NestedWebView.java

package com.prettifier.pretty;

import android.annotation.SuppressLint;
import android.content.Context;
import android.os.Build;
import android.support.annotation.RequiresApi;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.NestedScrollingChild;
import android.support.v4.view.NestedScrollingChildHelper;
import android.support.v4.view.ViewCompat;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.webkit.WebView;

public class NestedWebView extends WebView implements NestedScrollingChild {
    private int mLastY;
    private final int[] mScrollOffset = new int[2];
    private final int[] mScrollConsumed = new int[2];
    private int mNestedOffsetY;
    private NestedScrollingChildHelper mChildHelper;
    private boolean firstScroll = true;


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

    public NestedWebView(Context context, AttributeSet attrs) {
        this(context, attrs, android.R.attr.webViewStyle);
    }

    public NestedWebView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        mChildHelper = new NestedScrollingChildHelper(this);
        setNestedScrollingEnabled(true);
    }

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    public NestedWebView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    @SuppressLint("ClickableViewAccessibility") @Override public boolean onTouchEvent(MotionEvent ev) {
        boolean returnValue;

        MotionEvent event = MotionEvent.obtain(ev);
        final int action = MotionEventCompat.getActionMasked(event);
        if (action == MotionEvent.ACTION_DOWN) {
            mNestedOffsetY = 0;
        }
        int eventY = (int) event.getY();
        event.offsetLocation(0, mNestedOffsetY);
        switch (action) {
            case MotionEvent.ACTION_MOVE:
                int deltaY = mLastY - eventY;
                // NestedPreScroll
                if (dispatchNestedPreScroll(0, deltaY, mScrollConsumed, mScrollOffset)) {
                    deltaY -= mScrollConsumed[1];
                    mLastY = eventY - mScrollOffset[1];
                    event.offsetLocation(0, -mScrollOffset[1]);
                    mNestedOffsetY += mScrollOffset[1];
                }
                returnValue = super.onTouchEvent(event);

                // NestedScroll
                if (dispatchNestedScroll(0, mScrollOffset[1], 0, deltaY, mScrollOffset)) {
                    event.offsetLocation(0, mScrollOffset[1]);
                    mNestedOffsetY += mScrollOffset[1];
                    mLastY -= mScrollOffset[1];
                }
                break;
            case MotionEvent.ACTION_DOWN:
                returnValue = super.onTouchEvent(event);
                if (firstScroll) {
                    mLastY = eventY - 5;
                    firstScroll = false;
                } else {
                    mLastY = eventY;
                }
                startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL);
                break;
            default:
                returnValue = super.onTouchEvent(event);
                // end NestedScroll
                stopNestedScroll();
                break;
        }
        return returnValue;
    }

    @Override public void setNestedScrollingEnabled(boolean enabled) {
        mChildHelper.setNestedScrollingEnabled(enabled);
    }

    @Override public boolean isNestedScrollingEnabled() {
        return mChildHelper.isNestedScrollingEnabled();
    }

    @Override public boolean startNestedScroll(int axes) {
        return mChildHelper.startNestedScroll(axes);
    }

    @Override public void stopNestedScroll() {
        mChildHelper.stopNestedScroll();
    }

    @Override public boolean hasNestedScrollingParent() {
        return mChildHelper.hasNestedScrollingParent();
    }

    @Override public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed,
                                                  int[] offsetInWindow) {
        return mChildHelper.dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed, offsetInWindow);
    }

    @Override public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
        return mChildHelper.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow);
    }

    @Override public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
        return mChildHelper.dispatchNestedFling(velocityX, velocityY, consumed);
    }

    @Override public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
        return mChildHelper.dispatchNestedPreFling(velocityX, velocityY);
    }

}