📱 Detecting Keyboard Activities with Notification Center (iOS – Xcode)

✅ 1. Definition

The Notification Center in iOS is a system that allows apps to observe (listen to) events happening in the system or app, such as keyboard appearance and disappearance.

👉 In simple terms: It lets your app know when the keyboard shows, hides, or changes size.

🧠 2. Key Concepts

🔹 NotificationCenter

• A built-in iOS system class
• Sends and receives event notifications

🔹 Keyboard Notifications

iOS provides predefined notifications for keyboard events:

🔹 Observer

• A listener that waits for notifications
• Your ViewController acts as an observer

⚙️ 3. How Notification Center Works

Keyboard Event → Notification Center → Observer (ViewController) → Action

🏗️ 4. Detecting Keyboard Events (Step-by-Step)

🔹 Step 1: Add Observers

override func viewDidLoad() {
    super.viewDidLoad()
    
    NotificationCenter.default.addObserver(self,
        selector: #selector(keyboardWillShow),
        name: UIResponder.keyboardWillShowNotification,
        object: nil)
    
    NotificationCenter.default.addObserver(self,
        selector: #selector(keyboardWillHide),
        name: UIResponder.keyboardWillHideNotification,
        object: nil)
}

🔹 Step 2: Handle Keyboard Appearing

@objc func keyboardWillShow(notification: Notification) {
    print("Keyboard is shown")
}

🔹 Step 3: Handle Keyboard Hiding

@objc func keyboardWillHide(notification: Notification) {
    print("Keyboard is hidden")
}

📏 5. Getting Keyboard Size (Important)

Sometimes we need keyboard height to adjust UI.

@objc func keyboardWillShow(notification: Notification) {
    if let userInfo = notification.userInfo,
       let keyboardFrame = userInfo[UIResponder.keyboardFrameEndUserInfoKey] as? CGRect {
        
        let keyboardHeight = keyboardFrame.height
        print("Keyboard height: \(keyboardHeight)")
    }
}

📊 6. Diagram Description (for Exams)

Draw:

Keyboard Appears
      ↓
NotificationCenter sends event
      ↓
ViewController receives notification
      ↓
Adjust UI / Move TextField

💡 7. Real Use Case Example

🎯 Chat App

Problem:

• Keyboard covers text field

Solution:

• Detect keyboard height
• Move input field upward

@objc func keyboardWillShow(notification: Notification) {
    if let keyboardFrame = notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? CGRect {
        self.view.frame.origin.y = -keyboardFrame.height / 2
    }
}

@objc func keyboardWillHide(notification: Notification) {
    self.view.frame.origin.y = 0
}

🧹 8. Removing Observers (Important Rule)

Always remove observers to avoid memory issues:

deinit {
    NotificationCenter.default.removeObserver(self)
}

📌 9. Important Rules / Tips

• Always register observers in viewDidLoad()
• Remove observers in deinit
• Use keyboard height to adjust UI layout
• Handle both show and hide events
• Avoid hardcoding UI shifts

⚠️ 10. Common Mistakes

• ❌ Not removing observers → memory leaks
• ❌ Ignoring keyboard height
• ❌ UI not adjusting properly → text fields hidden
• ❌ Using fixed values instead of dynamic keyboard size

🧠 11. Best Practices

• Use Auto Layout instead of manual frame shifting when possible
• Animate UI changes with keyboard
• Handle different device sizes
• Use keyboard notifications for responsive design

📝 12. Likely Exam Questions

1. What is Notification Center in iOS?
2. How do you detect keyboard appearance?
3. List keyboard-related notifications.
4. Write code to detect keyboard show event.
5. Why is keyboard height important?
6. Explain how to adjust UI when keyboard appears.
7. What is the role of observers?
8. Why should observers be removed?

📚 13. Quick Revision Summary

• Notification Center detects system events

• Used for keyboard tracking in apps

• Key notifications:

  • Show
  • Hide
  • Frame change

• Steps:

  1. Add observer
  2. Handle event
  3. Adjust UI
  4. Remove observer

• Used in chat apps, forms, login screens