[Android学习整理]之从源码分析mScrollX，scrollTo()，smoothScrollTo

/**
     * The offset, in pixels, by which the content of this view is scrolled
     * horizontally.
     * {@hide}
     */
    @ViewDebug.ExportedProperty(category = "scrolling")
    protected int mScrollX;
    /**
     * The offset, in pixels, by which the content of this view is scrolled
     * vertically.
     * {@hide}
     */
    @ViewDebug.ExportedProperty(category = "scrolling")
    protected int mScrollY;


     /**
     * Return the scrolled left position of this view. This is the left edge of
     * the displayed part of your view. You do not need to draw any pixels
     * farther left, since those are outside of the frame of your view on
     * screen.
     *
     * @return The left edge of the displayed part of your view, in pixels.
     */
    public final int getScrollX() {
        return mScrollX;
    }

    /**
     * Return the scrolled top position of this view. This is the top edge of
     * the displayed part of your view. You do not need to draw any pixels above
     * it, since those are outside of the frame of your view on screen.
     *
     * @return The top edge of the displayed part of your view, in pixels.
     */
    public final int getScrollY() {
        return mScrollY;
    }

<span style="font-family:SimSun;font-size:14px;"> /** 
     * 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()) {  
                postInvalidateOnAnimation();  
            }  
        }  
    }  

    /** 
     * 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);  
    }</span>

/**
     * Like {@link View#scrollBy}, but scroll smoothly instead of immediately.
     *
     * @param dx the number of pixels to scroll by on the X axis
     * @param dy the number of pixels to scroll by on the Y axis
     */
    public final void smoothScrollBy(int dx, int dy) {
        if (getChildCount() == 0) {
            // Nothing to do.
            return;
        }
        long duration = AnimationUtils.currentAnimationTimeMillis() - mLastScroll;
        if (duration > ANIMATED_SCROLL_GAP) {
            final int height = getHeight() - mPaddingBottom - mPaddingTop;  //获取部件高度
            final int bottom = getChildAt(0).getHeight();   //获取当前能看到的item高度
            final int maxY = Math.max(0, bottom - height);   
            final int scrollY = mScrollY;
            dy = Math.max(0, Math.min(scrollY + dy, maxY)) - scrollY;

            mScroller.startScroll(mScrollX, scrollY, 0, dy);
            postInvalidateOnAnimation();
        } else {
            if (!mScroller.isFinished()) {
                mScroller.abortAnimation();
                if (mFlingStrictSpan != null) {
                    mFlingStrictSpan.finish();
                    mFlingStrictSpan = null;
                }
            }
            scrollBy(dx, dy);
        }
        mLastScroll = AnimationUtils.currentAnimationTimeMillis();
    }

    /**
     * Like {@link #scrollTo}, but scroll smoothly instead of immediately.
     *
     * @param x the position where to scroll on the X axis
     * @param y the position where to scroll on the Y axis
     */
    public final void smoothScrollTo(int x, int y) {
        smoothScrollBy(x - mScrollX, y - mScrollY);
    }