Button onClick attribute is none if activity written in Kotlin - kotlin

Question

Follow this tutorial: Android - Start Another Activity if I made MainActivity.java button OnClick attribute has the sendMessage() method.
But if I made MainActivity.kt button OnClick attribute has nothing to show, just a none.
Is this an Android Studio 3 bug or I missed something for Kotlin?
Java mainActivity:
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
/** Called when the user taps the Send button */
public void sendMessage(View view) {
// Do something in response to button
}
}
Kotlin mainActivity:
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
}
/** Called when the user taps the Send button */
fun sendMessage(view: View) {
// Do something in response to button
}
}
XML layout (Java and Kotlin project are the same)
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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="ir.bigbang.vahid.myapplication.MainActivity">
<Button
android:id="#+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button"
tools:layout_editor_absoluteX="148dp"
tools:layout_editor_absoluteY="81dp" />
</android.support.constraint.ConstraintLayout>

Answer 1

It seems like the designer does not support Kotlin yet. Here are some solution:
XML (Not Recommended)
Add the following line to your Button tag. This is exactly what the designer will do.
android:onClick="sendMessage"
Old Fashion
No need to add anything.
val button = findViewById<Button>(R.id.Button)
button.setOnClickListener {
}
kotlin-android-extensions (Recommended)
Add apply plugin: "kotlin-android-extensions" to your build.gradle
// button is the Button id
button.setOnClickListener {
}

Answer 2

Your code will like this:
button.setOnClickListener(){
Toast.makeText(this#MainActivity, "Its toast!", Toast.LENGTH_SHORT).show();
}
Here import will:
import kotlinx.android.synthetic.main. activity_main.*
Here "button" is the id of that Button in .xml file. Here the advantage is no need to create Button object in your java class.

Answer 3

Once defined the sendMessage class as :
/** Called when the user taps the Send button */
fun sendMessage(view: View) {
setContentView(R.layout.activity_second)
// Do something in response to button
}
And also defined a second activity as:
class SecondActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_second)
}
}
I added the SendMessage to the OnClick function:
And then it worked.

Answer 4

You can easily define this inside the XML itself. But using the android:onClick attribute is still a little expensive.
Instead you could consider using the Kotlin Android Extensions and synthetic properties:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
button.setOnClickListener {
// Do something in response to button
}
}

Answer 5

Button OnClick implementation it's can be done by some ways in Android
some of the possible ways are below in sample:
1>Using OnClickListener as a interface
Here we implement our main activity with OnClicklistener
and override the function onClick
override fun onClick(v: View?) {
when (v?.id){
(R.id.btn1) -> {
toastmsg("Button1");
}
R.id.btn2 -> {
toastmsg("Button2");
}
}
}
2>And create a function and pass the OnClickListener with
variable sample:
findViewById<Button>(R.id.btn3).setOnClickListener(btnClick);
var btnClick =
OnClickListener {
Toast.makeText(this, "BtnClick", Toast.LENGTH_SHORT).show() ;
}
3>Create OnClickListener in Oncreate()
btn1=findViewById(R.id.btn1);
btn1?.setOnClickListener {
toastmsg("test button1");
}
full sample Code of the example it contains all the possible implementation of the Button OnClickListener :
class MainActivity : AppCompatActivity() , OnClickListener{
lateinit var tv1:TextView;
lateinit var tv2:TextView;
lateinit var tv3:TextView;
var btn1: Button? =null;
var btn2: Button? =null;
var btn3: Button? =null;
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
btn1=findViewById(R.id.btn1);
btn1?.setOnClickListener {
toastmsg("test button1");
}
findViewById<Button>(R.id.btn2).setOnClickListener(this);
findViewById<Button>(R.id.btn3).setOnClickListener(btnClick);
}
var btnClick =
OnClickListener {
Toast.makeText(this, "BtnClick", Toast.LENGTH_SHORT).show() ;
}
override fun onClick(v: View?) {
when (v?.id){
(R.id.btn1) -> {
toastmsg("Button1");
}
R.id.btn2 -> {
toastmsg("Button2");
}
}
}
private fun toastmsg(msg: String){
Toast.makeText(this, "DaggerTest" + msg, Toast.LENGTH_SHORT).show();
}
}

Answer 6

Here's the solution I came up with in the MainActivity.kt file.
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val button = findViewById<Button>(R.id.button)
button.setOnClickListener {
sendMessage()
}
}
/** Called when the user taps the Send button */
private fun sendMessage() {
val editText = findViewById<EditText>(R.id.editText)
val message = editText.text.toString()
val intent = Intent(this, DisplayMessageActivity::class.java).apply
{
putExtra(EXTRA_MESSAGE, message)
}
startActivity(intent)
}