The power of protocols to avoid concrete implementations

protocolDemo.m

// Model
// OAStack.m
@protocol OAStack

- (void) pushData:(id)data;
- (id)   popData;

@optional

- (void) purgeStack;

@end

// Model
// OAStackWithArray.m

@implementation OAStackWithArray : NSObject <OAStack>

@property (nonatomic, strong) NSMutableArray *containerArray;
@property (nonatomic, assign) NSUInteger currentIndex;

- (id)init
{
  self = [[super alloc] init];
	if(self){
		self.containerArray = [NSMutableArray array];
		self.currentIndex = 0;
	}
	return self;
}

- (void) pushData:(id)data
{
	self.containerArray[self.currentIndex] = data;
	self.currentIndex++;
}

- (id) popData
{
	NSUInteger idx = self.currentIndex;
	self.currentIndex--;
	self.currentIndex = self.currentIndex < 0 ? 0 : self.currentIndex;
	return self.cointainerArray[idx];
}

@end

// Model
// OAStackWithDictionary.m

@implementation OAStackWithDictionary : NSObject <OAStack>

@property (nonatomic, strong) NSMutableDictionary *containerDictionary;
@property (nonatomic, assign) NSUInteger currentIndex;

- (id)init
{
  self = [[super alloc] init];
	if(self){
		self.containerDictionary = [NSMutableDictionary dictionary];
		self.currentIndex = 0;
	}
	return self;
}

- (void) pushData:(id)data
{

	[self.containerDictionary setObject:data forKey:[NSString stringWithFormat: @"%d", self.currentIndex]];
	self.currentIndex++;
}

- (id) popData
{
	NSUInteger idx = self.currentIndex;
	self.currentIndex--;
	self.currentIndex = self.currentIndex < 0 ? 0 : self.currentIndex;
	return [self.containerDictionary objectForKey:[NSString stringWithFormat: @"%d", idx]];
}

- (void) purgeStack
{
	self.currentIndex = 0;
	self.containerDictionary = [NSMutableDictionary dictionary];
}

@end

// Model
// OAStackHelper.m

@implementation OAStackHelper : NSObject

+ (id<OAStack>) createArrayBasedStack
{
	return [[OAStackWithArray alloc] init];
}

+ (id<OAStack>) createDictionaryBasedStack
{
	return [[OAStackWithDictionary alloc] init];
}


@end

// Controller
// OAStackViewController.m

@implementation OAStackViewController : UIViewController

@property (nonatomic, strong) id<OAStack> stack;

- (void) viewDidLoad
{
	NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
	NSString *stackType = [defaults objectForKey:@"stackType"];
	if([stackType isEqualToString:@"ARRAY"]){
		self.stack = [OAStackHelper createArrayBasedStack];
	} 
	else if ([stackType isEqualToString:@"DICT"]){
		self.stack = [OAStackHelper createDictionaryBasedStack];
	}
		
}

- (IBAction) pushDataToStack:(id)sender
{
	UIButton *addButton = (UIButton*)sender;
	[self.stack pushData:addButton.tag];
}

- (IBAction) popDataFromStack:(id)sender
{
	id data = [self.stack popData];
}

- (IBAction) clearData:(id)sender
{
	if(self.stack respondsToSelector:@selector(purgeStack)){
		[self.stack purgeStack];
	}
}

@end