Accessing TextView in a custom class in Kotlin

Question

I'm a newbie in Android and Kotlin and I'm facing a problem that I can't solve it. I have a activity_main.xml file that contains a seekBar and a TextView. When I'm changing the seekBar I want to display the changed value in the TextView in realtime. To learn new things and practicing Kotlin I create a new class that contains a Seekbar.OnSeekBarChangeListener and that class reacts the seekBar changes. But the problem is when I set the TextView to the new seekBar value it's cause a findViewById NullPointException.

FATAL EXCEPTION: main
Process: kotlindemo1.kristof.kotlin_1demo, PID: 24410 
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
at android.app.Activity.findViewById(Activity.java:2090)
at kotlindemo1.kristof.kotlin_1demo.seekB._$_findCachedViewById(seekB.kt:0)
at kotlindemo1.kristof.kotlin_1demo.seekB.onProgressChanged(seekB.kt:15)
at android.widget.SeekBar.onProgressRefresh(SeekBar.java:93)

This is the MainActivity class:

package kotlindemo1.kristof.kotlin_1demo

import android.app.Activity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*


open class MainActivity : Activity() {

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    sB1.setOnSeekBarChangeListener(seekB())
}
}

Class that handle the SeekBar changes:

package kotlindemo1.kristof.kotlin_1demo

import android.app.Activity
import android.os.Bundle
import android.widget.SeekBar
import kotlinx.android.synthetic.main.activity_main.*

class seekB : SeekBar.OnSeekBarChangeListener, Activity(){
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
}

override fun onProgressChanged(seekBar: SeekBar?, progress: Int, fromUser: Boolean) {
    if(fromUser) tW1.text = getString(R.string.max_gen_number, progress) //java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
}

override fun onStartTrackingTouch(seekBar: SeekBar?) {
}

override fun onStopTrackingTouch(seekBar: SeekBar?) {
}
}

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/cL"
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="kotlindemo1.kristof.kotlin_1demo.MainActivity"
>

<SeekBar
    android:id="@+id/sB1"
    android:progress="100"
    android:max="200"
    android:layout_width="182dp"
    android:layout_height="21dp"
    android:layout_marginTop="60dp"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    tools:layout_constraintRight_creator="1"
    tools:layout_constraintLeft_creator="1"
    />

<TextView
    android:id="@+id/tW1"
    android:layout_width="139dp"
    android:layout_height="wrap_content"
    android:text=""
    android:textAlignment="center"
    android:layout_marginTop="20dp"
    app:layout_constraintTop_toBottomOf="@+id/sB1"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintHorizontal_bias="0.502"
    tools:layout_constraintRight_creator="1"
    tools:layout_constraintLeft_creator="1" />

</android.support.constraint.ConstraintLayout>

In Java it's easy, using the FindViewById and SetContentView methods to referencing a widget but in Kotlin if we importing kotlinx.android.synthetic.main.activity_main.* than we don't need to use findViewById. I tried to pass the context from the MainActivity but that's not helped.

Answer 1

Bro i did from most easy way for you. My Android Studio version is 3.0 Canary 5.If you not update your Android Studio, you may get exception.

MainActivity.kt In onCreate

val textView = findViewById<TextView>(R.id.tv)
    val seekBar = findViewById<SeekBar>(R.id.seekBar)

    seekBar.setOnSeekBarChangeListener(object : SeekBar.OnSeekBarChangeListener{
        override fun onProgressChanged(p0: SeekBar?, p1: Int, p2: Boolean) {

            textView.text = p1.toString()

        }

        override fun onStartTrackingTouch(p0: SeekBar?) {
        }

        override fun onStopTrackingTouch(p0: SeekBar?) {
        }
    })

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.kt.seekbar.MainActivity">

<TextView
    android:id="@+id/tv"
    android:layout_centerInParent="true"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

<SeekBar
    android:id="@+id/seekBar"
    android:layout_centerInParent="true"
    android:max="100"
    android:layout_below="@id/tv"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />


</RelativeLayout>

Answer 2

Its working i tried.I added TextView reference to seekB's constructor.

MainActivity

class MainActivity : AppCompatActivity() {

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

    val textView = findViewById<TextView>(R.id.tv)
    val seekBar = findViewById<SeekBar>(R.id.seekBar)

    seekBar.setOnSeekBarChangeListener(seekB(textView))


}
}

seekB class

class seekB(val tv: TextView) : SeekBar.OnSeekBarChangeListener{

override fun onProgressChanged(seekBar: SeekBar?, progress: Int, fromUser: 
Boolean) {
    if(fromUser) tv.text = progress.toString()
}

override fun onStartTrackingTouch(seekBar: SeekBar?) {
}

override fun onStopTrackingTouch(seekBar: SeekBar?) {
}
}