) methodb; @end 

in document.m:

@interface documenta : document  - (void) methoda; - (void) methodb;  @end  @interface documentb : document  - (void) methoda; - (void) methodb;  @end  @implementation document  - (document *)initwithdocumenttype:(nsuinteger)documenttype; {     id instance = nil;     switch (documenttype) {         case documenttypea:             instance = [[documenta alloc] init];             break;         case documenttypeb:             instance = [[documentb alloc] init];             break;         default:             break;     }      return instance; }  - (void) methoda {     return nil; }  - (void) methodb {     return nil; }  @end  @implementation documenta  - (void) methoda {     // ... }  - (void) methodb {     // ... }  @end  @implementation documentb  - (void) methoda {     // ... }  - (void) methodb {     // ... }  @end 

how user interacts document

via menu item, user can switch between documenta , documentb @ will.

what happens when "switch" occurs

when user switches from, say, documenta documentb, need 2 things happen:

1. my primary nsviewcontroller (mainviewcontroller) needs able use new object.
2. my appdelegate needs update nstextfield happens located in content border of main window. (fwiw, can seem assign outlet nstextfield in appdelegate)

the question(s)

i've seen singletons mentioned quite bit way have global reference without cluttering one's appdelegate (primarily here , here). said, i've not seen info on overwriting such singleton (in our case, when user switches documenta documentb [or vice versa], global reference need hold new object). i'm not expert on design patterns, remember hearing singletons not meant destroyed , recreated...

so, given this, here questions:

1. how store class cluster (such mainviewcontroller , appdelegate can access appropriately)?
2. am mixing concerns having both mainviewcontroller (who uses document heavily) , appdelegate (who manages primary window [and thus, nstextfield]) have knowledge of document?

feel free let me know if i'm thinking problem incorrectly; want implementation orthogonal , correct possible.

thanks!

---

status update #1

thanks advice @jackyboy, here's route i've taken:

• document 1 that, upon "switching", "notifies" appdelegate , mainviewcontroller passing them newly created instance.
• both appdelegate , mainviewcontroller can update document object via singleton instance necessary.

here new files (dumbed down y'all can see crux of matter):

in document.h:

#import <foundation/foundation.h> @class appdelegate; @class mainviewcontroller;  typedef enum {     documenttypea,     documenttypeb } documenttype;  @interface document : nsobject  @property (weak, nonatomic) mainviewcontroller *mainviewcontrollerref; @property (weak, nonatomic) appdelegate *appdelegateref;  + (document *)sharedinstance; - (id)initwithparser:(nsuinteger)parsertype;  @end 

in document.m:

#import "appdelegate.h" #import "document.h" #import "mainviewcontroller.h"  @interface documenta : document  // ...  @end  @interface documentb : document  // ...  @end  @implementation document  @synthesize appdelegateref; @synthesize mainviewcontrollerref;  + (document *)sharedinstance {     static xparser *globalinstance;     static dispatch_once_t predicate;     dispatch_once(&predicate, ^{         // default, return documenta object (for no particular reason).         globalinstance = [[self alloc] initwithdocumenttype:documenta];     });      return globalinstance; }  - (id)initwithdocumenttype:(nsuinteger)documenttype {     document *instance = nil;     switch (parsertype) {         case documenttypea:             instance = [[documenta alloc] init];             break;         case documenttypeb:             instance = [[documentb alloc] init];             break;         default:             break;     }      // question: right? have store these references     // every time new document type initialized?         self.appdelegateref = (appdelegate *)[nsapp delegate];     self.mainviewcontrollerref = self.appdelegateref.mainviewcontroller;      [self.appdelegateref parserswitchedwithparser:instance];     [self.mainviewcontrollerref parserswitchedwithparser:instance];      return instance; }  @end  @implementation xparser_nsxml  // ...  @end  @implementation documenta  // ...  @end 

should bothered fact document has knowledge of existence of appdelegate , mainviewcontroller? additionally, should bothered fact when document object updates, re-notifies both appdelegate , mainviewcontroller (even though 1 of initiated update)?

as always, appreciate everyone's eyeballs on quest ideal implementation continues. :)

---

status update #2

a comment @caleb helped me understand nsnotification-based setup lot less unwieldy particular problem.

thanks, all!

i don't see need shared object here, less singleton. need find current document @ arbitrary times many different objects? seems more have 2 objects (app delegate , view controller) both need know current document. notifications provide easy way manage that: whenever switch happens, can post nsnotification includes new document. objects need know current document have registered "document switch" notification, , when notification arrives can stash pointer document in instance variable or property.