Protocol Composition Example

protocol FirstDependency {}
protocol SecondDependency {}
protocol ThirdDependency {}

protocol HasFirstDependency {
	var first: FirstDependency { get }
}

protocol HasSecondDependency {
	var second: SecondDependency { get }
}

protocol HasThirdDependency {
	var third: ThirdDependency { get }
}

class RealFirstDependency: FirstDependency {}
class RealSecondDependency: SecondDependency {}
class RealThirdDependency: ThirdDependency {}

struct AppDependencies: HasFirstDependency, HasSecondDependency, HasThirdDependency {
	var first: FirstDependency
	var second: SecondDependency
	var third: ThirdDependency
}

class SomeObject {
	typealias Dependencies = HasFirstDependency & HasSecondDependency
	
	init(dependencies: Dependencies) {
		self.dependencies = dependencies
	}
	
	var dependencies: Dependencies
}

let appDependencies = AppDependencies(
	first: RealFirstDependency(),
	second: RealSecondDependency(),
	third: RealThirdDependency()
)

let object = SomeObject(dependencies: appDependencies)

print(object.dependencies.first)
print(object.dependencies.second)
print(object.dependencies.third) // Error