CODEWITHSUNDEEP
android-studiokotlinandroid-viewpagerlistener
gi pi
gi pi

Reputation: 103

Kotlin AddOnPageChangeListener not working

I wanted to connect a viewPager with an adapter (like this: viewPager.addOnPageChangeListener()), but the letters of the pageChangeListener just turn red like it wasn't a valid code...What am I doing wrong?? Heres a screenshot: [1]: https://i.sstatic.net/IOYJY.png

Context: I'm currently working on a game with a few fragments where you can choose your game cards. I need the pageChangeListener to change the pictures of them cards. Maybe there could be another way to do this but i don't know how...

package com.suffv1

import android.os.Bundle
import androidx.appcompat.app.ActionBar
import androidx.appcompat.app.AppCompatActivity
import androidx.viewpager.widget.ViewPager
import com.example.suff_02.Adapter2
import com.example.suff_02.R
import com.example.suff_02.kartenmodell

import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity(){

    private lateinit var actionbar: ActionBar

    private lateinit var liste: ArrayList<kartenmodell>
    private lateinit var myAdapter: Adapter2

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

        actionbar = this.supportActionBar!!

        loadCards()

        viewpager2.addOnPageChangeListener(object: ViewPager.OnPageChangeListener{

            override fun onPageScrolled(
                position: Int,
                positionOffset: Float,
                positionOffsetPixels: Int
            ) {
                val title = liste[position].KartenImage
                actionbar.title = title
            }

        })

    }

    private fun loadCards() {
        liste = ArrayList()

        liste.add(kartenmodell(R.drawable.bier_radler_klein_level_1))
        liste.add(kartenmodell(R.drawable.bier_hopfentrunk_klein_level_1))
        liste.add(kartenmodell(R.drawable.bier_butt_light_klein_level_1))
        liste.add(kartenmodell(R.drawable.bier_becks_klein_level_1))
        liste.add(kartenmodell(R.drawable.bier_tyskie_klein_level_1))

        myAdapter = Adapter2(this, liste)

        viewpager2.adapter = myAdapter

        viewpager2.setPadding(100, 0, 100, 0)
    }
}

Upvotes: 4

Views: 3195

Answers (2)

Andrew
Andrew

Reputation: 10162

Looks like you are using ViewPager2 not the original Viewpager and Viewpager2 does not have Page Change Listeners it instead has Page Change Callbacks

So you are using the wrong method to get notified when a pages is changed.

Instead do something like

var myPageChangeCallback = object : ViewPager2.OnPageChangeCallback() {
  override fun onPageSelected(position: Int) {
    Toast.makeText(this@MainActivity, "Selected position: ${position}", 
      Toast.LENGTH_SHORT).show()
  }
}

viewpager.registerOnPageChangeCallback(myPageChangeCallback)

Though architecturally a lot of times it is bad form to use a OnPageChangeCallback as you are likely breaking the encapsulation idea of the Fragment and it can be better to use the lifecycle state change to Resumed of the Fragment to do things when a page(Fragment) is selected. e.g. put the code in the Fragments onResume method.

Though in this case of setting the actionbar title it is probably ok architecturally to use a OnPageChangeCallback

Upvotes: 7

Lalit Fauzdar
Lalit Fauzdar

Reputation: 6363

To create a AddOnPageChangeListener(), you do it as:

//See how, you have to use object to create an Object of the interface OnPageChangeListener.
//This is how you do for other listeners/interfaces/class as well when you've to implement its member functions instead of new in java.
viewPager.addOnPageChangeListener(object: ViewPager.OnPageChangeListener{
    override fun onPageScrolled(position: Int, positionOffset: Float, positionOffsetPixels: Int) {
        TODO("Not yet implemented")
    }

    override fun onPageSelected(position: Int) {
        TODO("Not yet implemented")
    }

   override fun onPageScrollStateChanged(state: Int) {
       TODO("Not yet implemented")
   }

})

But, further in Kotlin, you can use Kotlin functions as

//This is for onPageSelected only.
viewPager.onPageChangeListener{ position: Int ->
    //This position is the selected Page's position
}

Upvotes: 1

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