Can I change the type of a property in a class extension? - objective-c

Question

I wish to expose a property with a generic type in a public header, and then change its type to a more specific one in a private header. I'm using Clang, but though I'm able to change its read/write property, it doesn't accept a different type. So far this is what I tried:
A common client would import BKSystem.h:
#interface BKSystem : NSObject
#property(nonatomic, readonly) id<XYZWorker> worker;
#end
While a client for testing has access to internals by importing BKSystem+Testing.h:
#import "BKSystem.h"
#interface BKConfigurableWorker : NSObject<XYZWorker>
#property(nonatomic) BKConfiguration *config;
#end
#pragma mark -
#interface BKSystem ()
// Attempts to change worker to be writable and with a more specific type.
#property(nonatomic, readwrite) BKConfigurableWorker *worker;
#end
But on a testing client this is what I get:
#import "BKSystem+Testing.h"
BKSystem *system = [[BKSystem alloc] init];
// I am able to write to this property.
system.worker = [[BKConfigurableWorker alloc] init];
// ERROR: Property 'config' not found on object of type 'id<XYZWorker>'
system.worker.config = [[BKConfiguration alloc] init];

Answer 1

To accomplish what you're trying to do, I'd use BKWorker as a subclass rather than a protocol. See my example header file below:
#interface BKConfiguration : NSObject
#end
#interface BKWorker : NSObject
#property (nonatomic, strong, readonly) BKConfiguration *config;
#end
#interface BKConfigurableWorker : BKWorker
#property (nonatomic, strong, readwrite) BKConfiguration *config;
#end
Notice that the same property is declared again in the configurable worker, but as readwrite rather than readonly.
This produces the following results when used in an implementation:
Notice how the configurable worker can be written to but the standard one can not.
Hope this helps.