📱 Exploring the App Delegate & Adding New View Controllers (iOS – Xcode)

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🧩 PART 1: Exploring the App Delegate

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✅ 1. Definition

The App Delegate is a central class in an iOS app that handles the app’s lifecycle events such as launching, backgrounding, and termination.

👉 File name: AppDelegate.swift

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🧠 2. Key Concepts

🔹 App Lifecycle

The sequence of states an app goes through:

• Launch
• Active
• Background
• Terminated

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🔹 Role of App Delegate

• App startup configuration
• Handling system-level events
• Managing app state transitions

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⚙️ 3. Important Methods in App Delegate

func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
) -> Bool {
    // App setup code
    return true
}

📌 Common Methods

Method	Purpose
didFinishLaunchingWithOptions	Called when app launches
applicationDidEnterBackground	App goes to background
applicationWillEnterForeground	App returns to foreground
applicationWillTerminate	App is about to close

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📊 Diagram Description (for Exams)

Draw lifecycle flow:

Launch → Active → Background → Foreground → Terminated

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📌 Important Rules

• App Delegate runs before UI appears
• Used for global app settings
• Avoid placing UI logic here

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💡 Example

func applicationDidEnterBackground(_ application: UIApplication) {
    print("App in Background")
}

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🧭 PART 2: Adding New View Controllers

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✅ 1. Definition

A View Controller manages a single screen. Adding new view controllers allows you to create multiple screens in your app.

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🧠 2. Key Concepts

🔹 Multiple Screens

• Each screen = one view controller
• Navigation between screens is required

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🔹 Types of Navigation

• Push (Navigation Controller)
• Present (Modal)

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⚙️ 3. Steps to Add a New View Controller

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🔹 Step 1: Open Storyboard

• Open Main.storyboard

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🔹 Step 2: Add View Controller

• Drag View Controller from Objects Library

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🔹 Step 3: Design UI

• Add labels, buttons, etc.

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🔹 Step 4: Create Swift File

• File → New → Cocoa Touch Class
• Name it (e.g., SecondViewController)

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🔹 Step 5: Link Class to View Controller

• Select new screen → Identity Inspector
• Set class name

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🔹 Step 6: Add Navigation

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🔸 Option 1: Segue (Storyboard)

• Control + Drag from button → new view controller

• Select:

  • Show (Push)
  • Present Modally

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🔸 Option 2: Programmatically

let vc = storyboard?.instantiateViewController(withIdentifier: "SecondVC") as! SecondViewController
self.present(vc, animated: true)

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🔹 Step 7: Set Storyboard ID

• In Identity Inspector → set identifier (e.g., "SecondVC")

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🔁 4. Passing Data Between View Controllers

vc.data = "Hello from First Screen"

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📊 Diagram Description (for Exams)

Draw:

ViewController1 → (Button Click) → ViewController2

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📌 5. Important Rules / Tips

• Always assign Storyboard ID
• Use Navigation Controller for push navigation
• Keep each ViewController focused on one task
• Avoid strong coupling between controllers

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💡 6. Example

🎯 Two-Screen App

Screen 1:

• Button → "Next"

Screen 2:

• Label → "Welcome"

👉 On button click → navigate to screen 2

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⚠️ 7. Common Mistakes

• ❌ Not setting Storyboard ID
• ❌ Not linking class properly
• ❌ Wrong segue type
• ❌ Force unwrapping causing crash

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📝 8. Likely Exam Questions

1. Define App Delegate.
2. Explain the role of App Delegate in iOS.
3. Describe app lifecycle states.
4. What is didFinishLaunchingWithOptions?
5. How do you add a new view controller in Xcode?
6. Explain different navigation methods.
7. Write code to navigate to another screen.
8. How is data passed between view controllers?

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📚 9. Quick Revision Summary

🔹 App Delegate

• Manages app lifecycle
• Handles system-level events
• Key method: didFinishLaunchingWithOptions

🔹 View Controllers

• Each screen = one controller

• Add via storyboard

• Navigate using:

  • Segue
  • Code

👉 Multi-screen apps require multiple view controllers

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