(Android)Create a ScrollView synchronizer

layout.xml

<!-- Use this in place of the classic ScrollView you want to use -->

<path.to.package.SynchronizerScrollView
    android:id="@+id/synchronizer_scrollview"
    android:...
    >
    ...
</path.to.package.SynchronizerScrollView>

ScrollViewListener.java

public interface ScrollViewListener
{
    void onScrollChanged(SynchronizerScrollView scrollView, int x, int y, int oldx, int oldy);
}

SynchronizedScrollActivity.java

import android.app.Activity;
import android.os.Bundle;

public class SynchronizedScrollActivity extends Activity implements ScrollViewListener
{
    private SynchronizerScrollView scrollView1 = null;
    private ScrollView scrollView2 = null;
    // or
    // private SynchronizerScrollView scrollView2 = null;

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

        scrollView1 = (SynchronizerScrollView) findViewById(R.id.scrollview1);
        scrollView1.setScrollViewListener(this);

        scrollView2 = (ScrollView) findViewById(R.id.scrollview2);
        // or
        // scrollView2 = (SynchronizerScrollView) findViewById(R.id.scrollview2);
        // scrollView2.setScrollViewListener(this);
    }

    public void onScrollChanged(SynchronizerScrollView scrollView, int x, int y, int oldx, int oldy)
    {
        scrollView2.scrollTo(x, y);
        // or
        // if(scrollView == scrollView1)
        // {
        //     scrollView2.scrollTo(x, y);
        // }
        // else if(scrollView == scrollView2)
        // {
        //     scrollView1.scrollTo(x, y);
        // }
    }

}

SynchronizerScrollView.java

import android.content.Context;
import android.util.AttributeSet;
import android.widget.ScrollView;

public class SynchronizerScrollView extends ScrollView
{
    private ScrollViewListener scrollViewListener = null;

    public SynchronizerScrollView(Context context)
    {
        super(context);
    }

    public SynchronizerScrollView(Context context, AttributeSet attrs, int defStyle)
    {
        super(context, attrs, defStyle);
    }

    public SynchronizerScrollView(Context context, AttributeSet attrs)
    {
        super(context, attrs);
    }

    public void setScrollViewListener(ScrollViewListener scrollViewListener) {
        this.scrollViewListener = scrollViewListener;
    }

    @Override
    protected void onScrollChanged(int x, int y, int oldx, int oldy) {
        super.onScrollChanged(x, y, oldx, oldy);
        if(scrollViewListener != null) {
            scrollViewListener.onScrollChanged(this, x, y, oldx, oldy);
        }
    }
}

THANKS

Thanks to @Andy on StackOverflow question: http://stackoverflow.com/questions/3948934/synchronise-scrollview-scroll-positions-android