笔记6 | 从源码理解分析mScrollX和mScrollY，scrollTo()和scrollBy()，smoothScrollTo和smoothScrollBy

前言

我想开头先说说

a.度娘一下什么都知道了，整理这些东西有什么用？还费时费力的！如何才能完全掌握一个知识？当你能把它讲清楚的时候，你才算掌握了他。

想完全把mScrollX和mScrollY，scrollTo()和scrollBy()，smoothScrollTo和smoothScrollBy弄明白并不容易，但查阅他们的源代码基本就能明白个大概，这篇文章就是从源码分析他们究竟有什么作用和区别，读懂了基本就会用！

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公众号

目录

• mScrollX和mScrollY
• scrollTo()和scrollBy()
• smoothScrollBy和smoothScrollTo
• view和viewgroup

一.mScrollX和mScrollY

/**
     * 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;
    }

直接翻译就可以得知

mScrollX：表示离视图起始位置的x水平方向的偏移量

mScrollY：表示离视图起始位置的y垂直方向的偏移量

通过getScrollX() 和getScrollY()方法获这兄弟俩。注意：mScrollX和mScrollY指的并不是坐标，而是偏移量。

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二.scrollTo()和scrollBy()

<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> 

源码可以看出，scrollTo 和 scrollBy区别，其实2者的效果是一样的，只是过程不同而已。

scrollTo（int x,int y）：

如果偏移位置发生了改变，就会给mScrollX和mScrollY赋新值，改变当前位置。注意：x,y代表的不是坐标点，而是偏移量。例如：我要移动view到坐标点（100，100），那么我的偏移量就是(0,，0) - （100，100） = （-100 ，-100） ，我就要执行view.scrollTo(-100,-100),达到这个效果。

scrollBy（int x,int y）：

从源码中看出，它实际上是调用了scrollTo(mScrollX + x, mScrollY + y);mScrollX + x和mScrollY + y，即表示在原先偏移的基础上在发生偏移，通俗的说就是相对我们当前位置偏移。根据父类VIEW里面移动，如果移动到了超出的地方，就不会显示。

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三.smoothScrollTo和smoothScrollBy

ScrollView.smoothScrollBy和smoothScrollTo的源码

 /**
     * 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);
    }

源码可以看出，是ScrollBy和ScrollTo增加滚动动画的升级方案；

moothScrollBy(int dx, int dy) ：

 dy = Math.max(0, Math.min(scrollY + dy, maxY)) - scrollY;为方法的核心，比较绕，大致为计算当前滑动状态下可划动距离；

if(duration > ANIMATEDSCROLLGAP) 这句是判断当前是否在滚动，当还在滚动状态下，执行mScroller.startScroll(mScrollX, scrollY, 0, dy)；当不在滚动，就立马打断。

smoothScrollTo(int x, int y)：

 它实际上是调用了smoothScrollBy(x - mScrollX, y - mScrollY);方法根据x,y的值来计算剩余可滚动的位移量；

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四.view和viewgroup

最后讲讲view和viewgroup这俩，完全吃透有点难，通俗讲，Android的UI界面都是由View和ViewGroup及其派生类组合而成的，View是所有UI组件的基类，而ViewGroup是容纳这些组件的容器；View类是ViewGroup的父类，ViewGroup具有View的所有特性，ViewGroup主要用来充当View的容器，将其中的View作为自己孩子，并对其进行管理，当然孩子也可以是ViewGroup类型。

View派生出的直接子类有：AnalogClock,ImageView,KeyboardView, ProgressBar,SurfaceView,TextView,ViewGroup,ViewStubView派生出的间接子类有：AbsListView,AbsSeekBar, AbsSpinner, AbsoluteLayout, AdapterView,AdapterViewAnimator, AdapterViewFlipper, AppWidgetHostView, AutoCompleteTextView,Button,CalendarView, CheckBox, CheckedTextView, Chronometer, CompoundButton

ViewGroup派生出的直接子类有：AbsoluteLayout,AdapterView,FragmentBreadCrumbs,FrameLayout,LinearLayout,RelativeLayout,SlidingDrawer；

ViewGroup派生出的间接子类有：AbsListView,AbsSpinner, AdapterViewAnimator, AdapterViewFlipper, AppWidgetHostView, CalendarView, DatePicker, DialerFilter, ExpandableListView, Gallery, GestureOverlayView,GridView,HorizontalScrollView, ImageSwitcher,ListView,

当然，随着Android版本不断地更新，这些派生出来的子类也是在不段增加的！

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