向上拖动时，可以惯性滑动显示到下一页的控件DragLayout

import android.annotation.SuppressLint;
import android.content.Context;
import android.support.v4.view.GestureDetectorCompat;
import android.support.v4.view.ViewCompat;
import android.support.v4.widget.ViewDragHelper;
import android.util.AttributeSet;
import android.view.GestureDetector.SimpleOnGestureListener;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;

/**
 * 这是一个viewGroup容器，实现上下两个frameLayout拖动切换
 * 
 * @author sistone.Zhang
 */
@SuppressLint("NewApi")
public class DragLayout extends ViewGroup {

	/* 拖拽工具类 */
	private final ViewDragHelper mDragHelper;
	private GestureDetectorCompat gestureDetector;

	/* 上下两个frameLayout，在Activity中注入fragment */
	private View frameView1, frameView2;
	private int viewHeight;
	private static final int VEL_THRESHOLD = 100; // 滑动速度的阈值，超过这个绝对值认为是上下
	private static final int DISTANCE_THRESHOLD = 100; // 单位是像素，当上下滑动速度不够时，通过这个阈值来判定是应该粘到顶部还是底部
	private int downTop1; // 手指按下的时候，frameView1的getTop值
	private ShowNextPageNotifier nextPageListener; // 手指松开是否加载下一页的notifier

	public DragLayout(Context context) {
		this(context, null);
	}

	public DragLayout(Context context, AttributeSet attrs) {
		this(context, attrs, 0);
	}

	public DragLayout(Context context, AttributeSet attrs, int defStyle) {
		super(context, attrs, defStyle);
		mDragHelper = ViewDragHelper
				.create(this, 10f, new DragHelperCallback());
		mDragHelper.setEdgeTrackingEnabled(ViewDragHelper.EDGE_BOTTOM);
		gestureDetector = new GestureDetectorCompat(context,
				new YScrollDetector());
	}

	@Override
	protected void onFinishInflate() {
		// 跟findviewbyId一样，初始化上下两个view
		frameView1 = getChildAt(0);
		frameView2 = getChildAt(1);
	}

	class YScrollDetector extends SimpleOnGestureListener {

		@Override
		public boolean onScroll(MotionEvent e1, MotionEvent e2, float dx,
				float dy) {
			// 垂直滑动时dy>dx，才被认定是上下拖动
			return Math.abs(dy) > Math.abs(dx);
		}
	}

	@Override
	public void computeScroll() {
		if (mDragHelper.continueSettling(true)) {
			ViewCompat.postInvalidateOnAnimation(this);
		}
	}

	/**
	 * 这是拖拽效果的主要逻辑
	 */
	private class DragHelperCallback extends ViewDragHelper.Callback {

		@Override
		public void onViewPositionChanged(View changedView, int left, int top,
				int dx, int dy) {
			int childIndex = 1;
			if (changedView == frameView2) {
				childIndex = 2;
			}

			// 一个view位置改变，另一个view的位置要跟进
			onViewPosChanged(childIndex, top);
		}

		@Override
		public boolean tryCaptureView(View child, int pointerId) {
			// 两个子View都需要跟踪，返回true
			return true;
		}

		@Override
		public int getViewVerticalDragRange(View child) {
			// 这个用来控制拖拽过程中松手后，自动滑行的速度，暂时给一个随意的数值
			return 1;
		}

		@Override
		public void onViewReleased(View releasedChild, float xvel, float yvel) {
			// 滑动松开后，需要向上或者乡下粘到特定的位置
			animTopOrBottom(releasedChild, yvel);
		}

		@Override
		public int clampViewPositionVertical(View child, int top, int dy) {
			int finalTop = top;
			if (child == frameView1) {
				// 拖动的时第一个view
				if (top > 0) {
					// 不让第一个view往下拖，因为顶部会白板
					finalTop = 0;
				}
			} else if (child == frameView2) {
				// 拖动的时第二个view
				if (top  VEL_THRESHOLD
					|| (downTop1 == -viewHeight && releasedChild.getTop() > DISTANCE_THRESHOLD)) {
				// 保持原地不动
				finalTop = viewHeight;
			}
		}

		if (mDragHelper.smoothSlideViewTo(releasedChild, 0, finalTop)) {
			ViewCompat.postInvalidateOnAnimation(this);
		}
	}

	/* touch事件的拦截与处理都交给mDraghelper来处理 */
	@Override
	public boolean onInterceptTouchEvent(MotionEvent ev) {

		if (frameView1.getBottom() > 0 && frameView1.getTop() 

private void initView() {
		fragment1 = new VerticalFragment1();
		fragment2 = new VerticalFragment2();
//		fragment3 = new VerticalFragment3();

		getSupportFragmentManager().beginTransaction()
				.add(R.id.first, fragment1)
				.add(R.id.second, fragment2)
//				.add(R.id.second, fragment3)//只支持两页
				.commit();

		ShowNextPageNotifier nextIntf = new ShowNextPageNotifier() {
			@Override
			public void onDragNext() {
//				fragment3.initView();
			}
		};
		draglayout = (DragLayout) findViewById(R.id.draglayout);
		draglayout.setNextPageListener(nextIntf);
	}