Unit 1: Kotlin basics for Android

Android Basics: Introduction to Kotlin

Write your first program in Kotlin

• https://developer.android.com/training/kotlinplayground is an interactive code editor on the web where you can practice writing Kotlin programs.
• All Kotlin programs need to have a main() function: fun main() {}
• Use the println() function to print a line of text.
• Place text you want to print between double quotes. For example "Hello".
• Repeat the println() instruction to print multiple lines of text.
• Errors are marked red in the program. There is an error message in the output pane to help you figure out where the error is and what might be causing it.

fun main() {
    println("Happy Birthday!")
}

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Create a birthday message in Kotlin

• Create a variable using the val keyword and a name. Once set, this value cannot be changed. Assign a value to a variable using the equal sign. Examples of values are text and numbers.

val age = 5

• Use ${} to surround variables and calculations in the text of print statements. For example: ${age} where age is a variable.* A String is text surrounded by quotes, such as "Hello".

println("You are already ${age} days old, ${name}!")

• An Int is a whole positive or negative number, such as 0, 23, or -1024.
• You can pass one or more arguments into a function for the function to use, for example:

fun printCakeBottom(age: Int, layers: Int) {}

• Use a repeat() {} statement to repeat a set of instructions several times. For example:

repeat (23) { print("%") }
repeat (layers) { print("@@@@@@@@@@") }

• A loop is an instruction to repeat instructions multiple times. A repeat() statement is an example of a loop.
• You can nest loops, that is, put loops within loops. For example, you can create a repeat() statement within a repeat() statement to print a symbol a number of times for a number of rows, like you did for the cake layers.

fun printCakeBottom(age: Int, layers: Int) {
    repeat(layers) {
        repeat(age + 2) {
            print("@")        
        }
        println()
    }
}

Summary of using function arguments: To use arguments with a function, you need to do three things:

• Add the argument and type to the function definition: printBorder(border: String)
• Use the argument inside the function: println(border)
• Supply the argument when you call the function: printBorder(border)

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Android Basics: Create your first Android app

Create and run your first Android app

• To create a new project, start Android Studio, click Start a new Android Studio project, name your project, choose a template (Usually Empty Activity), and fill in the details (Choose API 19: Android 4.4 (KitKat) as Minimum SDK).
• To create an Android virtual device (an emulator) to run your app, choose Tools > AVD Manager and then use the AVD Manager to select a hardware device and system image.
• To run your app on a virtual device, make sure you have created a device, select the device from the toolbar dropdown menu, and then run your app by clicking the Run icon! choose Run > Run app or click the Run icon in the toolbar.
• To find your project files, in the Project window, select Project Source Files from the dropdown.

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Android Basics: Build a basic layout

Create a Birthday Card app

• The Layout Editor helps you create the UI for your Android app.
• Almost everything you see on the screen of your app is a View. Views can be interactive, like a clickable button or an editable input field.
• A TextView is a UI element for displaying text in your app.

<TextView
    android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginStart="16dp"
    android:layout_marginTop="16dp"
    android:fontFamily="sans-serif-light"
    android:text="Happy Birthday Sam!"
    android:textColor="@android:color/black"
    android:textSize="36sp"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

• A ConstraintLayout is a container for other UI elements.

<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout\_width="match\_parent"
    android:layout\_height="match\_parent"
    tools:context=".MainActivity">
</androidx.constraintlayout.widget.ConstraintLayout>

• Views need to be constrained horizontally and vertically within a ConstraintLayout.
• One way to position a View is with a margin.
• A margin says how far a View is from an edge of the container it’s in.
• You can set attributes on a TextView like the font, text size, and color.

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Add images to your Android app

• TheResource Managerin Android Studio helps you add and organize your images and other resources.
• An ImageView is a UI element for displaying images in your app.
• ImageViews should have a content description to help make your app more accessible.

<ImageView
    android:id="@+id/imageView"
    android:layout_width="0dp"
    android:layout_height="0dp"
    android:importantForAccessibility="no"
    android:scaleType="centerCrop"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:srcCompat="@drawable/androidparty" />

• Text that is shown to the user like the birthday greeting should be extracted into a string resource to make it easier to translate your app into other languages.

android:text="@string/happy_birthday_text"

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Android Basics: Add a button to an app

Classes and object instances in Kotlin

• Call the random() function on an IntRange to generate a random number: (1..6).random()

fun main() {
    val diceRange = 1..6
    val randomNumber = diceRange.random()
    println("Random number: ${randomNumber}")
}

• Classes are like a blueprint of an object. They can have properties and behaviors, implemented as variables and functions.

class Dice {
    var sides = 6
    fun roll() {
        val randomNumber = (1..6).random()
        println(randomNumber)
    }
}

• An instance of a class represents an object, often a physical object, such as a dice. You can call the actions on the object and change its attributes.

fun main() {
    val myFirstDice = Dice()
    println(myFirstDice.sides)
    myFirstDice.roll()
}

• You can supply values to a class when you create an instance. For example: class Dice(val numSides: Int) and then create an instance with Dice(6).

class Dice (val numSides: Int) {
 
    fun roll(): Int {
        return (1..numSides).random()
    }
}

• Functions can return something. Specify the data type to be returned in the function definition, and use areturnstatement in the function body to return something. For example: fun example(): Int { return 5 }

fun roll(): Int {
     val randomNumber = (1..6).random()
     return randomNumber
}

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Create an interactive Dice Roller app

• The top-level or first activity is often called the MainActivity and is provided by the project template. For example, when the user scrolls through the list of apps on their device and taps on the “Dice Roller” app icon, the Android System will start up the MainActivity of the app.

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

• Add a Button in an Android app using the Layout Editor.

<Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/roll"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/textView" />

• Modify the MainActivity.kt class to add interactive behavior to the app.

val rollButton: Button = findViewById(R.id.button)
rollButton.setOnClickListener {
    // code here
}

• Pop up a Toast message as a temporary solution to verify you’re on the right track.

val toast = Toast.makeText(this, "Dice Rolled!", Toast.LENGTH_SHORT)
toast.show()
// Or you could combine the two lines in the click listener into a single line without a variable.
Toast.makeText(this, "Dice Rolled!", Toast.LENGTH_SHORT).show()

• Set an on-click listener for a Button using setOnClickListener() to add behavior for when a Button is clicked.

rollButton.setOnClickListener {
        val resultTextView: TextView = findViewById(R.id.textView)
        resultTextView.text = "6"
    }

class Dice(val numSides: Int) {
 
   fun roll(): Int {
       return (1..numSides).random()
   }
}
rollButton.setOnClickListener { rollDice() }
/**
* Roll the dice and update the screen with the result.
*/
private fun rollDice() {
   // Create new Dice object with 6 sides and roll it
   val dice = Dice(6)
   val diceRoll = dice.roll()
 
   // Update the screen with the dice roll
   val resultTextView: TextView = findViewById(R.id.textView)
   resultTextView.text = diceRoll.toString()
}

• When the app is running, you can update the screen by calling methods on the TextView, Button, or other UI elements in the layout.
• Comment your code to help other people who are reading your code understand what your approach was.
• Reformat your code and clean up your code.

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Add conditional behavior in Kotlin

• Use an if statement to set a condition for executing some instructions. For example, if the user rolls the lucky number, print a winning message.

fun main() {
   val num = 5
   if (num > 4) {
       println("The variable is greater than 4")
   }
}

• The Boolean data type has values of true and false and can be used for decision making.
• Compare values using operators such as greater than (>), less than (<), and equal to (-).
• Use a chain of else if statements to set multiple conditions. For example, print a different message for each possible dice roll.
• Use an else statement at the end of a chain of conditions to catch any cases that may not be covered explicitly. If you cover the cases for 6-sided dice, an else statement would catch the 7 and 8 numbers rolled with an 8-sided dice.

fun main() {
    val num = 4
    if (num > 4) {
        println("The variable is greater than 4")
    } else if (num == 4) {
        println("The variable is equal to 4")
    } else {
        println("The variable is less than 4")
    }
}

• Use a when statement as a compact form of executing code based on comparing a value.

when (rollResult) {
    luckyNumber -> println("You won!")
    1 -> println("So sorry! You rolled a 1. Try again!")
    2 -> println("Sadly, you rolled a 2. Try again!")
    3 -> println("Unfortunately, you rolled a 3. Try again!")
    4 -> println("No luck! You rolled a 4. Try again!")
    5 -> println("Don't cry! You rolled a 5. Try again!")
    6 -> println("Apologies! you rolled a 6. Try again!")
}

General form of if-else

if (condition-is-true) {

execute-this-code

} else if (condition-is-true) {

execute-this-code

} else {

execute-this-code

}

When statement

when (variable) {

matches-value -> execute-this-code

matches-value -> execute-this-code

}

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Add images to the Dice Roller app

• UsesetImageResource()to change the image that’s displayed in an ImageView

private fun rollDice() {
    val dice = Dice(6)
    val diceRoll = dice.roll()
    val diceImage: ImageView = findViewById(R.id.imageView)
    diceImage.setImageResource(R.drawable.dice_2)
}

• Use control flow statements like if / else expressions or when expressions to handle different cases in your app, for example, showing different images under different circumstances.

/**
* Roll the dice and update the screen with the result.
*/
private fun rollDice() {
    // Create new Dice object with 6 sides and roll the dice
    val dice = Dice(6)
    val diceRoll = dice.roll()
 
    // Find the ImageView in the layout
    val diceImage: ImageView = findViewById(R.id.imageView)
 
    // Determine which drawable resource ID to use based on the dice roll
    val drawableResource = when (diceRoll) {
        1 -> R.drawable.dice_1
        2 -> R.drawable.dice_2
        3 -> R.drawable.dice_3
        4 -> R.drawable.dice_4
        5 -> R.drawable.dice_5
        else -> R.drawable.dice_6
    }
 
    // Update the ImageView with the correct drawable resource ID
    diceImage.setImageResource(drawableResource)
 
    // Update the content description
    diceImage.contentDescription = diceRoll.toString()
}

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