Building Android Apps with Kotlin

Introduction to building Android apps using the Kotlin Programming Language

Updated: 03 September 2023

From this Udacity Corse

Building your First App

Create a Project

To get started you will need to first install Android Studio and an Emulator if needed

To get started create a new Android Studio Project with the following settings, select the Empty Activity and name the application DiceRoller with the Root Package com.example.DiceRoller. Select the API Level 19 and

Name	Dice Roller
Package	com.nabeelvalley.diceroller
Save Location	C:\Repos\AndroidStudio\DiceRoller
Language	Kotlin
Minimum API Level	API Level 19: Android 4.4
	Use `androidx.*` artifacts

Set up an Emulator

In the debug toolbar click Run (Green Play Button) > Add new Virtual Device And select the Pixel 2 with Play Store and then select the newest API Emulator after downloading the image
Once the Emulator is downloaded you can select it
Click Okay and the app should build and open on your Emulator

To use an app on your device configure the debugging See Intro To Android Notes

What’s in an App

The application when using the Android project view, if you look at the application there are the following folders

java is your application code
res are the static resources for your app
generatedJava contains build outputs
manifests contains manifest files with application details
Gradle Scripts are generated files that configure the application build and installation

The MainActivity.kt file and the activity_main.xml files are the stuff we see when the application runs the Main Activity, this is declared in the AndroidManifest.xml file:

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<activity android:name=".MainActivity">
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    <intent-filter>
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        <action android:name="android.intent.action.MAIN"/>
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        <category android:name="android.intent.category.LAUNCHER"/>
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    </intent-filter>
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</activity>

The Objects in our Layout File (activity_main.xml) are transformed into Kotlin Objects during the inflation process that we can work with and manipulate in the `MainActivity.xml

An Activity has a set of Lifecycle Methods that are called during the application setup, for our activity we have the following code:

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class MainActivity : AppCompatActivity() {
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    override fun onCreate(savedInstanceState: Bundle?) {
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        super.onCreate(savedInstanceState)
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        setContentView(R.layout.activity_main)
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    }
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}

The R.layout.activity_main is the layout that is being used, in the Layout file we can have:

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<?xml version="1.0" encoding="utf-8"?>
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<androidx.constraintlayout.widget.ConstraintLayout
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        xmlns:android="http://schemas.android.com/apk/res/android"
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        xmlns:tools="http://schemas.android.com/tools"
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        xmlns:app="http://schemas.android.com/apk/res-auto"
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        android:layout_width="match_parent"
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        android:layout_height="match_parent"
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        tools:context=".MainActivity">
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    <TextView
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            android:layout_width="wrap_content"
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            android:layout_height="wrap_content"
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            android:text="Hello World!"
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            app:layout_constraintBottom_toBottomOf="parent"
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            app:layout_constraintLeft_toLeftOf="parent"
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            app:layout_constraintRight_toRightOf="parent"
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            app:layout_constraintTop_toTopOf="parent"/>
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</androidx.constraintlayout.widget.ConstraintLayout>

The above layout can be previewed on Android Studio, but is essentially a ConstraintLayout with just a TextView in the center. The different nodes in the Layout file are called Views, there are many types of views

Update the Layout

For our first layout we’ll use a LinearLayout with an Image and Button in it. For now just replace the current View COntents with:

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<?xml version="1.0" encoding="utf-8"?>
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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
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              xmlns:tools="http://schemas.android.com/tools"
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              android:layout_width="match_parent"
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              android:layout_height="wrap_content"
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              tools:context=".MainActivity">
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    <TextView
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            android:layout_width="wrap_content"
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            android:layout_height="wrap_content"
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            android:text="Hello World!" />
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</LinearLayout>

Looking at the XML we can see the different properties of the LinearLayout and TextView that are on the screen

Now let’s update the text and positioning of the layout to add a button and center everything on the screen, we can use the following code to do so:

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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
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              xmlns:tools="http://schemas.android.com/tools"
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              android:layout_width="match_parent"
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              android:layout_height="wrap_content"
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              android:layout_gravity="center_vertical"
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              android:orientation="vertical"
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              tools:context=".MainActivity">
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    <TextView
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            android:layout_width="wrap_content"
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            android:layout_height="wrap_content"
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            android:layout_gravity="center_horizontal"
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            android:text="1"
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            android:textSize="30sp"/>
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    <Button
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            android:layout_width="wrap_content"
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            android:layout_height="wrap_content"
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            android:layout_gravity="center_horizontal"
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            android:text="Roll"/>
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</LinearLayout>

Lastly double click on the word Roll, and a lightbulb should appear in the menu, select the Extract string resource and it will be added to the values/strings.xml file:

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<resources>
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    <string name="app_name">DiceRoller</string>
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    <string name="roll">Roll</string>
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</resources>

We can then reference it in the layout by replacing Roll with @string/roll

Handle the Button

In order for us to access objects in the layout file we need to have a way to refer to the different elements, we can do this by searching for the relevant layout item. To make this easy we can define an ID for a view and this is generated as a constant in the R class and we can use findViewById in the Kotlin code and we can access this object

Set the Id for the button to modify the text when the activity starts up

Set the ID on the button with android:id="@+id/roll_button"
Get the button from the onCreate method with

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var rollButton = findViewById<Button>(R.id.roll_button)

Update the button text with

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rollButton.text = "Let's Roll"

We can then add a listener to handle the button click using and simply show a toast message. We make use of the setOnClickListener to do this

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rollButton.setOnClickListener {
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    Toast.makeText(this, "button clicked", Toast.LENGTH_SHORT)
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        .show()
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}

We can make use of this handler to change the TextView, set the ID to @+id/result_text

Next we’ll move the handler to it’s own logic. Create a function in the class called handleRollClick which will just replace the text for now, first be sure to add import kotlin.random.Random and use that Random

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import kotlin.random.Random
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...
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private fun handleDiceRoll() {
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    val randomInt = Random.nextInt(6) + 1
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    val resultText = findViewById<TextView>(R.id.result_text)
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    resultText.text = randomInt.toString()
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}

Then reference this from the onClickHandler with:

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rollButton.setOnClickListener { handleDiceRoll() }

Clicking the button now should display a random number

Use an Image

First, download all the images from here and unzip the folder

The images we’re using are all vectors. Switch to Project view and go to app > src > main > res > Drawable and drag the images into the Drawable folder

You can then preview these images in Android Studio

Now replace the result_text with the below Image View that shows the empty_dice image until the dice has been rolled

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<ImageView
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        android:id="@+id/dice_image"
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        android:layout_width="wrap_content"
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        android:layout_height="wrap_content"
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        android:layout_gravity="center_horizontal"
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        android:src="@drawable/empty_dice" />

Then in the handler get the dice_image view and set the imageResource to the image based on the number we have

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private fun handleDiceRoll() {
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    val randomInt = Random.nextInt(6) + 1
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    val drawableResource = when (randomInt) {
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        1 -> R.drawable.dice_1
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        2 -> R.drawable.dice_2
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        3 -> R.drawable.dice_3
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        4 -> R.drawable.dice_4
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        5 -> R.drawable.dice_5
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        else -> R.drawable.dice_6
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    }
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    val diceImage = findViewById<ImageView>(R.id.dice_image)
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    diceImage.setImageResource(drawableResource)
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}

We should minimize the calls to findViewById as this can be expensive. We should keep this reference in a field, we should create a lateinit variable which allows us to say that a variable will not be used before we assign it initially

The MainActivity can be updated to use the lateinit variable by inializing in with the onCreate function and then using that reference in the handleDiceRoll function

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class MainActivity : AppCompatActivity() {
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    lateinit var diceImage: ImageView
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    override fun onCreate(savedInstanceState: Bundle?) {
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        ...
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        rollButton.setOnClickListener { handleDiceRoll() }
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        diceImage = findViewById(R.id.dice_image)
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    }
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    private fun handleDiceRoll() {
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        ...
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        diceImage.setImageResource(drawableResource)
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    }
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}

We can also set the ImageView to set some data for the preview data, this is helpful for when we are designing and our data is based on some image or data that will maybe be fetched during some loading process, we can show the data for this with the tools namespace in the XML. For the ImageView we can change the image src for the preview by adding this:

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android:src="@drawable/dice_1"

So our resulting ImageView will be:

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<Button
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        android:layout_width="wrap_content"
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        android:layout_height="wrap_content"
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        android:layout_gravity="center_horizontal"
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        android:text="@string/roll"
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        android:id="@+id/roll_button"/>

Gradle

Gradle is the build system that is used for android, it does a few different things:

Specify what devices can run an app
Compile that application
Dependency management
App Signing
Automated Testing

When running an application the Gradle file compiles the application into an APK, there are two Gradle file types in the project, one for the entire project and one for each module

The Gradle website has some detail into what each of the different parts of this file are

Looking at the Project Gradle file in repositories we can see which remote servers libraries will be downloaded from:

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repositories {
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    google()
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    jcenter()
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}

Next we specify the dependencies for the Project, these are reference the plugins needed:

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dependencies {
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    classpath 'com.android.tools.build:gradle:3.4.0'
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    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"  "
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    // NOTE: Do not place your application dependencies here; they belong
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    // in the individual module build.gradle files
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}

If we look at the app module build.gradle we can see the different plugins needed for a project

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apply plugin: 'com.android.application'
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apply plugin: 'kotlin-android'
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apply plugin: 'kotlin-android-extensions'

Next we can see the configuration for our module:

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compileSdkVersion 28
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defaultConfig {
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    applicationId "com.nabeelvalley.diceroller"
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    minSdkVersion 19
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    targetSdkVersion 28
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    versionCode 1
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    versionName "1.0"
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    testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
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}

If we look further down we can see the dependencies needed for a specific module:

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dependencies {
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    implementation fileTree(dir: 'libs', include: ['*.jar'])
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    implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"  "
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    implementation 'androidx.appcompat:appcompat:1.1.0'
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    implementation 'androidx.core:core-ktx:1.1.0'
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    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
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    testImplementation 'junit:junit:4.12'
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    androidTestImplementation 'androidx.test:runner:1.2.0'
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    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
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}

Updating App Compatability

Note that androidx is included as a dependency for the application, this is a collection of libraries that are used to provide compatability across different android API levels

For us to make use of this for better image support, we can do the following:

Enable the support library in the app build.gradle file in the defaultConfig:

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vectorDrawables.useSupportLibrary = true

Replace app:src with app:srcCompat
Add the XML namespace to the app:

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xmlns:app="http://schemas.android.com/apk/res-auto"

Not sure how necessary this is but I also needed to change the ImageView to androidx.appcompat.widget.AppCompatImageView because the preview no longer worked after changing to use srcCompat

The resulting ImageView is:

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<androidx.appcompat.widget.AppCompatImageView
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        android:id="@+id/dice_image"
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        android:layout_width="wrap_content"
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        android:layout_height="wrap_content"
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        android:layout_gravity="center_horizontal"
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        app:srcCompat="@drawable/empty_dice"
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        tools:src="@drawable/dice_1"/>

If we want to be more correct about how we’re handling the Image view we can also change the type of our diceImage property to be AppCompatImageView which we import from androidx, so our declaration looks like so:

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lateinit var diceImage: AppCompatImageView

Developing Layouts

All visual elements are views and are children of the View class and have some common properties like height, width, background, and interactivity

Units for expressing dimensions are dp which are Density Independent Pixels which allow size to be consistent over different device sizes

Views are organised as a heirarchy and ViewGroups are views that are used to contain other views, some view groups are:

LinearLayout Horizontal or Vertial layouts
ScrollView for scrollable content

Deeper view heirarchies are more complex to render and can impact performance, the ConstraintLayout helps you to arrange more complex layouts without deep nesting of Views

Create Project

Create a new project called About Me with an Empty Activity

Name	About Me
Package	com.nabeelvalley.aboutme
Save Location	C:\Repos\AndroidStudio\DiceRoller
Language	Kotlin
Minimum API Level	API Level 19: Android 4.4
	Use `androidx.*` artifacts

Set up Linear Layout

Replace the existing layout for activity_main.xml and set the Root element to be a LinearLayout

To get started let’s use the following layout:

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<?xml version="1.0" encoding="utf-8"?>
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<LinearLayout
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        xmlns:android="http://schemas.android.com/apk/res/android"
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        xmlns:tools="http://schemas.android.com/tools"
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        xmlns:app="http://schemas.android.com/apk/res-auto"
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        android:layout_width="match_parent"
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        android:layout_height="match_parent"
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        tools:context=".MainActivity" android:id="@+id/linearLayout">
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        <TextView
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                android:textSize="16sp"
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                android:layout_height="wrap_content"
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                android:layout_width="wrap_content"
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                android:text="Hello World"/>
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</LinearLayout>

Will be using a mixture of the layout editor and the XML view, you can drag and edit elements using the layout editore

We’ll also create a dimension and string resource for the text element size and content using the refactorings (light bulb) in the Android Studio XML view for the layout

The final layout should be like:

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<?xml version="1.0" encoding="utf-8"?>
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<LinearLayout
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        xmlns:android="http://schemas.android.com/apk/res/android"
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        xmlns:tools="http://schemas.android.com/tools"
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        xmlns:app="http://schemas.android.com/apk/res-auto"
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        android:layout_width="match_parent"
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        android:layout_height="match_parent"
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        tools:context=".MainActivity" android:id="@+id/linearLayout">
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        <TextView
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                android:textSize="@dimen/text_size"
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                android:layout_height="wrap_content"
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                android:layout_width="match_parent"
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                android:text="@string/text_name" android:id="@+id/nameText" android:textAlignment="center"
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                android:textColor="@android:color/black"/>
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</LinearLayout>

With the following in the:

values/strings.xml

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<resources>
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    <string name="app_name">About Me</string>
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    <string name="text_name">Nabeel Valley</string>
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</resources>

values/dimens.xml

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<?xml version="1.0" encoding="utf-8"?>
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<resources>
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    <dimen name="text_size">20sp</dimen>
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</resources>

View Styling

We can modify the styles for a component, for spacing on a view we have the following:

Padding
Border
Margin

We can update the text view to add some spacing (using a dimension) and select the Roboto font. We can then use Android studio to extract the styling to a style with the following steps:

Right click text view > Refactor > Extract Style
Keep margins, colour, and font
The style should be created and applied:

In the styles.xml file you can see the created style:

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<style name="text_body">
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    <item name="android:textSize">@dimen/text_size</item>
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    <item name="android:layout_height">wrap_content</item>
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    <item name="android:layout_width">match_parent</item>
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    <item name="android:textColor">@android:color/black</item>
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    <item name="android:layout_marginTop">@dimen/layout_margin</item>
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    <item name="android:fontFamily">@font/roboto</item>
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</style>

And we can apply it to a View using the style attribute:

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<TextView
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        android:text="@string/text_name"
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        android:id="@+id/nameText"
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        android:textAlignment="center"
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        style="@style/text_body"/>

Checkpoint:

https://classroom.udacity.com/courses/ud9012/lessons/4f6d781c-3803-4cb9-b08b-8b5bcc318d1c/concepts/6efde730-a337-4d8e-b295-659d116fe9b8