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"QQT @yashigani ؔϞό #13 

yashigani @yashigani http://yashigani.hatenablog.com 

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• Depend interface, not depend implementations • Make less dependency between components • Improve testability • Make easy to mock dependency objects 

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// Constructor injection class ViewController: UIViewController { let client: APIClient init(client: APIClient) { self.client = APIClient } } 

• Pros • Easy to control • Cons • Don’t fit storyboard • Need dependency objects to create 

// Property injection override func prepareForSegue() { if let vc = segue.dVC as? NextViewController { vc.client = client } } 

• Pros • Don’t need to dependency objects to create • Fit view controller with storyboard • Cons • Too difficult to stay simple • Overtime pass to dependency objects 

// Hold all dependency objects in app class Context { // Singleton!!! private static let context = Context() static func get() -> Context { return context } let client = APIClient() } class ViewController: UIViewController { // Depend to singleton pattern let context = Context.get() } 

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View Controller Context 

View Controller ViewController: WithContext WithContext Context ContextType 

// protocol for Context protocol ContextType { var client: APIClientType { get } } class Context: ContextType { private static let context = Context() static func get() -> Context { return context } let client: APIClientType = APIClient() } 

protocol WithContext { var context: ContextType { get } } extension WithContext { var context: ContextType { return Context.get() } } class ViewController: UIViewController, WithContext {} 

• Default behavior, WithContext return static Context object • In unit test, add extension ViewController.context • Change context object to mock • Easy to mock context because it depend to ContextType • Solve strong dependency to singleton pattern 

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• Don’t depend to implementations, depend to protocols • DI makes easy and flexible to inject dependency objects • Depending protocols makes easy to change implementations • Fit View Controller with storyboard very much ✨ 

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