在Android中，我们通常说View在屏幕上的坐标，其实就是view的左上的坐标。调用View的invalidate()方法会促使View重绘。

在分析scrollBy()和scrollTo()之前，先上一段源码片段：

    /**
     * Set the scrolled position of your view. This will cause a call to
     * {@link #onScrollChanged(int, int, int, int)} and the view will be
     * invalidated.
     * @param x the x position to scroll to
     * @param y the y position to scroll to
     */
    public void scrollTo(int x, int y) {
        if (mScrollX != x || mScrollY != y) {
            int oldX = mScrollX;
            int oldY = mScrollY;
            mScrollX = x;
            mScrollY = y;
            invalidateParentCaches();
            onScrollChanged(mScrollX, mScrollY, oldX, oldY);
            if (!awakenScrollBars()) {
                invalidate(true);
            }
        }
    }

    /**
     * Move the scrolled position of your view. This will cause a call to
     * {@link #onScrollChanged(int, int, int, int)} and the view will be
     * invalidated.
     * @param x the amount of pixels to scroll by horizontally
     * @param y the amount of pixels to scroll by vertically
     */
    public void scrollBy(int x, int y) {
        scrollTo(mScrollX + x, mScrollY + y);
    }

scrollBy()和scrollTo()的参数正负影响滚动问题

scrollBy()和scrollTo()在参数为负的时候，向坐标轴正方向滚动；当参数为正的时候，向坐标轴负方向滚动。而作为我们的认知，应该是参数为负的时候，向坐标轴负方向滚动；当参数为正的时候，向坐标轴正方向滚动。

那为什么这两个方法传入参数和引起的滚动方向和我们平常的认知不同呢？

下面就让我们带着这个问题跟随源码分析。如果不想从它的执行过程一步步的去分析，可以直接看本文的最后一段源码。

源码执行过程分析

因为scrollBy(x, y)方法体只有一行，并且是调用scrollTo(x, y)，所以我们只要通过scrollTo(x, y)来进行分析就可以了。

在scrollTo(x, y)中，x和y分别被赋值给了mScrollX和mScrollY，最后调用了方法invalidate(true)。貌似到了这里就无路可走了，其实不然，我们知道invalidate这个方法会通知View进行重绘。

那么接下来，我们就可以跳过scrollTo(x, y)去分析View的draw()方法了。照例，在分析onDraw方法之前上一段源码片段：

    /**
     * Manually render this view (and all of its children) to the given Canvas.
     * The view must have already done a full layout before this function is
     * called.  When implementing a view, implement
     * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
     * If you do need to override this method, call the superclass version.
     *
     * @param canvas The Canvas to which the View is rendered.
     */
    public void draw(Canvas canvas) {
        if (ViewDebug.TRACE_HIERARCHY) {
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);
        }

        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~DIRTY_MASK) | DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas‘ layers to prepare for fading
         *      3. Draw view‘s content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            final Drawable background = mBGDrawable;
            if (background != null) {
                final int scrollX = mScrollX;
                final int scrollY = mScrollY;

                if (mBackgroundSizeChanged) {
                    background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
                    mBackgroundSizeChanged = false;
                }

                if ((scrollX | scrollY) == 0) {
                    background.draw(canvas);
                } else {
                    canvas.translate(scrollX, scrollY);
                    background.draw(canvas);
                    canvas.translate(-scrollX, -scrollY);
                }
            }
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            // Step 6, draw decorations (scrollbars)
            onDrawScrollBars(canvas);

            // we‘re done...
            return;
        }

        /*
         * Here we do the full fledged routine...
         * (this is an uncommon case where speed matters less,
         * this is why we repeat some of the tests that have been
         * done above)
         */

        boolean drawTop = false;
        boolean drawBottom = false;
        boolean drawLeft = false;
        boolean drawRight = false;

        float topFadeStrength = 0.0f;
        float bottomFadeStrength = 0.0f;
        float leftFadeStrength = 0.0f;
        float rightFadeStrength = 0.0f;

        // Step 2, save the canvas‘ layers
        int paddingLeft = mPaddingLeft;

        final boolean offsetRequired = isPaddingOffsetRequired();
        if (offsetRequired) {
            paddingLeft += getLeftPaddingOffset();
        }

        int left = mScrollX + paddingLeft;
        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
        int top = mScrollY + getFadeTop(offsetRequired);
        int bottom = top + getFadeHeight(offsetRequired);

        if (offsetRequired) {
            right += getRightPaddingOffset();
            bottom += getBottomPaddingOffset();
        }

        final ScrollabilityCache scrollabilityCache = mScrollCache;
        final float fadeHeight = scrollabilityCache.fadingEdgeLength;        
        int length = (int) fadeHeight;

        // clip the fade length if top and bottom fades overlap
        // overlapping fades produce odd-looking artifacts
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }

        // also clip horizontal fades if necessary
        if (horizontalEdges && (left + length > right - length)) {
            length = (right - left) / 2;
        }

        if (verticalEdges) {
            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
            drawTop = topFadeStrength * fadeHeight > 1.0f;
            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
        }

        if (horizontalEdges) {
            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
            drawLeft = leftFadeStrength * fadeHeight > 1.0f;
            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
            drawRight = rightFadeStrength * fadeHeight > 1.0f;
        }

        saveCount = canvas.getSaveCount();

        int solidColor = getSolidColor();
        if (solidColor == 0) {
            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

            if (drawTop) {
                canvas.saveLayer(left, top, right, top + length, null, flags);
            }

            if (drawBottom) {
                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
            }

            if (drawLeft) {
                canvas.saveLayer(left, top, left + length, bottom, null, flags);
            }

            if (drawRight) {
                canvas.saveLayer(right - length, top, right, bottom, null, flags);
            }
        } else {
            scrollabilityCache.setFadeColor(solidColor);
        }

        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        // Step 5, draw the fade effect and restore layers
        final Paint p = scrollabilityCache.paint;
        final Matrix matrix = scrollabilityCache.matrix;
        final Shader fade = scrollabilityCache.shader;

        if (drawTop) {
            matrix.setScale(1, fadeHeight * topFadeStrength);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            canvas.drawRect(left, top, right, top + length, p);
        }

        if (drawBottom) {
            matrix.setScale(1, fadeHeight * bottomFadeStrength);
            matrix.postRotate(180);
            matrix.postTranslate(left, bottom);
            fade.setLocalMatrix(matrix);
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }

        if (drawLeft) {
            matrix.setScale(1, fadeHeight * leftFadeStrength);
            matrix.postRotate(-90);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            canvas.drawRect(left, top, left + length, bottom, p);
        }

        if (drawRight) {
            matrix.setScale(1, fadeHeight * rightFadeStrength);
            matrix.postRotate(90);
            matrix.postTranslate(right, top);
            fade.setLocalMatrix(matrix);
            canvas.drawRect(right - length, top, right, bottom, p);
        }

        canvas.restoreToCount(saveCount);

        // Step 6, draw decorations (scrollbars)
        onDrawScrollBars(canvas);
    }

在这段代码片中，我们直接定位到onDrawScrollBars(canvas)方法，找到了这个方法离真相就不远了。上源码：

    /**
     * <p>Request the drawing of the horizontal and the vertical scrollbar. The
     * scrollbars are painted only if they have been awakened first.</p>
     *
     * @param canvas the canvas on which to draw the scrollbars
     *
     * @see #awakenScrollBars(int)
     */
    protected final void onDrawScrollBars(Canvas canvas) {
        // scrollbars are drawn only when the animation is running
        final ScrollabilityCache cache = mScrollCache;
        if (cache != null) {

            int state = cache.state;

            if (state == ScrollabilityCache.OFF) {
                return;
            }

            boolean invalidate = false;

            if (state == ScrollabilityCache.FADING) {
                // We‘re fading -- get our fade interpolation
                if (cache.interpolatorValues == null) {
                    cache.interpolatorValues = new float[1];
                }

                float[] values = cache.interpolatorValues;

                // Stops the animation if we‘re done
                if (cache.scrollBarInterpolator.timeToValues(values) ==
                        Interpolator.Result.FREEZE_END) {
                    cache.state = ScrollabilityCache.OFF;
                } else {
                    cache.scrollBar.setAlpha(Math.round(values[0]));
                }

                // This will make the scroll bars inval themselves after
                // drawing. We only want this when we‘re fading so that
                // we prevent excessive redraws
                invalidate = true;
            } else {
                // We‘re just on -- but we may have been fading before so
                // reset alpha
                cache.scrollBar.setAlpha(255);
            }


            final int viewFlags = mViewFlags;

            final boolean drawHorizontalScrollBar =
                (viewFlags & SCROLLBARS_HORIZONTAL) == SCROLLBARS_HORIZONTAL;
            final boolean drawVerticalScrollBar =
                (viewFlags & SCROLLBARS_VERTICAL) == SCROLLBARS_VERTICAL
                && !isVerticalScrollBarHidden();

            if (drawVerticalScrollBar || drawHorizontalScrollBar) {
                final int width = mRight - mLeft;
                final int height = mBottom - mTop;

                final ScrollBarDrawable scrollBar = cache.scrollBar;

                final int scrollX = mScrollX;
                final int scrollY = mScrollY;
                final int inside = (viewFlags & SCROLLBARS_OUTSIDE_MASK) == 0 ? ~0 : 0;

                int left, top, right, bottom;

                if (drawHorizontalScrollBar) {
                    int size = scrollBar.getSize(false);
                    if (size <= 0) {
                        size = cache.scrollBarSize;
                    }

                    scrollBar.setParameters(computeHorizontalScrollRange(),
                                            computeHorizontalScrollOffset(),
                                            computeHorizontalScrollExtent(), false);
                    final int verticalScrollBarGap = drawVerticalScrollBar ?
                            getVerticalScrollbarWidth() : 0;
                    top = scrollY + height - size - (mUserPaddingBottom & inside);
                    left = scrollX + (mPaddingLeft & inside);
                    right = scrollX + width - (mUserPaddingRight & inside) - verticalScrollBarGap;
                    bottom = top + size;
                    onDrawHorizontalScrollBar(canvas, scrollBar, left, top, right, bottom);
                    if (invalidate) {
                        invalidate(left, top, right, bottom);
                    }
                }

                if (drawVerticalScrollBar) {
                    int size = scrollBar.getSize(true);
                    if (size <= 0) {
                        size = cache.scrollBarSize;
                    }

                    scrollBar.setParameters(computeVerticalScrollRange(),
                                            computeVerticalScrollOffset(),
                                            computeVerticalScrollExtent(), true);
                    switch (mVerticalScrollbarPosition) {
                        default:
                        case SCROLLBAR_POSITION_DEFAULT:
                        case SCROLLBAR_POSITION_RIGHT:
                            left = scrollX + width - size - (mUserPaddingRight & inside);
                            break;
                        case SCROLLBAR_POSITION_LEFT:
                            left = scrollX + (mUserPaddingLeft & inside);
                            break;
                    }
                    top = scrollY + (mPaddingTop & inside);
                    right = left + size;
                    bottom = scrollY + height - (mUserPaddingBottom & inside);
                    onDrawVerticalScrollBar(canvas, scrollBar, left, top, right, bottom);
                    if (invalidate) {
                        invalidate(left, top, right, bottom);
                    }
                }
            }
        }
    }

上述代码，我们直接定位到if (drawVerticalScrollBar || drawHorizontalScrollBar)结构语句块。在水平方向滚动与垂直方向滚动语句块中，能够找到一行关键性代码invalidate(left, top, right, bottom)，接着上源码：

    /**
     * Mark the area defined by the rect (l,t,r,b) as needing to be drawn.
     * The coordinates of the dirty rect are relative to the view.
     * If the view is visible, {@link #onDraw(android.graphics.Canvas)}
     * will be called at some point in the future. This must be called from
     * a UI thread. To call from a non-UI thread, call {@link #postInvalidate()}.
     * @param l the left position of the dirty region
     * @param t the top position of the dirty region
     * @param r the right position of the dirty region
     * @param b the bottom position of the dirty region
     */
    public void invalidate(int l, int t, int r, int b) {
        if (ViewDebug.TRACE_HIERARCHY) {
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE);
        }

        if (skipInvalidate()) {
            return;
        }
        if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS) ||
                (mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID ||
                (mPrivateFlags & INVALIDATED) != INVALIDATED) {
            mPrivateFlags &= ~DRAWING_CACHE_VALID;
            mPrivateFlags |= INVALIDATED;
            mPrivateFlags |= DIRTY;
            final ViewParent p = mParent;
            final AttachInfo ai = mAttachInfo;
            //noinspection PointlessBooleanExpression,ConstantConditions
            if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {
                if (p != null && ai != null && ai.mHardwareAccelerated) {
                    // fast-track for GL-enabled applications; just invalidate the whole hierarchy
                    // with a null dirty rect, which tells the ViewAncestor to redraw everything
                    p.invalidateChild(this, null);
                    return;
                }
            }
            if (p != null && ai != null && l < r && t < b) {
                final int scrollX = mScrollX;
                final int scrollY = mScrollY;
                final Rect tmpr = ai.mTmpInvalRect;
                tmpr.set(l - scrollX, t - scrollY, r - scrollX, b - scrollY);
                p.invalidateChild(this, tmpr);
            }
        }
    }

invalidate(left, top, right, bottom)方法体中，倒数第5行tmpr.set(l - scrollX, t - scrollY, r - scrollX, b - scrollY)设置一个view需要绘制的脏矩形，这个方法的传入参数不觉得很奇怪吗？
mScrollX和mScrollY都是作为参数的减数（负负得正，负正得负），再结合开头的Android屏幕直角坐标系的概念，通过简单的逻辑分析或者计算就可以证明：当scrollTo()的传入参数为负的时候，view就向坐标轴正方向滚动；当为正的时候，view就向坐标轴负方向滚动。