Почему `onTouchEvent ()` в `NestedScrollView` вызывает метод` dispatchNestedPreScroll` перед распознаванием скользящего жеста?

Question

Интересно, почему onTouchEvent() в NestedScrollView вызывает метод dispatchNestedPreScroll перед распознаванием скользящего жеста, а расстояние, используемое этим методом, повлияет на суждение о скользящем жесте, что приведет к неправильной прокрутке scroll parent и заставьте requestDisallowInterceptTouchEvent() быть вызванным слишком поздно.

Thx!

Вот источник:

@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 (getChildCount() == 0) {
                return false;
            }
            if ((mIsBeingDragged = !mScroller.isFinished())) {
                final ViewParent parent = getParent();
                if (parent != null) {
                    parent.requestDisallowInterceptTouchEvent(true);
                }
            }

            /*
             * If being flinged and user touches, stop the fling. isFinished
             * will be false if being flinged.
             */
            if (!mScroller.isFinished()) {
                mScroller.abortAnimation();
            }

            // Remember where the motion event started
            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];
                vtev.offsetLocation(0, mScrollOffset[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) {
                // Scroll to follow the motion event
                mLastMotionY = y - mScrollOffset[1];

                final int oldY = getScrollY();
                final int range = getScrollRange();
                final int overscrollMode = getOverScrollMode();
                boolean canOverscroll = overscrollMode == View.OVER_SCROLL_ALWAYS
                        || (overscrollMode == View.OVER_SCROLL_IF_CONTENT_SCROLLS && range > 0);

                // Calling overScrollByCompat will call onOverScrolled, which
                // calls onScrollChanged if applicable.
                if (overScrollByCompat(0, deltaY, 0, getScrollY(), 0, range, 0,
                        0, true) && !hasNestedScrollingParent(ViewCompat.TYPE_TOUCH)) {
                    // Break our velocity if we hit a scroll barrier.
                    mVelocityTracker.clear();
                }

                final int scrolledDeltaY = getScrollY() - oldY;
                final int unconsumedY = deltaY - scrolledDeltaY;
                if (dispatchNestedScroll(0, scrolledDeltaY, 0, unconsumedY, mScrollOffset,
                        ViewCompat.TYPE_TOUCH)) {
                    mLastMotionY -= mScrollOffset[1];
                    vtev.offsetLocation(0, mScrollOffset[1]);
                    mNestedYOffset += mScrollOffset[1];
                } else if (canOverscroll) {
                    ensureGlows();
                    final int pulledToY = oldY + deltaY;
                    if (pulledToY < 0) {
                        EdgeEffectCompat.onPull(mEdgeGlowTop, (float) deltaY / getHeight(),
                                ev.getX(activePointerIndex) / getWidth());
                        if (!mEdgeGlowBottom.isFinished()) {
                            mEdgeGlowBottom.onRelease();
                        }
                    } else if (pulledToY > range) {
                        EdgeEffectCompat.onPull(mEdgeGlowBottom, (float) deltaY / getHeight(),
                                1.f - ev.getX(activePointerIndex)
                                        / getWidth());
                        if (!mEdgeGlowTop.isFinished()) {
                            mEdgeGlowTop.onRelease();
                        }
                    }
                    if (mEdgeGlowTop != null
                            && (!mEdgeGlowTop.isFinished() || !mEdgeGlowBottom.isFinished())) {
                        ViewCompat.postInvalidateOnAnimation(this);
                    }
                }
            }
            break;
        case MotionEvent.ACTION_UP:
            final VelocityTracker velocityTracker = mVelocityTracker;
            velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
            int initialVelocity = (int) velocityTracker.getYVelocity(mActivePointerId);
            if ((Math.abs(initialVelocity) > mMinimumVelocity)) {
                flingWithNestedDispatch(-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 && getChildCount() > 0) {
                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 true;
}

И я думаю, что это можно изменить так:

@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 (getChildCount() == 0) {
                return false;
            }
            if ((mIsBeingDragged = !mScroller.isFinished())) {
                final ViewParent parent = getParent();
                if (parent != null) {
                    parent.requestDisallowInterceptTouchEvent(true);
                }
            }

            /*
             * If being flinged and user touches, stop the fling. isFinished
             * will be false if being flinged.
             */
            if (!mScroller.isFinished()) {
                mScroller.abortAnimation();
            }

            // Remember where the motion event started
            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 (!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) {
                // Here is the difference
                if (dispatchNestedPreScroll(0, deltaY, mScrollConsumed, mScrollOffset,
                        ViewCompat.TYPE_TOUCH)) {
                    deltaY -= mScrollConsumed[1];
                    vtev.offsetLocation(0, mScrollOffset[1]);
                    mNestedYOffset += mScrollOffset[1];
                }

                // Scroll to follow the motion event
                mLastMotionY = y - mScrollOffset[1];

                final int oldY = getScrollY();
                final int range = getScrollRange();
                final int overscrollMode = getOverScrollMode();
                boolean canOverscroll = overscrollMode == View.OVER_SCROLL_ALWAYS
                        || (overscrollMode == View.OVER_SCROLL_IF_CONTENT_SCROLLS && range > 0);

                // Calling overScrollByCompat will call onOverScrolled, which
                // calls onScrollChanged if applicable.
                if (overScrollByCompat(0, deltaY, 0, getScrollY(), 0, range, 0,
                        0, true) && !hasNestedScrollingParent(ViewCompat.TYPE_TOUCH)) {
                    // Break our velocity if we hit a scroll barrier.
                    mVelocityTracker.clear();
                }

                final int scrolledDeltaY = getScrollY() - oldY;
                final int unconsumedY = deltaY - scrolledDeltaY;
                if (dispatchNestedScroll(0, scrolledDeltaY, 0, unconsumedY, mScrollOffset,
                        ViewCompat.TYPE_TOUCH)) {
                    mLastMotionY -= mScrollOffset[1];
                    vtev.offsetLocation(0, mScrollOffset[1]);
                    mNestedYOffset += mScrollOffset[1];
                } else if (canOverscroll) {
                    ensureGlows();
                    final int pulledToY = oldY + deltaY;
                    if (pulledToY < 0) {
                        EdgeEffectCompat.onPull(mEdgeGlowTop, (float) deltaY / getHeight(),
                                ev.getX(activePointerIndex) / getWidth());
                        if (!mEdgeGlowBottom.isFinished()) {
                            mEdgeGlowBottom.onRelease();
                        }
                    } else if (pulledToY > range) {
                        EdgeEffectCompat.onPull(mEdgeGlowBottom, (float) deltaY / getHeight(),
                                1.f - ev.getX(activePointerIndex)
                                        / getWidth());
                        if (!mEdgeGlowTop.isFinished()) {
                            mEdgeGlowTop.onRelease();
                        }
                    }
                    if (mEdgeGlowTop != null
                            && (!mEdgeGlowTop.isFinished() || !mEdgeGlowBottom.isFinished())) {
                        ViewCompat.postInvalidateOnAnimation(this);
                    }
                }
            }
            break;
        case MotionEvent.ACTION_UP:
            final VelocityTracker velocityTracker = mVelocityTracker;
            velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
            int initialVelocity = (int) velocityTracker.getYVelocity(mActivePointerId);
            if ((Math.abs(initialVelocity) > mMinimumVelocity)) {
                flingWithNestedDispatch(-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 && getChildCount() > 0) {
                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 true;
}