android - NestedWebView working properly with ScrollingViewBehavior

Question

I have an app with two activities extends same abstract - one with WebView, another with ViewPager with TabLayout and RecyclerViews - and on this level I've implemented "shared" showing/hiding toolbar with scroll/fling feature:

app:layout_scrollFlags="scroll|snap|enterAlways"

WebView is "pushing out" Toolbar just like RecyclerView under any tab in ViewPager (keeping TabLayout visible), in both cases any scroll to top brings back Toolbar. Just like on this example:

Now I've to change this behavior - Toolbar should be hidden when user is NOT on top of page, so: any scroll to bottom should hide Toolbar if present and only (scroll to top && getScrollY()<=0) should make Toolbar enter:

I've borrowed GIFs from THIS guide, which is suggesting (all other too) that my desired behavior is default with just scroll flag (and possibly snap). So I've removed enterAlways and my "native" Activity started to work as intended out-of-the-box. WebView - no change at all...

I'm suspecting that this bug is caused by my NestedWebView, I'm currently using THIS one. So I've tried to drop-in replace this class with these:

marshi - not scrollable webview content (toolbar shows/hides)

takahirom - toolbar enters even when 1px scroll in up direction almost immediate (kind of snap behavior, but during touch)

hanks - no nested scrolling (fixed toolbar), also with uncommented setNestedScrollingEnabled(true); line

How to achieve second GIF Toolbar behavior cooperating with WebView?

Answer 1

read edit on bottom and other answers/comments! don't use this class, also avoid WebView in any form if you can. You will get into trouble sooner or later, maintaining WebView is hell...

I've found few different implementations of NestedWebView and noticed that all of them are old and implements NestedScrollingChild when currently we have 3rd version of this interface: NestedScrollingChild3 (androidx). So I've penetrated current version of NestedSrcollView and I've added some new methods handling, and now it works. To be exact - I was trying to achieve smooth scroll through whole screen just like when WebView would fill whole screen and Toolbar was part of web content (always on top). Also snap is working. use wisely

import android.annotation.SuppressLint;
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewParent;
import android.webkit.WebView;
import android.widget.OverScroller;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.core.view.NestedScrollingChild3;
import androidx.core.view.NestedScrollingChildHelper;
import androidx.core.view.ViewCompat;

/**
 * WebView compatible with CoordinatorLayout by snachmsm
 * The implementation based on NestedScrollView of design library androidx v1.0.1
 */
public class NestedWebView extends WebView implements NestedScrollingChild3 {

    private static final String TAG = "NestedWebView";
    private static final int INVALID_POINTER = -1;

    private final int[] mScrollOffset = new int[2];
    private final int[] mScrollConsumed = new int[2];

    private int mLastMotionY;
    private NestedScrollingChildHelper mChildHelper;
    private boolean mIsBeingDragged = false;
    private VelocityTracker mVelocityTracker;
    private int mTouchSlop;
    private int mActivePointerId = INVALID_POINTER;
    private int mNestedYOffset;
    private OverScroller mScroller;
    private int mMinimumVelocity;
    private int mMaximumVelocity;
    private int mLastScrollerY;

    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);
        setOverScrollMode(WebView.OVER_SCROLL_NEVER);
        initScrollView();
        mChildHelper = new NestedScrollingChildHelper(this);
        setNestedScrollingEnabled(true);
    }

    private void initScrollView() {
        mScroller = new OverScroller(getContext());
        final ViewConfiguration configuration = ViewConfiguration.get(getContext());
        mTouchSlop = configuration.getScaledTouchSlop();
        mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
        mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        final int action = ev.getAction();
        if ((action == MotionEvent.ACTION_MOVE) && (mIsBeingDragged)) { // most common
            return true;
        }

        switch (action & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_MOVE:
                final int activePointerId = mActivePointerId;
                if (activePointerId == INVALID_POINTER) {
                    break;
                }

                final int pointerIndex = ev.findPointerIndex(activePointerId);
                if (pointerIndex == -1) {
                    Log.e(TAG, "Invalid pointerId=" + activePointerId
                            + " in onInterceptTouchEvent");
                    break;
                }

                final int y = (int) ev.getY(pointerIndex);
                final int yDiff = Math.abs(y - mLastMotionY);
                if (yDiff > mTouchSlop
                        && (getNestedScrollAxes() & ViewCompat.SCROLL_AXIS_VERTICAL) == 0) {
                    mIsBeingDragged = true;
                    mLastMotionY = y;
                    initVelocityTrackerIfNotExists();
                    mVelocityTracker.addMovement(ev);
                    mNestedYOffset = 0;
                    final ViewParent parent = getParent();
                    if (parent != null) {
                        parent.requestDisallowInterceptTouchEvent(true);
                    }
                }
                break;
            case MotionEvent.ACTION_DOWN:
                mLastMotionY = (int) ev.getY();
                mActivePointerId = ev.getPointerId(0);

                initOrResetVelocityTracker();
                mVelocityTracker.addMovement(ev);

                mScroller.computeScrollOffset();
                mIsBeingDragged = !mScroller.isFinished();

                startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL);
                break;
            case MotionEvent.ACTION_CANCEL:
            case MotionEvent.ACTION_UP:
                mIsBeingDragged = false;
                mActivePointerId = INVALID_POINTER;
                recycleVelocityTracker();
                if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0, getScrollRange())) {
                    ViewCompat.postInvalidateOnAnimation(this);
                }
                stopNestedScroll();
                break;
            case MotionEvent.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                break;
        }

        return mIsBeingDragged;
    }

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

        MotionEvent vtev = MotionEvent.obtain(ev);

        final int actionMasked = ev.getActionMasked();

        if (actionMasked == MotionEvent.ACTION_DOWN) {
            mNestedYOffset = 0;
        }
        vtev.offsetLocation(0, mNestedYOffset);

        switch (actionMasked) {
            case MotionEvent.ACTION_DOWN:
                if ((mIsBeingDragged = !mScroller.isFinished())) {
                    final ViewParent parent = getParent();
                    if (parent != null) {
                        parent.requestDisallowInterceptTouchEvent(true);
                    }
                }

                if (!mScroller.isFinished()) {
                    abortAnimatedScroll();
                }

                mLastMotionY = (int) ev.getY();
                mActivePointerId = ev.getPointerId(0);
                startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL, ViewCompat.TYPE_TOUCH);
                break;
            case MotionEvent.ACTION_MOVE:
                final int activePointerIndex = ev.findPointerIndex(mActivePointerId);
                if (activePointerIndex == -1) {
                    Log.e(TAG, "Invalid pointerId=" + mActivePointerId + " in onTouchEvent");
                    break;
                }

                final int y = (int) ev.getY(activePointerIndex);
                int deltaY = mLastMotionY - y;
                if (dispatchNestedPreScroll(0, deltaY, mScrollConsumed, mScrollOffset,
                        ViewCompat.TYPE_TOUCH)) {
                    deltaY -= mScrollConsumed[1];
                    mNestedYOffset += mScrollOffset[1];
                }
                if (!mIsBeingDragged && Math.abs(deltaY) > mTouchSlop) {
                    final ViewParent parent = getParent();
                    if (parent != null) {
                        parent.requestDisallowInterceptTouchEvent(true);
                    }
                    mIsBeingDragged = true;
                    if (deltaY > 0) {
                        deltaY -= mTouchSlop;
                    } else {
                        deltaY += mTouchSlop;
                    }
                }
                if (mIsBeingDragged) {
                    mLastMotionY = y - mScrollOffset[1];

                    final int oldY = getScrollY();
                    final int range = getScrollRange();

                    // Calling overScrollByCompat will call onOverScrolled, which
                    // calls onScrollChanged if applicable.
                    if (overScrollByCompat(0, deltaY, 0, oldY, 0, range, 0,
                            0, true) && !hasNestedScrollingParent(ViewCompat.TYPE_TOUCH)) {
                        mVelocityTracker.clear();
                    }

                    final int scrolledDeltaY = getScrollY() - oldY;
                    final int unconsumedY = deltaY - scrolledDeltaY;

                    mScrollConsumed[1] = 0;

                    dispatchNestedScroll(0, scrolledDeltaY, 0, unconsumedY, mScrollOffset,
                            ViewCompat.TYPE_TOUCH, mScrollConsumed);

                    mLastMotionY -= mScrollOffset[1];
                    mNestedYOffset += mScrollOffset[1];
                }
                break;
            case MotionEvent.ACTION_UP:
                final VelocityTracker velocityTracker = mVelocityTracker;
                velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
                int initialVelocity = (int) velocityTracker.getYVelocity(mActivePointerId);
                if ((Math.abs(initialVelocity) > mMinimumVelocity)) {
                    if (!dispatchNestedPreFling(0, -initialVelocity)) {
                        dispatchNestedFling(0, -initialVelocity, true);
                        fling(-initialVelocity);
                    }
                } else if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0,
                        getScrollRange())) {
                    ViewCompat.postInvalidateOnAnimation(this);
                }
                mActivePointerId = INVALID_POINTER;
                endDrag();
                break;
            case MotionEvent.ACTION_CANCEL:
                if (mIsBeingDragged) {
                    if (mScroller.springBack(getScrollX(), getScrollY(), 0, 0, 0,
                            getScrollRange())) {
                        ViewCompat.postInvalidateOnAnimation(this);
                    }
                }
                mActivePointerId = INVALID_POINTER;
                endDrag();
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
                final int index = ev.getActionIndex();
                mLastMotionY = (int) ev.getY(index);
                mActivePointerId = ev.getPointerId(index);
                break;
            case MotionEvent.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                mLastMotionY = (int) ev.getY(ev.findPointerIndex(mActivePointerId));
                break;
        }

        if (mVelocityTracker != null) {
            mVelocityTracker.addMovement(vtev);
        }
        vtev.recycle();
        return super.onTouchEvent(ev);
    }

    private void abortAnimatedScroll() {
        mScroller.abortAnimation();
        stopNestedScroll(ViewCompat.TYPE_NON_TOUCH);
    }

    private void endDrag() {
        mIsBeingDragged = false;

        recycleVelocityTracker();
        stopNestedScroll();
    }

    private void onSecondaryPointerUp(MotionEvent ev) {
        final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK)
                >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
        final int pointerId = ev.getPointerId(p