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Android Activity Lifecycle – The Complete Guide

Android activity lifecycle if one of the most asked questions in Android Developer interviews. It is also the one to mess up easily.

You must have already seen this activity lifecycle diagram from developer.android.com and many android development books.

 

android activity Lifecycle
source: developer.android.com

 

But this is not enough. This diagram just outlines the activity lifecycle callback sequence through it being created to being destroyed. But it doesn’t show how it behaves with more than one activity.

In this post, we’ll take an in-depth look at the android activity lifecycle by actually creating an application and using Log Statements. We’ll put log statements all over the app in lifecycle methods to check their behaviour.

These are the activity lifecycle callbacks we’ll be taking a look at: 

  1. onCreate
  2. onStart
  3. onResume
  4. onSaveInstanceState
  5. onPause
  6. onStop
  7. onDestroy
  8. onRestoreInstanceState

 

The Setup

We’ll be having two activities: 1. ActivityA(MainActivity)  2. ActivityB and we’ll be exploring the following situations in the activity lifecycle.

  1. Starting ActivityA (on app launch)
  2. Starting ActivityB over ActivityA
  3. Pressing back (returning) from ActivityB to ActivityA
  4. Pressing back (exiting app) from ActivityA
  5. Rotating phone in ActivityA (changing orientation)
  6. Pressing home button

First create an Android Studio project. In the MainActivity.kt file, add the following methods:

class MainActivity : AppCompatActivity() {
    var TAG = "AndroidVille"

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        title = "Activity A"
        Log.d(TAG, "Activity A: onCreate")
    }

    override fun onStart() {
        super.onStart()
        Log.d(TAG, "Activity A: onStart() ")
    }

    override fun onResume() {
        super.onResume()
        Log.d(TAG, "Activity A: onResume() ")
    }

    override fun onPause() {
        super.onPause()
        Log.d(TAG, "Activity A: onPause() ")
    }

    override fun onSaveInstanceState(outState: Bundle) {
        outState.putString("androidville", "Hello there")
        super.onSaveInstanceState(outState)
        Log.d(TAG, "Activity A: onSaveInstanceState() ")
    }

    override fun onRestoreInstanceState(savedInstanceState: Bundle) {
        super.onRestoreInstanceState(savedInstanceState)
        Log.d(TAG, "Activity A: onRestoreInstanceState() ")
    }

    override fun onRestart() {
        super.onRestart()
        Log.d(TAG, "Activity A: onRestart() ")
    }

    override fun onStop() {
        super.onStop()
        Log.d(TAG, "Activity A: onStop() ")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.d(TAG, "Activity A: onDestroy() ")
    }

    fun launchNewActivity(v: View) {
        startActivity(Intent(this, ActivityB::class.java))
    }

Now create another activity (ActivityB), and add the same methods, but this time, replace the “ActivityA” text with “ActivityB” in all the log statements.

package com.example.androidactivitylifecycle

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log

class ActivityB : AppCompatActivity() {
    var TAG = "AndroidVille"

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_b)
        title = "Activity B"
        Log.d(TAG, "Activity B: onCreate")
    }


    override fun onStart() {
        super.onStart()
        Log.d(TAG, "Activity B: onStart() ")
    }

    override fun onResume() {
        super.onResume()
        Log.d(TAG, "Activity B: onResume() ")
    }

    override fun onPause() {
        super.onPause()
        Log.d(TAG, "Activity B: onPause() ")
    }

    override fun onSaveInstanceState(outState: Bundle) {
        outState.putString("androidville", "Hello there")
        super.onSaveInstanceState(outState)
        Log.d(TAG, "Activity B: onSaveInstanceState() ")
    }

    override fun onRestoreInstanceState(savedInstanceState: Bundle) {
        super.onRestoreInstanceState(savedInstanceState)
        Log.d(TAG, "Activity B: onRestoreInstanceState() ")
    }

    override fun onRestart() {
        super.onRestart()
        Log.d(TAG, "Activity B: onRestart() ")
    }

    override fun onStop() {
        super.onStop()
        Log.d(TAG, "Activity B: onStop() ")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.d(TAG, "Activity B: onDestroy() ")
    }

}

Now, we’ll create a Log Filter to just focus on our custom log statements. For this, 

  1. Click on the “Logcat” tab on the bottom of the Android Studio. 
  2. Then click on the dropdown menu next to where it says “Regex”. Then select “Edit Filter Configuration”
  3. In the popup window, enter the name of the Filter (whatever you like). Enter the TAG of your log statement in “Log Tag”. It should be the same as the TAG variable inside the activities. Then click on “Ok”.

android logcat filter

This will help you filter out the unnecessary log statements and focus on the activity lifecycle ones.

 

Activity Lifecycle (on app launch)

With this setup, we’re ready to explore the activity lifecycle method’s call sequence. Go ahead and run the app, make sure your logcat window is opened up and clean. 

When the app launches, this is the callback sequence:

android activity lifecycle

  1. onCreate
  2. onStart
  3. onResume

First, the ActivityA’s onCreate method is called, which sets up the layout of the activity. Then we have onStart which is the place to set up things such as broadcast receivers. And finally onResume when Activity is interactive.

 

Launching ActivityB over ActivityA *Important*

This is the most commonly asked question about android activity lifecycle. What are the callback methods when one activity is launched above another. 

I cannot stress enough the importance of this question and also one that people can get wrong. Because it’s so simple, I used to commit this mistake of saying all methods of ActivityA and then B.

Let’s see what happens here:

android activity lifecycle

 

This is the activity lifecycle callback sequence: 

  1. onPause – ActivityA
  2. onCreate – ActivityB
  3. onStart – ActivityB
  4. onResume – ActivityB
  5. onStop – ActivityA
  6. onSaveInstanceState – ActivityA

First, the ActivityA is paused, then lifecycle methods of ActivityB are called. But finally the ActivityA is stopped.

Important point to note here is that ActivityA is stopped (not destroyed) after ActivityB has been created and is interactive.

 

Pressing Back Button from ActivityB 

Now when pressing back from ActivityB we land on ActivityA and this is the callback sequence:

android activity lifecycle

  1. onPause – ActivityB
  2. onRestart – ActivityA
  3. onStart – ActivityA
  4. onResume – ActivityA
  5. onStop – ActivityB
  6. onDestroy – ActivityB

Here we see that at the end, ActivityB is destroyed. Notice that when launching B on A, ActivityA wasn’t destroyed but stopped, and it’s process lived on.

 

Pressing Back Button from ActivityA

Let’s see what happens when we press the back button from ActivityA (MainActivity).

android activity lifecycle

  1. onPause – ActivityA
  2. onStop – ActivityA
  3. onDestroy – ActivityA

Since ActivityA is the last activity, on pressing back button, the app exits and the process of ActivityA is destroyed.

 

Rotating the device (activity lifecycle in orientation change) *Important*

This is another important scenario in these questions. And this one is a bit unique, which makes use of onRestoreInstanceState. We’ll be rotating the device from ActivityA.

Here are the callback method call sequence:

  1. onPause – ActivityA
  2. onStop – ActivityA
  3. onSaveInstanceState – ActivityA
  4. onDestroy – ActivityA
  5. onCreate – ActivityA
  6. onStart – ActivityA
  7. onRestoreInstanceState – ActivityA
  8. onResume – ActivityA

You can get the key value pairs you store from onSaveInstanceState, inside onRestoreInstanceState.

 

Pressing home button from ActivityA

On pressing home button from ActivityA, you’ll get the following callback sequence:

android activity lifecycle

  1. onPause – ActivityA
  2. onStop – ActivityA
  3. onSaveInstanceState – ActivityA

And when you come back from paused state (by pressing recents button on android), following is the sequence:

android activity lifecycle

  1. onRestart – ActivityA
  2. onStart – ActivityA
  3. onResume – ActivityA

So these were some of the scenarios in Android Activity Lifecycle. Let me know if you want me to post some other scenarios as well. In the upcoming posts, I’ll try to make one with fragments lifecycle and how they work with activity lifecycle.

 

Activity Lifecycle in Android – Hindi Tutorial

 

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