Lifecycle awareness in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ lifecycle

intermediate

What is the output when the Activity is rotated?

Consider this Android Activity code snippet. What will be printed in the log when the device is rotated (causing configuration change)?

Android Kotlin

class MainActivity : AppCompatActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    Log.d("Lifecycle", "onCreate called")
  }
  override fun onDestroy() {
    super.onDestroy()
    Log.d("Lifecycle", "onDestroy called")
  }
}

A onDestroy called followed by onCreate called

B Only onCreate called

C Only onDestroy called

D Neither onCreate nor onDestroy called

💡 Hint

Think about what happens to an Activity during a configuration change like rotation.

When the device rotates, the current Activity is destroyed and recreated. So onDestroy is called first, then onCreate is called again.

❓ ui_behavior

intermediate

Which lifecycle method is best to start a camera preview?

You want to start the camera preview when the user can see the app screen. Which lifecycle method should you use?

A onStart()

B onCreate()

C onPause()

D onResume()

💡 Hint

Consider when the app is fully visible and interactive.

onResume() is called when the Activity is in the foreground and ready for user interaction, so it's the best place to start camera preview.

🔧 Debug

advanced

Why does this app crash on rotation?

This Activity crashes when rotated. What is the cause?

Android Kotlin

class MainActivity : AppCompatActivity() {
  private lateinit var textView: TextView
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    textView = findViewById(R.id.textView)
    textView.text = "Hello"
  }
}

A textView is null because it is not declared

B Missing call to super.onCreate()

C textView is used before setContentView, so it is not initialized

D setContentView should be called after onCreate

💡 Hint

Think about when views are available after setContentView.

textView is accessed before setContentView, so it is not yet linked to the layout and causes a crash.

❓ navigation

advanced

What happens to the Activity stack after this navigation?

In an app, MainActivity starts DetailActivity with this code: startActivity(Intent(this, DetailActivity::class.java)) finish() What is the state of the Activity stack after this runs?

A MainActivity is removed, DetailActivity is on top

B Both MainActivity and DetailActivity are in the stack

C Only MainActivity remains in the stack

D DetailActivity is removed immediately

💡 Hint

Consider what finish() does after starting a new Activity.

finish() removes MainActivity from the stack, so only DetailActivity remains on top.

🧠 Conceptual

expert

What is the correct order of lifecycle callbacks when an Activity goes to background and then returns?

Put these lifecycle methods in the order they are called when the user presses Home (app goes to background) and then returns to the app.

A 1,2,4,3,5

B 1,2,3,4,5

C 2,1,3,4,5

D 1,3,2,4,5

💡 Hint

Remember the lifecycle flow when app goes background and foreground.

When going to background: onPause then onStop. When returning: onRestart, onStart, onResume.

Practice

1. What is the main purpose of lifecycle awareness in an Android app?

easy

A. To add animations to UI components

B. To change the app's theme colors dynamically

C. To increase the app's download speed

D. To respond to app state changes like start and stop

Solution

1. Step 1: Understand lifecycle awareness concept

   Lifecycle awareness means the app knows when it starts, stops, or pauses.

2. Step 2: Identify the main goal

   The goal is to respond properly to these state changes to save resources and improve user experience.

3. Final Answer:

   To respond to app state changes like start and stop -> Option D

4. Quick Check:

   Lifecycle awareness = respond to state changes [OK]

Hint: Lifecycle awareness means reacting to app start/stop events [OK]

Common Mistakes:

• Confusing lifecycle with UI styling
• Thinking it controls app download speed
• Assuming it only adds animations

2. Which annotation is used to mark a method that listens to lifecycle events in Android Kotlin?

easy

A. @OnClick

B. @OnLifecycleEvent

C. @Override

D. @Subscribe

Solution

1. Step 1: Recall lifecycle event annotations

   In Android Kotlin, @OnLifecycleEvent marks methods that respond to lifecycle changes.

2. Step 2: Compare options

   @OnClick is for button clicks, @Override is for overriding methods, @Subscribe is for event bus patterns.

3. Final Answer:

   @OnLifecycleEvent -> Option B

4. Quick Check:

   Lifecycle event listener annotation = @OnLifecycleEvent [OK]

Hint: Use @OnLifecycleEvent to handle lifecycle changes [OK]

Common Mistakes:

• Using @OnClick for lifecycle events
• Confusing @Override with lifecycle annotations
• Thinking @Subscribe is for lifecycle

3. Given this code snippet, what lifecycle event will trigger the onStartEvent() method?

class MyObserver : LifecycleObserver {
  @OnLifecycleEvent(Lifecycle.Event.ON_START)
  fun onStartEvent() {
    println("Started")
  }
}

medium

A. When the app starts or comes to foreground

B. When the app is paused

C. When the app is stopped

D. When the app is destroyed

Solution

1. Step 1: Identify the lifecycle event annotation

   The method is annotated with @OnLifecycleEvent(Lifecycle.Event.ON_START), which means it triggers on start.

2. Step 2: Understand ON_START meaning

   ON_START happens when the app moves to the foreground or becomes visible.

3. Final Answer:

   When the app starts or comes to foreground -> Option A

4. Quick Check:

   ON_START event triggers on app foreground [OK]

Hint: ON_START means app is visible or foregrounded [OK]

Common Mistakes:

• Confusing ON_START with ON_PAUSE or ON_STOP
• Thinking ON_START means app destroyed
• Assuming it triggers on app pause

4. What is wrong with this observer class?

class MyObserver : LifecycleObserver {
  fun onResumeEvent() {
    println("Resumed")
  }
}

medium

A. Missing @OnLifecycleEvent annotation on the method

B. Class should extend Activity instead of LifecycleObserver

C. Method name must be onStartEvent instead of onResumeEvent

D. println cannot be used inside lifecycle methods

Solution

1. Step 1: Check method annotation

   The method onResumeEvent lacks the @OnLifecycleEvent annotation, so it won't be called on lifecycle changes.

2. Step 2: Verify class and method correctness

   The class correctly implements LifecycleObserver, method name can be any valid name, and println is allowed.

3. Final Answer:

   Missing @OnLifecycleEvent annotation on the method -> Option A

4. Quick Check:

   Lifecycle methods need @OnLifecycleEvent annotation [OK]

Hint: Always add @OnLifecycleEvent to lifecycle methods [OK]

Common Mistakes:

• Forgetting the annotation
• Thinking method name must be fixed
• Assuming println is disallowed

5. You want to save battery by stopping a sensor when the app is not visible. Which lifecycle event should you observe to stop the sensor properly?

hard

A. Lifecycle.Event.ON_CREATE

B. Lifecycle.Event.ON_RESUME

C. Lifecycle.Event.ON_PAUSE

D. Lifecycle.Event.ON_DESTROY

Solution

1. Step 1: Understand when to stop sensors

   Sensors should stop when the app is no longer visible to save battery, which happens at ON_PAUSE.

2. Step 2: Match lifecycle event to stopping action

   ON_PAUSE is called when the app loses focus but is not yet destroyed, ideal for stopping sensors.

3. Final Answer:

   Lifecycle.Event.ON_PAUSE -> Option C

4. Quick Check:

   Stop sensors on ON_PAUSE to save battery [OK]

Hint: Stop sensors on ON_PAUSE to save battery [OK]

Common Mistakes:

• Stopping sensors too late at ON_DESTROY
• Stopping sensors too early at ON_CREATE
• Confusing ON_RESUME with stopping event