Lifecycle awareness in Android Kotlin - Mini App: Build & Ship

Build: Lifecycle Demo Screen

This screen shows how to observe Android lifecycle events and display them in a list.

Target UI

-------------------------
| Lifecycle Demo Screen  |
-------------------------
| Events:               |
| - onCreate            |
| - onStart             |
| - onResume            |
|                       |
| [Clear Events Button] |
-------------------------

Display a list of lifecycle events as they happen.

Observe lifecycle events: onCreate, onStart, onResume, onPause, onStop, onDestroy.

Add each event name to the list when it occurs.

Add a button to clear the list of events.

Use AndroidX Lifecycle components for observing lifecycle.

UI should update automatically when events are added or cleared.

Starter Code

Android Kotlin

package com.example.lifecycleawareness

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.items
import androidx.compose.material3.Button
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.mutableStateListOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleEventObserver

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            LifecycleDemoScreen()
        }
    }
}

@Composable
fun LifecycleDemoScreen() {
    // TODO: Implement lifecycle event list and observer
    // TODO: Display events in a LazyColumn
    // TODO: Add a button to clear events
}

Task 1

Task 2

Task 3

Task 4

Task 5

Solution

Android Kotlin

package com.example.lifecycleawareness

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.items
import androidx.compose.material3.Button
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.mutableStateListOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.Composable
import androidx.compose.runtime.DisposableEffect
import androidx.compose.ui.Modifier
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleEventObserver

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            LifecycleDemoScreen(lifecycle = lifecycle)
        }
    }
}

@Composable
fun LifecycleDemoScreen(lifecycle: Lifecycle) {
    val events = remember { mutableStateListOf<String>() }

    DisposableEffect(lifecycle) {
        val observer = LifecycleEventObserver { _, event ->
            when(event) {
                Lifecycle.Event.ON_CREATE -> events.add("onCreate")
                Lifecycle.Event.ON_START -> events.add("onStart")
                Lifecycle.Event.ON_RESUME -> events.add("onResume")
                Lifecycle.Event.ON_PAUSE -> events.add("onPause")
                Lifecycle.Event.ON_STOP -> events.add("onStop")
                Lifecycle.Event.ON_DESTROY -> events.add("onDestroy")
                else -> {}
            }
        }
        lifecycle.addObserver(observer)

        onDispose {
            lifecycle.removeObserver(observer)
        }
    }

    Surface(modifier = Modifier.fillMaxSize(), color = MaterialTheme.colorScheme.background) {
        Column {
            LazyColumn(modifier = Modifier.weight(1f)) {
                items(events) { event ->
                    Text(text = "- $event")
                }
            }
            Button(onClick = { events.clear() }) {
                Text(text = "Clear Events")
            }
        }
    }
}

This solution uses Jetpack Compose to build the UI and AndroidX Lifecycle components to observe lifecycle events.

We create a mutableStateListOf to hold event names so the UI updates automatically when the list changes.

The LifecycleEventObserver listens to lifecycle changes and adds the event name to the list.

We use DisposableEffect to add the observer when the composable enters composition and remove it when it leaves, avoiding leaks.

The UI shows the events in a scrollable list and a button to clear the list.

Final Result

Completed Screen

-------------------------
| Lifecycle Demo Screen  |
-------------------------
| Events:               |
| - onCreate            |
| - onStart             |
| - onResume            |
|                       |
| [Clear Events Button] |
-------------------------

When the screen starts, lifecycle events like onCreate, onStart, onResume appear in the list.

If the app is backgrounded and resumed, onPause, onStop, onStart, onResume events appear accordingly.

Tapping 'Clear Events' empties the list.

Stretch Goal

Add a Text showing the total count of lifecycle events recorded.

💡 Hint

Use events.size to get the count and display it above the list.