Activity in Android

14th December, 2022

In this tutorial we will learn one of the most important topics of Android app development, which is Android Activity.

What is Activity?

Activity is referred to as a screen of an Android Application. Activity is one of the building blocks of Android OS. An Android app consists of one or more screens or activities. We can say that Activity is a class pre-written in Kotlin or Java programing.

In other words, Android Activity provide a frame for users to interact with and perform their tasks.

Android Activity Lifecycle

Every Android Activity goes through a lifecycle or different stages. The various stages of each activity in an Android application are managed by the Android Activity Stack.

There are four major stages in an Activity.

⇒ Running stage
When an activity is shown on the user’s phone screen, it is in the running state.
⇒ Paused state
An Activity is not in focus but still alive for the user. It is in paused state.
⇒ Resumed state
When the Activity goes from paused state to the foreground. This is the resumed state.
⇒ Stopped state
Activity is in the stopped state when it is not in the activity stack and is not visible to the users.

Android Activity Lifecycle Methods

Activity has lifecycle methods like created, started, resumed, paused, stopped or destroyed. Please see the image below that describes the lifecycle of Android Activity.

Take a look at the code of the Android activity lifecycle methods in MainActivity.

package com.example.myapplication1

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

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        //Called when the activity is first created
    }

    override fun onStart() {
        super.onStart()
        //Called after it's creation or by restart method after onStop().
        //Activity is not yet rendered on screen
        //but is about to become visible to the user.
        //You can perform operation related to UI
    }

    override fun onResume() {
        super.onResume()
        //Called when Activity is visible to user
        //Activity is in the active state
        //user can interact with Activity
    }

    override fun onPause() {
        super.onPause()
        //Called when Activity losses its focus
        //Activity is going into the background but has not yet been killed
        //You can commit database transactions and perform light-weight processing
    }

    override fun onStop() {
        super.onStop()
        //Called when activity is getting destroyed or
        //another existing activity comes back to resume state
        //Activity is no longer visible to the user
    }

    override fun onRestart() {
        super.onRestart()
        //Called when user comes on screen or resume the activity which was stopped
    }

    override fun onDestroy() {
        super.onDestroy()
        //Called when an activity is destroyed by a user or Android system
        //Activity is not in background
    }
}

Declare an Activity In Androidmanifest.xml

To implement in the application we have to register each and every Android activity in Androidmanifest.xml file. The <activity> tag is used to declare an activity.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" >

    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.MyApplication1"
        tools:targetApi="31" >

        <!--Declare an Activity-->
        <activity
            android:name=".MainActivity"
            android:exported="true" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
            <meta-data
                android:name="android.app.lib_name"
                android:value="" />
        </activity>
    </application>

</manifest>

Happy coding!