Formal protocols in Objective-C are a powerful feature that define a set of methods that a class can choose to implement. They serve as a contract between classes, ensuring that certain methods are available for use.

What are Formal Protocols?

Formal protocols declare a list of methods that any conforming class must implement. They're similar to interfaces in other programming languages and play a crucial role in Objective-C Polymorphism.

Defining a Formal Protocol

To define a formal protocol in Objective-C, use the @protocol keyword followed by the protocol name and a list of method declarations:

@protocol MyProtocol
- (void)requiredMethod;
@optional
- (void)optionalMethod;
@end

In this example, requiredMethod must be implemented by any class adopting the protocol, while optionalMethod is optional.

Adopting and Implementing Protocols

To adopt a protocol, list it in angle brackets after the superclass name in the class declaration:

@interface MyClass : NSObject <MyProtocol>
@end

@implementation MyClass
- (void)requiredMethod {
    // Implementation here
}
@end

Benefits of Formal Protocols

Enforce a contract between classes
Enable polymorphism without inheritance
Improve code organization and modularity
Facilitate delegation patterns

Protocol Composition

Objective-C allows a class to adopt multiple protocols, enabling a form of multiple inheritance for method declarations:

@interface MyClass : NSObject <ProtocolA, ProtocolB>
@end

Protocol Extensions

While not as powerful as Swift protocol extensions, Objective-C allows you to provide default implementations for protocol methods using Objective-C Categories:

@interface NSObject (MyProtocolDefaults) <MyProtocol>
@end

@implementation NSObject (MyProtocolDefaults)
- (void)optionalMethod {
    // Default implementation
}
@end

Best Practices

Use protocols to define interfaces for delegates and data sources
Keep protocols focused on a single responsibility
Consider using Objective-C Informal Protocols for backward compatibility
Document protocol methods thoroughly, especially optional ones

Conclusion

Formal protocols are a cornerstone of Objective-C programming, enabling flexible and modular code design. They're extensively used in iOS and macOS development, particularly in delegate patterns and framework APIs. Understanding and effectively using protocols is crucial for writing clean, maintainable Objective-C code.