Correctly implementing PagerAdapter in Android

Question

I have problems with implementation of my custom PagerAdapter and using it with a ViewPager. This sample PagerAdapter has 10 items, every item is a button with it's index as text. When I run my program, I see a button with text '1' insted of '0'. And when I swipe to other items I get only blank views. When I swipe backwards sometimes I see a button with some number, but it disappears (maybe it is destroying and I remove it from the container), and sometimes I see a button with a number, but the number changes after the swipe (I think I create a new Button and I add it to the container, and for some reasons the viewpager shows this new button).

How can I fix this implementation? I haven't seen difference in examples.

My PagerAdapter implementation:

public class MyPagerAdapter extends PagerAdapter {

    @Override
    public int getCount() {
        return 10;
    }

    @Override
    public boolean isViewFromObject(View view, Object o) {
        return o.getClass()==view.getClass();
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Button button = new Button(container.getContext());
        ViewGroup.LayoutParams params = new ActionBar.LayoutParams(-1,-1);
        button.setLayoutParams(params);
        button.setText(String.valueOf(position));
        container.addView(button);
        return button;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((Button)object);
    }
}

And my Activity:

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ViewPager pager = (ViewPager) findViewById(R.id.pager);
        pager.setAdapter(new MyPagerAdapter());
    }
}

Answer 1

Here is complete code:

xml layout:

<LinearLayout 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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:orientation="vertical"
    tools:context="com.example.androidviewpagerapp.MainActivity" >

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</LinearLayout>

MyPagerAdapter class:

import android.support.v4.view.PagerAdapter;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.Toast;

public class MyPagerAdapter extends PagerAdapter {

    @Override
    public int getCount() {
        return 10;
    }

    @Override
    public boolean isViewFromObject(View view, Object o) {
        return o==view;
    }

    @Override
    public Object instantiateItem(final ViewGroup container, int position) {
        Button button = new Button(container.getContext());
        ViewGroup.LayoutParams params = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
        button.setLayoutParams(params);
        button.setText(String.valueOf(position));

        final int page = position;
        button.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                Toast.makeText(container.getContext(), "You clicked: " + page + ". page.", Toast.LENGTH_SHORT).show();
            }
        });

        container.addView(button);
        return button;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((Button)object);
    }
}

MainActivity:

import android.support.v4.view.ViewPager;
import android.app.Activity;
import android.os.Bundle;

public class MainActivity extends Activity {
    ViewPager viewPager;
    MyPagerAdapter myPagerAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        viewPager = (ViewPager)findViewById(R.id.pager);
        myPagerAdapter = new MyPagerAdapter();
        viewPager.setAdapter(myPagerAdapter);
    }
}

You will see that Buttons are full screen. To avoid that you need to create some layout (like LinearLayout) and add button to that layout.

Example:

import android.support.v4.view.PagerAdapter;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.Toast;

public class MyPagerAdapter extends PagerAdapter {

    @Override
    public int getCount() {
        return 10;
    }

    @Override
    public boolean isViewFromObject(View view, Object o) {
        return o==view;
    }

    @Override
    public Object instantiateItem(final ViewGroup container, int position) {
        Button button = new Button(container.getContext());
        ViewGroup.LayoutParams params = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
        button.setLayoutParams(params);
        button.setText(String.valueOf(position));

        LinearLayout layout = new LinearLayout(container.getContext());
        layout.setOrientation(LinearLayout.VERTICAL);
        ViewGroup.LayoutParams layoutParams = new ViewGroup.LayoutParams(
                ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT);

        //add buton to layout
        layout.addView(button);

        final int page = position;
        button.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                Toast.makeText(container.getContext(), "You clicked: " + page + ". page.", Toast.LENGTH_SHORT).show();
            }
        });
        //to container add layout instead of button
        container.addView(layout);
        //return layout instead of button
        return layout;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        //cast to LinearLayout
        container.removeView((LinearLayout)object);
    }
}