Kotlin : 100 Interview Questions and Answers PART8

37. What is LiveData in Kotlin?

LiveData is a lifecycle-aware observable data holder class used in Android development with Kotlin (and Java). It’s part of Android Jetpack's Architecture Components.

Purpose of LiveData

LiveData is used to observe data changes in a way that respects the lifecycle of UI components (e.g., Activity, Fragment), so that:

• UI updates only happen when the component is active (started/resumed)
• It prevents memory leaks and crashes from observing when the view is not alive

Key Features :

• Lifecycle-aware: Observers are only notified when in an active lifecycle state
• Automatic cleanup: No need to manually unregister observers
• Observable: Similar to Flow or Rx, but designed for UI and ViewModel communication
• Thread-safe: Can be safely updated from any thread using postValue

Basic Example

1. ViewModel with LiveData

class MyViewModel : ViewModel() {
    private val _counter = MutableLiveData<Int>()
    val counter: LiveData<Int> get() = _counter

    fun increment() {
        _counter.value = (_counter.value ?: 0) + 1
    }
}

2. Fragment observing LiveData

class MyFragment : Fragment() {
    private val viewModel: MyViewModel by viewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        viewModel.counter.observe(viewLifecycleOwner) { count ->
            textView.text = "Count: $count"
        }

        button.setOnClickListener {
            viewModel.increment()
        }
    }
}

When to Use LiveData

Use LiveData when:

• You're working with Jetpack components like ViewModel, Fragment, Activity
• You want to keep UI in sync with data changes
• You want automatic lifecycle awareness

38. Describe the scope function “apply” in Kotlin.

In Kotlin, the apply scope function is used to configure an object. It allows you to access the object using this and returns the object itself.

apply:

• Context object: this
• Returns: the object itself
• Use case: configuring or initializing an object

Syntax

val obj = MyClass().apply {
    property1 = "value"
    property2 = 42
    doSomething()
}

Equivalent to:

val obj = MyClass()
obj.property1 = "value"
obj.property2 = 42
obj.doSomething()

When to Use apply:

• 1. Initialize and configure an object
• 2. Builder-style configuration
• 3. Avoid repeating the object name
• 4. Return the object itself after applying some setup

39. Describe the scope function “run” in Kotlin.

In Kotlin, the run scope function is used to execute a block of code within the context of an object or as a standalone block, returning the last expression's result.

run :

Syntax:

1. As an object extension

val result = "Hello".run {
    println(length)
    length * 2
}
// result = 10

Here:

• "Hello" is the object
• this refers to "Hello"
• Returns length * 2

2. As a standalone block (no object)

val result = run {
    val a = 5
    val b = 10
    a + b
}
// result = 15

Used like a lambda to group and return a computed value.

run is great when you want to use an object and compute a result or group a block of expressions into a single return value.

40. Comparison between Scope Functions .