Kotlin Android Tutorial Shorten findViewById setOnClickListener Basic Kotlin Syntax

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Step 1: Create an Android Studio project with Kotlin support.

If you are not using Android Studio 3.0 or above version then you need to upgrade your Android Studio.

I have downloaded Android Studio 3.0 version (canary version) from this link.

Follow steps in this link to run 3.0 version.

When you are creating new project, include kotlin support as per below image.

Step 2: Updating activity_main.xml file

Add below source code into activity_main.xml 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.exampledemo.parsaniahardik.basicskotlin.MainActivity">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/btn"
        android:text="Hello World!"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/text"
        android:text="hello kotlin" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/btnfunction"
        android:text="What is 2 + 3 ?"/>

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/img"/>


</LinearLayout>

Step 3: Updating MainActivity.kt

Copy following source code into MainActivity.kt class

import android.app.Activity
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.support.annotation.IdRes
import android.view.View
import android.widget.Button
import android.widget.ImageView
import android.widget.TextView
import android.widget.Toast

class MainActivity : AppCompatActivity() {

    // replace findViewById() by following
    private val button: Button by bind(R.id.btn)
    private val buttonFunction: Button by bind(R.id.btnfunction)
    private val textview: TextView by bind(R.id.text)
    private val imageView: ImageView by bind(R.id.img)

    //declare variables like below
    private var stringVariable : String? = null //declaring string variable
    private var integerVariable : Int? = null //value of this variable can be changed
    private val readOnlyVariable: Int = 1 //value of this variable can not be changed

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

        button.text = "give string to button by this way!"

        stringVariable = button.text as String //giving button's string to stringVariable

        textview.text = stringVariable //setting textview's text same as button's text

        integerVariable = 1  //giving any integer value to integer variable

        buttonFunction.setOnClickListener(){
            giveAddition(2,3)
           Toast.makeText(this, "This answer is from Kotlin function -->"+giveAddition(2,3), Toast.LENGTH_LONG).show()
        }

        imageView.setImageResource(R.mipmap.ic_launcher)

    }

    fun <T : View> Activity.bind(@IdRes res : Int) : Lazy<T> {
        @Suppress("UNCHECKED_CAST")
        return lazy(LazyThreadSafetyMode.NONE){ findViewById(res) as T }
    }

    fun giveAddition(a: Int, b: Int): Int {
        return a + b
    }

}

Step 4: Description of Various Syntaxes

Removing findViewById

Till now you are setting ids to views by findViewById. But you need to use findViewById  for every single view in whole Java file. In Kotlin, you can say good bye to findViewById. But How? 

First, put following function

fun <T : View> Activity.bind(@IdRes res : Int) : Lazy<T> {
        @Suppress("UNCHECKED_CAST")
        return lazy(LazyThreadSafetyMode.NONE){ findViewById(res) as T }
}

Now to give ids to every views, you just need to do is

 private val button: Button by bind(R.id.btn)

Give id at the last and view type(Button, ImageView, TextView etc.) at middle as shown in above code.

Declaring Variables

Define variables in Kotlin as follow

String variable

 private var stringVariable : String? = null //declaring string variable 
 private var stringVariables : String = "hello"

Integer variable

private var integerVariable : Int? = null //value of this variable can be changed   
private val readOnlyVariable: Int = 1 //value of this variable can not be changed

Now take attention at above two lines. First line starts with var and second with val.

There are two types of variables in Kotlin. Mutable and Immutable.

 var vs val in Kotlin

var is mutable variable, that means value of this type of variable can be changed.(Read and Write)

val is immutable variable, that means value of this type of variable can not be changed. (Read only)

Setting Button’s label or text

button.text = "give string to button by this way!"

conventional method setText() is not used here.

Giving button’s text to String Variable

stringVariable = button.text as String //giving button's string to stringVariable

Setting TextView’s text from String Variable

textview.text = stringVariable //setting textview's text same as button's text

Giving value to integer variable

integerVariable = 1  //giving any integer value to integer variable

No need of semicolon(;) at the end.

Reducing lines for setOnClickListener

 buttonFunction.setOnClickListener(){
            giveAddition(2,3)
           Toast.makeText(this, "This answer is from Kotlin function -->"+giveAddition(2,3), Toast.LENGTH_LONG).show()
}

Syntax for function

 fun giveAddition(a: Int, b: Int): Int {
        return a + b
    }

As you can see there is little difference than Java code. In Java, we used to give return type at starting where here it is written at the end of first line.

Void in Java is written as Unit in Kotlin. For example

fun printHello(name: String?): Unit {
    if (name != null)
        println("Hello ${name}")
    else
        println("Hi there!")
    // `return Unit` or `return` is optional
}

Setting Image Resource

 imageView.setImageResource(R.mipmap.ic_launcher)

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