FlexScrollView.java

package com.example.app.myapplication;

import android.content.Context;
import android.support.v4.widget.ViewDragHelper;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;

public abstract class FlexScrollView extends ViewGroup {
    private static final String TAG = "FlexScrollView";

    private final View mDragView;
    private int mDeltaY;

    public FlexScrollView(Context context) {
        this(context, null, 0);
    }

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

    public FlexScrollView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        mDragView = new View(context);
        addView(mDragView);
    }

    private ViewDragHelper.Callback mCallback = new ViewDragHelper.Callback() {
        @Override
        public int clampViewPositionVertical(View child, int top, int dy) {
            top = Math.min(top, 0);
            top = Math.max(top, getHeight() - child.getHeight());
            return top;
        }

        @Override
        public boolean tryCaptureView(View child, int pointerId) {
            return child == mDragView;
        }

        @Override
        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
            mDeltaY -= dy;
            requestLayout();
        }

        @Override
        public void onViewCaptured(View capturedChild, int activePointerId) {
            mDeltaY = 0;
        }

        @Override
        public void onViewReleased(View releasedChild, float xvel, float yvel) {
            mDragHelper.flingCapturedView(0, getHeight() - releasedChild.getHeight(), 0, 0);
            invalidate();
        }
    };
    private ViewDragHelper mDragHelper = ViewDragHelper.create(this, mCallback);

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        measureChildren(widthMeasureSpec, heightMeasureSpec);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        int action = ev.getAction();
        if (action == MotionEvent.ACTION_MOVE && mDragHelper.checkTouchSlop(ViewDragHelper.DIRECTION_VERTICAL)) {
            mDragHelper.captureChildView(mDragView, ev.getPointerId(0));
            return true;
        }
        return mDragHelper.shouldInterceptTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mDragHelper.processTouchEvent(event);
        return true;
    }

    @Override
    public void computeScroll() {
        boolean rc = mDragHelper.continueSettling(true);
        if (rc) {
            invalidate();
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        if (mDragView.getHeight() == 0) {
            int h = getMaxHeight();
            mDragView.layout(0, 0, getWidth(), h);
            mDragView.bringToFront();
        }
        layoutChildren(-mDragView.getTop(), mDeltaY);
    }

    abstract public int getMaxHeight();
    abstract public void layoutChildren(int yOffset, int yDelta);
}