Protocol — what it is, protocols and delegates in iOS

protocol Drawable {
    var boundingRect: CGRect { get }
    mutating func draw(in context: CGContext)
}

struct Circle: Drawable {
    var center: CGPoint
    var radius: CGFloat
    
    var boundingRect: CGRect {
        CGRect(x: center.x - radius, y: center.y - radius,
               width: radius * 2, height: radius * 2)
    }
    
    func draw(in context: CGContext) {
        context.addEllipse(in: boundingRect)
    }
}

protocol Container {
    associatedtype Item
    var count: Int { get }
    mutating func append(_ item: Item)
    subscript(i: Int) -> Item { get }
}

struct IntStack: Container {
    var items: [Int] = []
    var count: Int { items.count }
    typealias Item = Int
    
    mutating func append(_ item: Int) {
        items.append(item)
    }
    
    subscript(i: Int) -> Int {
        items[i]
    }
}

func sum<C: Container>(_ container: C) -> C.Item where C.Item: Numeric {
    // function body
}

protocol Named { var name: String { get } }
protocol Aged { var age: Int { get } }

struct Person: Named, Aged {
    let name: String
    let age: Int
}

func greet(_ entity: Named & Aged) {
    print("Hello, \(entity.name), age \(entity.age)!")
}

// Protocol extension — default implementation
extension Named {
    func introduce() {
        print("My name is \(name)")
    }
}

@protocol Loggable
@required
- (void)logMessage: (NSString *)message;
@optional
- (NSString *)logPrefix;
@end

@interface ConsoleLogger : NSObject 
@end

@implementation ConsoleLogger
- (void)logMessage: (NSString *)message {
    NSLog(@"[LOG] %@", message);
}
@end