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首先了解view的绘制流程：

所以onmeasure ---测量view  

onlayout---确定view大小----》所以继承ViewGroup必须要重写onlayout，确定子view

而onDraw----是继承view时候需要操作的。

所以：自定义ViewGroup一般是利用现有的组件根据特定的布局方式来组成新的组件。

              自定义View，一般是没有现成的view

序列化，大概有这个意思，不一定对。
自定义序列化： IOT
协议比如：物联网：蓝牙：传递的数据 串口 协议：

onmeasure的测量 是先从子布局开始还是先从父布局开始的？

----根据算法来控制的，比如view pageer就是父布局开始

MeasureSpec是什么

public static class MeasureSpec {private static final int MODE_SHIFT = 30;private static final int MODE_MASK  = 0x3 << MODE_SHIFT;/** @hide */@IntDef({UNSPECIFIED, EXACTLY, AT_MOST})@Retention(RetentionPolicy.SOURCE)public @interface MeasureSpecMode {}

----是view里面的一个类---我们知道int 是32位

------上面代码里的30，就是高两位是00，后面30位---》所以这组成里MeasureSpec

-------高两位表示UNSPECIFIED，EXACTLY，AT_MOST

关于getChildMeasureSpec(int spec, int padding, int childDimension)算法

第一个参数，父亲给的，

第二个参数，父亲的

第三个参数，孩子需要的

-----》根据UNSPECIFIED，EXACTLY，AT_MOST来计算

 public static int getChildMeasureSpec(int spec, int padding, int childDimension) {int specMode = MeasureSpec.getMode(spec);int specSize = MeasureSpec.getSize(spec);int size = Math.max(0, specSize - padding);int resultSize = 0;int resultMode = 0;switch (specMode) {// Parent has imposed an exact size on uscase MeasureSpec.EXACTLY:if (childDimension >= 0) {resultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size. So be it.resultSize = size;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.WRAP_CONTENT) {// Child wants to determine its own size. It can't be// bigger than us.resultSize = size;resultMode = MeasureSpec.AT_MOST;}break;// Parent has imposed a maximum size on uscase MeasureSpec.AT_MOST:if (childDimension >= 0) {// Child wants a specific size... so be itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size, but our size is not fixed.// Constrain child to not be bigger than us.resultSize = size;resultMode = MeasureSpec.AT_MOST;} else if (childDimension == LayoutParams.WRAP_CONTENT) {// Child wants to determine its own size. It can't be// bigger than us.resultSize = size;resultMode = MeasureSpec.AT_MOST;}break;// Parent asked to see how big we want to becase MeasureSpec.UNSPECIFIED:if (childDimension >= 0) {// Child wants a specific size... let them have itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size... find out how big it should// beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;resultMode = MeasureSpec.UNSPECIFIED;} else if (childDimension == LayoutParams.WRAP_CONTENT) {// Child wants to determine its own size.... find out how// big it should beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;resultMode = MeasureSpec.UNSPECIFIED;}break;}//noinspection ResourceTypereturn MeasureSpec.makeMeasureSpec(resultSize, resultMode);}

下面是一个流式布局的例子：

 * desc   : 官方FlexboxLayout 流式布局*/
class FlowLayout(context: Context) : ViewGroup(context) {private val mHorizontalSpacing = dp2px(16f) //每个item横向间距private val mVerticalSpacing = dp2px(8f) //每个item竖向间距private  var allLines:MutableList<MutableList<View>> = ArrayList<MutableList<View>>() //记录所有的行，一行一行保存，用于layoutvar lineHeights:ArrayList<Int> = ArrayList()//记录每一行的行高，用于layoutconstructor(context: Context,attrs:AttributeSet):this(context){// 在这里处理从 XML 布局 --序列化格式--建值对}constructor(context: Context, attrs: AttributeSet, defStyle: Int) : this(context, attrs) {// 在这里处理从 XML 布局文件中传入的属性和样式}//初始化，因为是onMeasure递归的方式，所以要放在onMeasureprivate fun clearMeasureParams(){
//        allLines =ArrayList<MutableList<View>>()
//        lineHeights = ArrayList()allLines.clear()lineHeights.clear()}override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {clearMeasureParams()// 内存抖动
//        super.onMeasure(widthMeasureSpec, heightMeasureSpec)//先度量孩子val childCount =childCount//ViewGroup解析父控件给我的宽高val selfWidth = MeasureSpec.getSize(widthMeasureSpec)val selfHeight =MeasureSpec.getSize(heightMeasureSpec)//measure过程中 子view要求的父viewgroup的宽高var parentNeededWidth =0;var parentNeededHeight =0;val linView = ArrayList<View>()var lineWidthUsed =0 //记录这一行的以及使用了多宽的sizevar lineHeight =0 //一行的杭高for (i in 0 until childCount){val childview = getChildAt(i)val childlayoutParams = childview.layoutParamsif (childview.visibility != GONE) {val childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,paddingLeft + paddingRight,childlayoutParams.width)val childHeightMeasureSpec1 = getChildMeasureSpec(heightMeasureSpec,paddingTop + paddingBottom,childlayoutParams.height)childview.measure(childWidthMeasureSpec, childHeightMeasureSpec1)//获取子view的度量高度val childMeasureWidth = childview.measuredWidthval childmeasuredHeight = childview.measuredHeightlinView.add(childview)//是否药换行if (childMeasureWidth + lineWidthUsed + mHorizontalSpacing > selfWidth) {//换行判断当前行所需要的宽和高，所以要记录下来allLines.add(linView)lineHeights.add(lineHeight) //行高。。这个if语句会缺少最后一行，parentNeededHeight = parentNeededHeight + lineHeight + mVerticalSpacingparentNeededWidth =Math.max(parentNeededWidth, lineWidthUsed + mHorizontalSpacing)linView.clear()lineWidthUsed = 0lineHeight = 0}//每行自己的宽高lineWidthUsed = lineWidthUsed + childMeasureWidth + mHorizontalSpacinglineHeight = Math.max(lineHeight, childmeasuredHeight)//最后一行，if (i == childCount - 1) {allLines.add(linView)lineHeights.add(lineHeight)parentNeededHeight = parentNeededHeight + lineHeight + mVerticalSpacingparentNeededWidth =Math.max(parentNeededWidth, lineWidthUsed + mHorizontalSpacing)}}}//度量自己//根据子view的度量结果，来重新度量自己的viewGroup//作为一个viewgroup，他自己也是个view，他的大小也需要根据他的父亲提供的宽高来度量var withmode: Int = MeasureSpec.getMode(widthMeasureSpec)var heightmode: Int = MeasureSpec.getMode(heightMeasureSpec)//确切的值 EXACTLYval realWidth =if(withmode  ==MeasureSpec.EXACTLY) selfWidth else parentNeededWidthval realHeight =if(heightmode  ==MeasureSpec.EXACTLY) selfHeight else parentNeededWidthsetMeasuredDimension(realWidth,realHeight)}override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {val lineCount =allLines.sizevar curl =paddingLeft //左边届var curT= paddingTop //上边界for(i in 0 until lineCount){val Lineviews = allLines.get(i)val lineHeight = lineHeights.get(i)for (j in 0 until Lineviews.size){//其中一行val view = Lineviews.get(j)//这一行的view//计算边界val left =curl //左边届val top= curT //上边界val right = left+measuredWidth//左边届val botton = top +measuredHeight  //上边界view.layout(left,top,right,botton)curl =right+mHorizontalSpacing//下一个左边}curT =curT +lineHeight+mVerticalSpacing //下一个topcurl = paddingLeft //每一个新的行要重制左边届}//        view.layout(left,top,right,bottom)}
}