Start of a Flingable ViewPager

FlingableViewPager.java

import android.content.Context;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v4.widget.ScrollerCompat;
import android.support.v4.view.ViewCompat;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.ViewConfiguration;
import android.view.animation.DecelerateInterpolator;


public class FlingableViewPager extends ViewPager
{
	private ScrollerCompat  mScroller;
	private VelocityTracker mVelocityTracker;
	private int             mMinimumFlingVelocity;
	private int             mMaximumFlingVelocity;
	private int             mLastFlingX;
	
	
	public FlingableViewPager(Context context)
	{
		this(context, null);
	}
	
	public FlingableViewPager(Context context, AttributeSet attrs)
	{
		super(context, attrs);
		final ViewConfiguration configuration = ViewConfiguration.get(context);
		mMinimumFlingVelocity = configuration.getScaledMinimumFlingVelocity();
		mMaximumFlingVelocity = configuration.getScaledMaximumFlingVelocity();
		mScroller = ScrollerCompat.create(getContext(), new DecelerateInterpolator());
	}
	
	@Override
	public boolean onTouchEvent(MotionEvent ev)
	{
		boolean result = super.onTouchEvent(ev);
		
		PagerAdapter adapter = getAdapter();
		if(adapter == null || adapter.getCount() == 0){
			// Nothing to present or scroll; nothing to touch.
			return false;
		}
		
		final int action = ev.getAction();
		switch(action & MotionEventCompat.ACTION_MASK)
		{
			case MotionEvent.ACTION_DOWN:
				if(mVelocityTracker == null){
					mVelocityTracker = VelocityTracker.obtain();
				}
				else{
					mVelocityTracker.clear();
				}
				mVelocityTracker.addMovement(ev);
				break;
				
			case MotionEvent.ACTION_MOVE:
				mVelocityTracker.addMovement(ev);
				break;
				
			case MotionEvent.ACTION_UP:
				final VelocityTracker velocityTracker = mVelocityTracker;
				velocityTracker.computeCurrentVelocity(1000, mMaximumFlingVelocity);
				int initialVelocityX = (int)velocityTracker.getXVelocity();
				if((Math.abs(initialVelocityX) > mMinimumFlingVelocity)){
					startFling(initialVelocityX);
					result = true;
				}
				break;
				
			case MotionEvent.ACTION_CANCEL:
				mVelocityTracker.recycle();
				mVelocityTracker = null;
				break;
		}
		
		return result;
	}
	
	@Override
	protected void onDetachedFromWindow()
	{
		removeCallbacks(mFlingRunnable);
		super.onDetachedFromWindow();
	}
	
	public void startFling(int initialVelocityX)
	{
		if(initialVelocityX == 0){
			return;
		}
		removeCallbacks(mFlingRunnable);
		if(beginFakeDrag()){
			mScroller.abortAnimation();
			mLastFlingX = getScrollX();
			//fakeDragBy(0);
			mScroller.fling(mLastFlingX, 0, initialVelocityX, 0, 0, Integer.MAX_VALUE, 0, 0);
			ViewCompat.postOnAnimation(this, mFlingRunnable);
		}
	}
	
	private Runnable mFlingRunnable = new Runnable()
	{
		@Override
		public void run()
		{
			boolean finished = mScroller.computeScrollOffset();
			int newX = mScroller.getCurrX();
			fakeDragBy(newX - mLastFlingX);
			if(finished){
				mLastFlingX = newX;
				ViewCompat.postOnAnimation(FlingableViewPager.this, this);
			}
			else{
				endFakeDrag();
			}
		}
	};
}