Button onClick attribute is none if activity written in Kotlin

问题

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>

回答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 {

}

回答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.

回答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.

回答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
    }
}

回答5:

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)
}

来源：https://stackoverflow.com/questions/46970565/button-onclick-attribute-is-none-if-activity-written-in-kotlin