Dependency injection in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is dependency injection in simple terms?

Dependency injection means giving an object what it needs instead of letting it create those things itself. It helps keep code clean and easy to change.

beginner

Why is dependency injection useful in iOS Swift apps?

It makes testing easier because you can swap real parts with fake ones. It also helps organize code so parts don’t depend too much on each other.

intermediate

What are the common ways to do dependency injection in Swift?

You can inject dependencies through initializer methods, by setting properties after creating an object, or by using a dependency container that provides needed parts.

beginner

Show a simple Swift example of initializer injection.

```swift
protocol Service {
  func doWork()
}

class RealService: Service {
  func doWork() { print("Working") }
}

class Client {
  let service: Service
  init(service: Service) {
    self.service = service
  }
  func start() {
    service.doWork()
  }
}

let service = RealService()
let client = Client(service: service)
client.start() // prints "Working"
```

intermediate

What problem does dependency injection solve compared to creating dependencies inside a class?

It avoids tight coupling, so classes don’t depend on specific implementations. This makes code easier to change, test, and reuse.

What is the main goal of dependency injection?

A To make objects create their own dependencies

B To give objects their dependencies from outside

C To hide dependencies inside methods

D To remove all dependencies

Dependency injection means providing dependencies from outside, not creating them inside.

Which of these is NOT a common way to inject dependencies in Swift?

A Method injection

B Property injection

C Inheritance injection

D Initializer injection

Inheritance injection is not a standard term or method for dependency injection.

Why does dependency injection help with testing?

A It hides bugs automatically

B It makes code run faster

C It removes the need for tests

D It allows replacing real parts with fake ones easily

Dependency injection lets you swap real dependencies with mocks or fakes for testing.

In the example: `init(service: Service)`, what is happening?

A Dependency is injected via initializer

B Dependency is created inside the class

C Dependency is ignored

D Dependency is deleted

The service is passed into the class through the initializer, which is initializer injection.

What does tight coupling mean in code?

A Classes depend too much on specific implementations

B Classes are independent and flexible

C Classes have no dependencies

D Classes are loosely connected

Tight coupling means classes rely heavily on specific details, making changes harder.

Explain dependency injection and why it is helpful in iOS Swift development.

Think about how giving objects what they need from outside helps keep code clean.

Describe how you would use initializer injection in a Swift class with a simple example.

Remember the example with a service protocol and a client class.

Practice

1. What is the main purpose of dependency injection in Swift programming?

easy

A. To provide objects with their dependencies from outside rather than creating them inside

B. To make the app run faster by caching data

C. To automatically generate user interfaces

D. To encrypt data before saving

Solution

1. Step 1: Understand dependency injection concept

   Dependency injection means giving an object the things it needs (dependencies) from outside instead of creating them inside the object.

2. Step 2: Identify the main purpose

   This approach helps keep code flexible, easier to test, and cleaner by separating concerns.

3. Final Answer:

   To provide objects with their dependencies from outside rather than creating them inside -> Option A

4. Quick Check:

   Dependency injection = provide dependencies from outside [OK]

Hint: Dependency injection means giving dependencies from outside [OK]

Common Mistakes:

• Thinking it speeds up app performance
• Confusing with UI generation
• Mixing with data encryption

2. Which of the following is the correct syntax for constructor injection in Swift?

easy

A. class Client { func inject(service: Service) { self.service = service } }

B. class Service { init() { } } class Client { let service: Service; init(service: Service) { self.service = service } }

C. class Client { var service = Service() }

D. class Service { func start() {} }

Solution

1. Step 1: Identify constructor injection pattern

   Constructor injection means passing dependencies through the initializer (init) of a class.

2. Step 2: Check which code passes dependency via init

   class Service { init() { } } class Client { let service: Service; init(service: Service) { self.service = service } } shows a Client class receiving a Service instance through its init method and storing it.

3. Final Answer:

   class Service { init() { } } class Client { let service: Service; init(service: Service) { self.service = service } } -> Option B

4. Quick Check:

   Constructor injection uses init to pass dependencies [OK]

Hint: Constructor injection passes dependencies via init method [OK]

Common Mistakes:

• Creating dependencies inside the class instead of passing them
• Using methods instead of init for injection
• Not storing the injected dependency

3. Given the code below, what will be printed?

protocol Logger { func log(_ message: String) }
class ConsoleLogger: Logger { func log(_ message: String) { print("Console: \(message)") } }
class Client {
  let logger: Logger
  init(logger: Logger) { self.logger = logger }
  func doWork() { logger.log("Work done") }
}
let client = Client(logger: ConsoleLogger())
client.doWork()

medium

A. No output

B. Work done

C. Console: Work done

D. Logger: Work done

Solution

1. Step 1: Understand the Logger protocol and ConsoleLogger class

   ConsoleLogger implements Logger and prints messages prefixed with "Console:".

2. Step 2: Trace the Client usage

   Client is initialized with ConsoleLogger and calls logger.log("Work done"), which prints "Console: Work done".

3. Final Answer:

   Console: Work done -> Option C

4. Quick Check:

   Injected ConsoleLogger prints with "Console:" prefix [OK]

Hint: Injected logger prints message with prefix [OK]

Common Mistakes:

• Ignoring the prefix in print statement
• Assuming no output because of protocol
• Confusing method names

4. Identify the error in the following Swift code using dependency injection:

protocol DataService { func fetchData() -> String }
class RealDataService: DataService { func fetchData() -> String { "Real Data" } }
class ViewModel {
  var dataService: DataService
  init() { }
  func load() -> String { dataService.fetchData() }
}

medium

A. dataService is not initialized before use

B. fetchData() should return Int, not String

C. ViewModel should not have an init method

D. RealDataService does not conform to DataService

Solution

1. Step 1: Check property initialization

   ViewModel has a non-optional property dataService but its init() does not initialize it.

2. Step 2: Understand Swift initialization rules

   Non-optional properties must be initialized before use; missing initialization causes a compile error.

3. Final Answer:

   dataService is not initialized before use -> Option A

4. Quick Check:

   Non-optional properties must be initialized [OK]

Hint: Non-optional properties must be set in init [OK]

Common Mistakes:

• Assuming default nil for non-optional
• Confusing return types
• Ignoring protocol conformance

5. You want to make a Swift class NetworkManager testable by injecting a protocol NetworkService. Which approach best applies dependency injection and allows easy swapping of real and mock services?

hard

A. Create NetworkManager with a fixed real network service inside the class

B. Make NetworkManager inherit from NetworkService and override methods

C. Use global variables to hold the network service instance

D. Define a NetworkService protocol, make NetworkManager accept it via init, and pass either real or mock implementations when creating NetworkManager

Solution

1. Step 1: Understand testability with dependency injection

   To test NetworkManager easily, inject a protocol so you can swap real and mock implementations.

2. Step 2: Identify best practice

   Passing NetworkService via init allows NetworkManager to use any implementation, making testing flexible.

3. Final Answer:

   Define a NetworkService protocol, make NetworkManager accept it via init, and pass either real or mock implementations when creating NetworkManager -> Option D

4. Quick Check:

   Inject protocol via init for flexible testing [OK]

Hint: Inject protocol via init to swap real/mock easily [OK]

Common Mistakes:

• Hardcoding real service inside class
• Using global variables causing tight coupling
• Misusing inheritance instead of composition