Pinch to zoom zooms in to the top left corner

[JustinBis]

When you enable zooming, pinch to zoom gestures seem to always zoom to the top left corner. This makes zooming unintuitive for the user. This is made worse by horizontal scrolling not working (Issue #11), so if you want to zoom in on an element in the center of a page you can't.

[rahul-hexalitics]

Hi @JustinBis, Did you find any solution for this problem?

[watercyb]

I'm using nested webview from another author (https://github.com/gdutxiaoxu/nestedwebview), which has similar issues with horizontal scrolling and zooming in. I tried modifying it myself, and it seems to work. The code:

package io.github.gdutxiaoxu.nestedwebview;

import static androidx.core.view.ViewCompat.TYPE_TOUCH;

import android.annotation.SuppressLint;
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
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;

/**
 * 博客地址： https://blog.csdn.net/gdutxiaoxu
 * 公众号： 徐公
 */
public class NestedWebView extends WebView implements NestedScrollingChild3, ScaleGestureDetector.OnScaleGestureListener {

    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 mLastMotionX;
    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 mLastScrollerX;
    private int mLastScrollerY;

    private float scaleX, scaleY;

    ScaleGestureDetector mScaleDetector = new ScaleGestureDetector(this.getContext(), this);

    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:
                mLastMotionX = (int) ev.getX();
                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, getScrollRange(), 0, getScrollRange())) {
                    ViewCompat.postInvalidateOnAnimation(this);
                }
                stopNestedScroll();
                break;
            case MotionEvent.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                break;
        }

        return mIsBeingDragged;
    }


    @Override
    public boolean onScale(@NonNull ScaleGestureDetector detector) {
        scaleX *= detector.getScaleFactor();
        scaleY *= detector.getScaleFactor();
        setScrollX((int) (scaleX - detector.getFocusX()));
        setScrollY((int) (scaleY - detector.getFocusY()));
        return true;
    }

    @Override
    public boolean onScaleBegin(@NonNull ScaleGestureDetector detector) {
        scaleX = getScrollX() + detector.getFocusX();
        scaleY = getScrollY() + detector.getFocusY();
        return true;
    }

    @Override
    public void onScaleEnd(@NonNull ScaleGestureDetector detector) {
    }


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

        mScaleDetector.onTouchEvent(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();
                }

                mLastMotionX = (int) ev.getX();
                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 x = (int) ev.getX(activePointerIndex);
                final int y = (int) ev.getY(activePointerIndex);
                int deltaX = mLastMotionX - x;
                int deltaY = mLastMotionY - y;
                if (dispatchNestedPreScroll(deltaX, deltaY, mScrollConsumed, mScrollOffset,
                        ViewCompat.TYPE_TOUCH)) {

                    deltaX -= mScrollConsumed[0];
                    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) {
                    mLastMotionX = x - mScrollOffset[0];
                    mLastMotionY = y - mScrollOffset[1];

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

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

                    final int scrolledDeltaX = getScrollX() - oldX;
                    final int scrolledDeltaY = getScrollY() - oldY;
                    final int unconsumedX = deltaX - scrolledDeltaX;
                    final int unconsumedY = deltaY - scrolledDeltaY;

                    mScrollConsumed[0] = 0;
                    mScrollConsumed[1] = 0;

                    dispatchNestedScroll(scrolledDeltaX, scrolledDeltaY, unconsumedX, 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 initialVelocityX = (int) velocityTracker.getXVelocity(mActivePointerId);
                int initialVelocityY = (int) velocityTracker.getYVelocity(mActivePointerId);
                if ((Math.abs(initialVelocityX) > mMinimumVelocity) || (Math.abs(initialVelocityY) > mMinimumVelocity)) {
                    if (!dispatchNestedPreFling(initialVelocityX, -initialVelocityY)) {
                        dispatchNestedFling(initialVelocityX, -initialVelocityY, true);
                    }
                    fling(-initialVelocityX, -initialVelocityY, Math.abs(initialVelocityY) > mMinimumVelocity);
                } else if (mScroller.springBack(getScrollX(), getScrollY(), 0, getScrollRange(), 0,
                        getScrollRange())) {
                    postInvalidateOnAnimation();
                }
                mActivePointerId = INVALID_POINTER;
                endDrag();
                break;
            case MotionEvent.ACTION_CANCEL:
                if (mIsBeingDragged) {
                    if (mScroller.springBack(getScrollX(), getScrollY(), 0, getScrollRange(), 0,
                            getScrollRange())) {
                        postInvalidateOnAnimation();
                    }
                }
                mActivePointerId = INVALID_POINTER;
                endDrag();
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
                final int index = ev.getActionIndex();

                mLastMotionX = (int) ev.getX(index);
                mLastMotionY = (int) ev.getY(index);
                mActivePointerId = ev.getPointerId(index);
                break;
            case MotionEvent.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                mLastMotionX = (int) ev.getX(ev.findPointerIndex(mActivePointerId));
                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(pointerIndex);
        if (pointerId == mActivePointerId) {
            final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
            mLastMotionX = (int) ev.getX(newPointerIndex);
            mLastMotionY = (int) ev.getY(newPointerIndex);
            mActivePointerId = ev.getPointerId(newPointerIndex);
            if (mVelocityTracker != null) {
                mVelocityTracker.clear();
            }
        }
    }

    private void fling(int velocityX, int velocityY, boolean participateInNestedScrolling) {
        int width = getWidth();
        int height = getHeight();
        mScroller.fling(getScrollX(), getScrollY(), // start
                velocityX, velocityY, // velocities
                Integer.MIN_VALUE, Integer.MAX_VALUE, // x
                Integer.MIN_VALUE, Integer.MAX_VALUE, // y
                width / 2, height / 2);
        runAnimatedScroll(participateInNestedScrolling);
    }

    private void runAnimatedScroll(boolean participateInNestedScrolling) {
        if (participateInNestedScrolling) {
            startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL, ViewCompat.TYPE_NON_TOUCH);
        } else {
            stopNestedScroll(ViewCompat.TYPE_NON_TOUCH);
        }
        mLastScrollerX = getScrollX();
        mLastScrollerY = getScrollY();
        ViewCompat.postInvalidateOnAnimation(this);
    }

    @Override
    public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
        if (disallowIntercept) {
            recycleVelocityTracker();
        }
        super.requestDisallowInterceptTouchEvent(disallowIntercept);
    }

    private void initOrResetVelocityTracker() {
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        } else {
            mVelocityTracker.clear();
        }
    }

    private void initVelocityTrackerIfNotExists() {
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
    }

    private void recycleVelocityTracker() {
        if (mVelocityTracker != null) {
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }
    }

    @Override
    protected boolean overScrollBy(int deltaX, int deltaY,
                                   int scrollX, int scrollY,
                                   int scrollRangeX, int scrollRangeY,
                                   int maxOverScrollX, int maxOverScrollY,
                                   boolean isTouchEvent) {
        // this is causing double scroll call (doubled speed), but this WebView isn't overscrollable
        // all overscrolls are passed to appbar, so commenting this out during drag
        if (!mIsBeingDragged)
            overScrollByCompat(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY,
                    maxOverScrollX, maxOverScrollY, isTouchEvent);
        // without this call webview won't scroll to top when url change or when user pick input
        // (webview should move a bit making input still in viewport when "adjustResize")
        return true;
    }

    int getScrollRange() {
        //Using scroll range of webview instead of childs as NestedScrollView does.
        return computeVerticalScrollRange();
    }

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

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

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

    @Override
    public boolean startNestedScroll(int axes) {
        return startNestedScroll(axes, ViewCompat.TYPE_TOUCH);
    }

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

    @Override
    public void stopNestedScroll() {
        stopNestedScroll(ViewCompat.TYPE_TOUCH);
    }

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

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

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

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

    @Override
    public void dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed,
                                     @Nullable int[] offsetInWindow, int type, @NonNull int[] consumed) {
        mChildHelper.dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed,
                offsetInWindow, type, consumed);
    }

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


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

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

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

    @Override
    public int getNestedScrollAxes() {
        return ViewCompat.SCROLL_AXIS_VERTICAL;
    }

    @Override
    public void computeScroll() {
        if (mScroller.isFinished()) {
            return;
        }

        mScroller.computeScrollOffset();
        final int x = mScroller.getCurrX();
        final int y = mScroller.getCurrY();
        int unconsumedX = x - mLastScrollerX;
        int unconsumedY = y - mLastScrollerY;
        mLastScrollerX = x;
        mLastScrollerY = y;

        // Nested Scrolling Pre Pass
        mScrollConsumed[0] = 0;
        mScrollConsumed[1] = 0;
        dispatchNestedPreScroll(unconsumedX, unconsumedY, mScrollConsumed, null,
                ViewCompat.TYPE_NON_TOUCH);
        unconsumedX -= mScrollConsumed[0];
        unconsumedY -= mScrollConsumed[1];


        if (unconsumedX != 0 || unconsumedY != 0) {
            // Internal Scroll
            final int oldScrollX = getScrollX();
            final int oldScrollY = getScrollY();
            overScrollByCompat(unconsumedX, unconsumedY, oldScrollX, oldScrollY, getScrollRange(), getScrollRange(),
                    0, 0, false);
            final int scrolledByMeX = getScrollX() - oldScrollX;
            unconsumedX -= scrolledByMeX;
            final int scrolledByMeY = getScrollY() - oldScrollY;
            unconsumedY -= scrolledByMeY;

            // Nested Scrolling Post Pass
            mScrollConsumed[0] = 0;
            mScrollConsumed[1] = 0;
            dispatchNestedScroll(0, 0, unconsumedX, unconsumedY, mScrollOffset,
                    ViewCompat.TYPE_NON_TOUCH, mScrollConsumed);
            unconsumedX -= mScrollConsumed[0];
            unconsumedY -= mScrollConsumed[1];
        }

        if (unconsumedX != 0 && unconsumedY != 0) {
            abortAnimatedScroll();
        }

        if (!mScroller.isFinished()) {
            ViewCompat.postInvalidateOnAnimation(this);
        }
    }

    // copied from NestedScrollView exacly as it looks, leaving overscroll related code, maybe future use
    private boolean overScrollByCompat(int deltaX, int deltaY,
                                       int scrollX, int scrollY,
                                       int scrollRangeX, int scrollRangeY,
                                       int maxOverScrollX, int maxOverScrollY,
                                       boolean isTouchEvent) {
        final int overScrollMode = getOverScrollMode();
        final boolean canScrollHorizontal =
                computeHorizontalScrollRange() > computeHorizontalScrollExtent();
        final boolean canScrollVertical =
                computeVerticalScrollRange() > computeVerticalScrollExtent();
        final boolean overScrollHorizontal = overScrollMode == View.OVER_SCROLL_ALWAYS
                || (overScrollMode == View.OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollHorizontal);
        final boolean overScrollVertical = overScrollMode == View.OVER_SCROLL_ALWAYS
                || (overScrollMode == View.OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollVertical);

        int newScrollX = scrollX + deltaX;
        if (!overScrollHorizontal) {
            maxOverScrollX = 0;
        }

        int newScrollY = scrollY + deltaY;
        if (!overScrollVertical) {
            maxOverScrollY = 0;
        }

        // Clamp values if at the limits and record
        final int left = -maxOverScrollX;
        final int right = maxOverScrollX + scrollRangeX;
        final int top = -maxOverScrollY;
        final int bottom = maxOverScrollY + scrollRangeY;

        boolean clampedX = false;
        if (newScrollX > right) {
            newScrollX = right;
            clampedX = true;
        } else if (newScrollX < left) {
            newScrollX = left;
            clampedX = true;
        }

        boolean clampedY = false;
        if (newScrollY > bottom) {
            newScrollY = bottom;
            clampedY = true;
        } else if (newScrollY < top) {
            newScrollY = top;
            clampedY = true;
        }

        if (clampedY && !hasNestedScrollingParent(ViewCompat.TYPE_NON_TOUCH)) {
            mScroller.springBack(newScrollX, newScrollY, 0, getScrollRange(), 0, getScrollRange());
        }

        onOverScrolled(newScrollX, newScrollY, clampedX, clampedY);

        return clampedX || clampedY;
    }
}