@interface MySingleton : NSObject
{
  SomeClass* _someClass;
}

@property (class, readonly, strong) Class_B* shared;

@end

@protocol SomeProtocol
@end

@interface SomeClass : NSObject <SomeProtocol>
@end

@interface MySingleton : NSObject
{
  id<SomeProtocol> _someClass;
}
@property (class, readonly, strong)MySingleton* shared;

- (instancetype)initWithSomeClass:(id<SomeProtocol>)someClass;
@end

@implementation MySingleton

+ (MySingleton*)shared {

  static MySingleton* mySingleton;
  static dispatch_once_t predicate;

  dispatch_once(&predicate, ^{
    SomeClass* someClass = [[SomeClass alloc] init];
    mySingleton = [[MySingleton alloc] initWithSomeClass:someClass];
  });

  return mySingleton;
}

- (instancetype)initWithSomeClass:(id<SomeProtocol>)someClass {
  self = [super init];
  if (self) {
    _someClass = someClass;
  }
  return self;
}
@end

@interface someClassFake : NSObject <SomeProtocol>
@end

SomeClassFake* someFakeClass = [[SomeClassFake alloc] init];
MySingletone* mySingleton = [[MySingleton alloc] initWithObject: someClassFake];

@interface MySingleton : NSObject
{
  id<SomeProtocol> _someClass;
}
@property (class, readonly, strong)MySingleton* shared;

@end

@implementation MySingleton

+ (MySingleton*)shared {

  static MySingleton* mySingleton;
  static dispatch_once_t predicate;

  dispatch_once(&predicate, ^{
    mySingleton = [[MySingleton alloc] init];
  });

  return mySingleton;
}

- (void)setSomeClass:(id<SomeProtocol>)someClass {
  _someClass = someClass;
}

@end

SomeClass* someClass = [[SomeClass alloc] init];
[MySingleton.shared setSomeClass:someFakeClass];

SomeClassFake* someFakeClass = [[SomeClassFake alloc] init];
[MySingleton.shared setSomeClass:someFakeClass];

@interface MySingleton (Shared)
@property (class, readonly, strong)MySingleton* shared;
@end

@implementation MySingleton (Shared)

    + (MySingleton*)shared {

      static MySingleton* mySingleton;
      static dispatch_once_t predicate;

      dispatch_once(&predicate, ^{
        SomeClass* someClass = [[SomeClass alloc] init];
        mySingleton = [[MySingleton alloc] initWithSomeClass:someClass];
      });

      return mySingleton;
    }
    @end