仿QQ侧边栏滑动的实现

视频来源：http://www.imooc.com/video/4386
实现过程：主要是实现一个继承于HorizontalScrollView 的控件SlideMenu ，大致步骤：

1. 重写OnMeasure 方法来定义子View的大小
2. 重写onLayout 方法来初始化子View的位置
3. 重写onTouchEvent 方法来设置滑动事件

4. 重写onScrollChanged 方法来设置滑动动画，以达到菜单栏左端固定于屏幕左侧出现（而不是菜单栏从右侧开始慢慢出现），以及相应的缩放、透明度设置。这里需要引入NineOldAndroid包

   代码如下：

class SlideMenu:HorizontalScrollView {    var mWapper:LinearLayout?=null    var mMenu:ViewGroup?=null    var mContent:ViewGroup?=null    var mScreenWidth:Int=0    var once=0    //dp    var mMenuRightPadding=0    constructor(context: Context,attributeSet: AttributeSet) : super(context,attributeSet){        val wm:WindowManager= context.getSystemService(Context.WINDOW_SERVICE) as WindowManager        val outMetrics:DisplayMetrics= DisplayMetrics()        wm.defaultDisplay.getMetrics(outMetrics)        mScreenWidth=outMetrics.widthPixels        //把dp、sp转化为px        mMenuRightPadding= TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 50F,context.resources.displayMetrics ).toInt()    }    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {        if (once==0)        {            mWapper= getChildAt(0) as LinearLayout?            mMenu= mWapper?.getChildAt(0) as ViewGroup?            mContent= mWapper?.getChildAt(1) as ViewGroup?            mMenu?.layoutParams?.width=mScreenWidth-mMenuRightPadding            mContent?.layoutParams?.width=mScreenWidth            once=1        }        super.onMeasure(widthMeasureSpec, heightMeasureSpec)    }//    设置偏移量将menu隐藏    override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {        super.onLayout(changed, l, t, r, b)        if (changed)        {            scrollTo(mMenu?.layoutParams?.width!!,0)        }    }    override fun onTouchEvent(ev: MotionEvent?): Boolean {        val action=ev?.action        when (action)        {            MotionEvent.ACTION_UP->{                //隐藏部分大于菜单栏一般                if (scrollX >= (mMenu?.layoutParams?.width!! /2))                {                    smoothScrollTo(mMenu?.layoutParams?.width!!,0)                }                else                {                    smoothScrollTo(0,0)                }                return true            }        }        return super.onTouchEvent(ev)    }    override fun onScrollChanged(l: Int, t: Int, oldl: Int, oldt: Int) {        super.onScrollChanged(l, t, oldl, oldt)        val scale=l.toFloat()/mMenu?.layoutParams?.width!!        ViewHelper.setTranslationX(mMenu, scale*mMenu?.layoutParams?.width!!*0.7f)        val rightScale=0.7f+0.3f*scale        ViewHelper.setScaleX(mContent,rightScale)        ViewHelper.setScaleY(mContent,rightScale)        ViewHelper.setPivotX(mContent, 0f)        ViewHelper.setPivotY(mContent, (mContent?.height!! /2).toFloat())        val leftScale=1.0f-scale*0.3f        ViewHelper.setScaleX(mMenu,leftScale)        ViewHelper.setScaleY(mMenu,leftScale)        ViewHelper.setAlpha(mMenu,leftScale)        ViewHelper.setPivotX(mMenu, 0f)        ViewHelper.setPivotY(mMenu, (mMenu?.height!! /2).toFloat())    }}

效果如图：