Objective-C Protocols & Delegation

Protocols & Delegation in Objective-C

Protocols define interfaces that can be implemented by any class, while delegation is a design pattern that uses protocols to enable communication between objects.

Protocols declare methods that can be implemented by any class, similar to interfaces in other languages. Delegation is a design pattern where one object acts on behalf of another.

Declaring Protocols


// NetworkFetcher.h
@protocol NetworkFetcherDelegate <NSObject>

@required
- (void)fetcher:(id)fetcher didReceiveData:(NSData *)data;

@optional
- (void)fetcher:(id)fetcher didFailWithError:(NSError *)error;
- (void)fetcherDidStart:(id)fetcher;
- (void)fetcherDidFinish:(id)fetcher;

@end

@interface NetworkFetcher : NSObject

@property (weak, nonatomic) id<NetworkFetcherDelegate> delegate;

- (void)startFetching;

@end

Note: The <NSObject> syntax means the protocol conforms to the NSObject protocol, which is common practice for most protocols.

Implementing Protocols


// ViewController.h
#import "NetworkFetcher.h"

@interface ViewController : UIViewController <NetworkFetcherDelegate>
@end

// ViewController.m
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    NetworkFetcher *fetcher = [[NetworkFetcher alloc] init];
    fetcher.delegate = self;
    [fetcher startFetching];
}

#pragma mark - NetworkFetcherDelegate

- (void)fetcher:(id)fetcher didReceiveData:(NSData *)data {
    // Handle received data
    NSLog(@"Received %lu bytes", (unsigned long)data.length);
}

- (void)fetcher:(id)fetcher didFailWithError:(NSError *)error {
    // Handle error (optional)
    NSLog(@"Error: %@", error.localizedDescription);
}

@end

Checking Protocol Conformance


if ([self.delegate conformsToProtocol:@protocol(NetworkFetcherDelegate)]) {
    if ([self.delegate respondsToSelector:@selector(fetcherDidStart:)]) {
        [self.delegate fetcherDidStart:self];
    }
}

Protocol Inheritance


@protocol Scrollable <NSObject>
- (void)scrollToTop;
@end

@protocol Refreshable <Scrollable>
- (void)refreshData;
@end

// A class adopting Refreshable must implement:
// - refreshData
// - scrollToTop
// - NSObject protocol methods

Delegation Pattern

Delegation Pattern Components

Delegate Protocol: Defines the methods the delegate should implement
Delegate Property: A weak reference to the delegate object
Delegate Implementation: The object that implements the protocol methods


// TableViewManager.h
@protocol TableViewManagerDelegate <NSObject>

- (NSInteger)numberOfRows;
- (UITableViewCell *)cellForRowAtIndex:(NSInteger)index;
@optional
- (void)didSelectRowAtIndex:(NSInteger)index;

@end

@interface TableViewManager : NSObject

@property (weak, nonatomic) id<TableViewManagerDelegate> delegate;

- (void)reloadData;

@end

Multiple Protocol Conformance


@interface MyViewController : UIViewController <UITableViewDelegate, UITableViewDataSource, UIScrollViewDelegate>

@end

Protocols as Types


// Can use protocols as parameter and return types
- (id<Drawable>)createDrawableShape;
- (void)renderShape:(id<Drawable>)shape;

// Property with protocol type
@property (strong, nonatomic) id<Parser> dataParser;

Common Cocoa Delegation Patterns

Class	Protocol	Purpose
UITableView	UITableViewDelegate, UITableViewDataSource	Table view management
UICollectionView	UICollectionViewDelegate, UICollectionViewDataSource	Collection view management
UITextField	UITextFieldDelegate	Text field events
UIApplication	UIApplicationDelegate	App lifecycle events
CLLocationManager	CLLocationManagerDelegate	Location updates

Blocks vs Delegation

Criteria	Delegation	Blocks
Complexity	Better for complex, multi-method interfaces	Better for simple, one-off callbacks
Memory	Weak references prevent retain cycles	Need careful memory management
Readability	Methods grouped in protocol	Inline logic can be harder to follow
Flexibility	Single delegate	Multiple handlers possible

Best Practices

Always declare delegate properties as weak to avoid retain cycles
Use the respondsToSelector: check for optional methods
Name delegate methods clearly indicating the sender:
- Good: tableView:didSelectRowAtIndexPath:
- Bad: didSelectRow:
Group related protocol methods with #pragma mark
Consider using blocks for simple, one-time callbacks
Document whether your protocol methods are called on the main thread

Warning: Forgetting to set the delegate property or not implementing required methods are common sources of bugs. Always double-check these when debugging.

Advanced: Informal Protocols


// Before formal protocols existed, categories on NSObject were used
@interface NSObject (MyInformalProtocol)
- (void)informalMethod;
@end

// Implementation would check with respondsToSelector:
if ([object respondsToSelector:@selector(informalMethod)]) {
    [object informalMethod];
}

Note: Informal protocols are largely obsolete now that formal protocols support optional methods.

Real-World Example: Custom Delegation


// PrinterManager.h
@protocol PrinterManagerDelegate <NSObject>

- (void)printerManager:(PrinterManager *)manager didFinishPrinting:(BOOL)success;
- (NSInteger)numberOfPagesToPrint;
@optional
- (void)printerManagerDidStart:(PrinterManager *)manager;

@end

@interface PrinterManager : NSObject

@property (weak, nonatomic) id<PrinterManagerDelegate> delegate;

- (void)startPrinting;

@end

// PrinterManager.m
@implementation PrinterManager

- (void)startPrinting {
    if ([self.delegate respondsToSelector:@selector(printerManagerDidStart:)]) {
        [self.delegate printerManagerDidStart:self];
    }
    
    NSInteger pages = [self.delegate numberOfPagesToPrint];
    BOOL success = [self printPages:pages];
    
    [self.delegate printerManager:self didFinishPrinting:success];
}

@end

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