Java-enum style classes in Objective-C?

Question

I am new to Obj-C so forgive me if this is a stupid question:

How do I implement some in the style of Javas enums? Or to be more precise:

I want a class with some known properties which are fix at compile time and unique per instance. Additionally I only want one instance type.

Let me give an example in Java:

public enum MessageTypes {
  DEFAULT("white", "standard", 1),
  EXPRESS("red", "expressMessage", 2),
  BORADCAST("green", "broadcast", 3);

  String color; String tagName; int dbId;
  MessageTypes(String color, String tagName, int dbId) {
    // you get the idea
  }
  //some methonds like getEnumByTagName
}

How would you do something like this in Objective-C? Am I missing something? Is this a bad pattern at all?

Thanks in advance!

EDIT: I am sorry, if I did not made myself clear. I know, that obj-c enums are not what I am looking for (as they are only marginally more than a typedef to an int).

I would like to create a set of (kind-of-singleton, immutable) instances of a specific class. The singleton pattern in Apples Dev-Docs is of no use as I want multiple distinct instances of a class each with individual values in their properties.

The goal of that is to have multiple Message types (about 20) that can be assigned to a Message as a property. Each of my Message types has a (fix and predefined) color, attribute-value (in an XML-representation) and a numerical ID.

In Java, I would use an enum as in my code sample. But how do I create different MessageTypes and associate them with their properties in Obj-C?

Creating 20 Sublcasses of MessageType (each with a singleton-instance holding the properties) seems like a lot of work for such a simple task and total overkill.

My current approach is to create a class with an NSArray holding the different instances. Up on first access of a method like +(id)messageTypeForId:NSInteger id_ the NSArray is prepopulated. But this feels totally clumsy and not at all elegant...

Is there a more satisfying approach?

Answer 1

There is not much in the way of a "more satisfying approach".

The normal Cocoa pattern would be to create methods like:

+ (MessageTypes*) sharedDefaultMessageType;
+ (MessageTypes*) sharedExpressMessageType;
+ (MessageTypes*) sharedBroadcastMessageType;
etc

and then implement them something like:

+ (MessageTypes*) sharedDefaultMessageType
{
   static MessageTypes* thisMessageType = nil;
   if ( !thisMessageType ) {
      thisMessageType = [[MessageTypes alloc] initWithColor:@"white" tagName:@"standard" dbId:1];
   }
   return thisMessageType;
}

Alternatively, storing the shared MessageType* in an NSMutableArray or NSMutableDictionary or precalculating them as you are doing are all equally valid approraches.

Note that the above "template" method could be generated via a macro such that you could write in the .m file:

CREATEMESSAGETYPE( Default, @"white", @"standard", 1 )
CREATEMESSAGETYPE( Express, @"red", @"expressMessage", 2 )
CREATEMESSAGETYPE( Broadcast, @"green", @"broadcast", 3 )

which might be "more satisfying" or more ugly, depending on your point of view.

Answer 2

I think I'd just use a standard C enum:

typedef enum { MT_WHITE, MT_RED, MT_GREEN } MessageType;

Then you just use it as you would any other data type:

@interface Blah {}

-(void) setMessageType:(MessageType)newMessageType;

@end