Achieving Loose Coupling with Delegates and Data Sources

Objects frequently need to communicate without creating tight dependencies. Objective‑C’s protocol‑based delegation pattern provides a clean12 way to define a contract that a receiving object agrees to fulfill. The delegate protocol typical follows CamelCase naming and ends with the word Delegate, e.g. DataLoaderDelegate.

A delegate protocol declares methods that the owning object will call when it needs to inform or request data from the delegate. Methods are usually marked @optional because not every delegate cares about all notifications.

@protocol DataLoaderDelegate <NSObject>
@optional
- (void)loader:(DataLoader *)loader didFinishWithResult:(id)result;
- (void)loader:(DataLoader *)loader didEncounterError:(NSError *)error;
@end

The object that accepts the delegate role holds a reference to the delegate through a property. To avoid retain cycles, this property must be weak.

@interface DataLoader : NSObject
@property (nonatomic, weak) id<DataLoaderDelegate> delegate;
- (void)startAsyncTask;
@end

When a class internally adopts a delegate protocol but does not wish to expose that fact publicly, it can conform in a class extension inside the implementation file. This hides the implementation detail.

@interface MyController () <DataLoaderDelegate>
@end

@implementation MyController

- (void)performAction {
    DataLoader *loader = [[DataLoader alloc] init];
    loader.delegate = self;
    [loader startAsyncTask];
}

- (void)loader:(DataLoader *)loader didFinishWithResult:(id)result {
    // Process result
}

- (void)loader:(DataLoader *)loader didEncounterError:(NSError *)error {
    // Handle error
}

@end

A similar pattern, the data source protocol, is31 used when one object needs to provide data to another. The naming convention changes to DataSource (e.g. TableViewDataSource) and the methods typically return values rather than being purely2 event-driven29. The relationship remains non‑owning, and the data source property is also declared weak.

Structuring Class Implementations with Categories

As a class grows, placing all methods in a single implementation file becomes unwieldy28. Objective‑C categories allow you to split a class’s code across multiple logical sections, improving readability and maintainability.

Consider a class that models a user profile and must handle parsing, network requests, and UI updates. Instead of one monolithic @implementation, use distinct categories in separate files:

// UserProfile+Parsing.m
@implementation UserProfile (Parsing)
- (instancetype)initWithJSON:(NSDictionary *)json { /* ... */ }
- (NSDictionary *)toDictionary { /* ... */ }
@end

// UserProfile+Networking.m
@implementation UserProfile (Networking)
- (void)syncWithServer { /* ... */ }
@end

// UserProfile+Interface.m
@implementation UserProfile (Interface)
- (void)prepareViewModels { /* ... */ }
@end

Each category groups related functionality, making it easier to navigate the codebase and isolate15 bugs. When using categories for internal organization,12 the methods can remain private by not exposing them in the public header, keeping the external interface clean7.

Delegation and data source patterns, combined with well‑organized categories, form a robust foundation for maintainable Objective‑C projects.