protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)

 int selfWidth = MeasureSpec.getSize(widthMeasureSpec);
 int selfHeight = MeasureSpec.getSize(heightMeasureSpec);

//	獲得LayoutParams
LayoutParams childParams = childView.getLayoutParams();
//	計(jì)算measureSpec
int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, parentLeft + parentRight, childParams.width);
int childHeightMeasureSpec = getChildMeasureSpec(widthMeasureSpec, parentTop + parentBottom, childParams.height);
//	測(cè)量
childView.measure(childWidthMeasureSpec, childHeightMeasureSpec);

	//	以橫向尺寸為例，第一個(gè)參數(shù)是父布局給的spec，第二個(gè)參數(shù)是扣除自己使用的尺寸，第三個(gè)是layoutParams
    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 us
        //	老王的錢是確定的，小王有三種可能
        case 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 us
        //	老王的錢最多有多少，小王有三種可能
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = 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 be
        //	老王的錢不確定，小王有三種可能
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = 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 be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

public class FlowLayout extends ViewGroup {

    private int mHorizontalSpacing = dp2px(16); //每個(gè)item橫向間距
    private int mVerticalSpacing = dp2px(8); //每個(gè)item橫向間距

    //  記錄所有的行
    private List<List<View>> allLines = new ArrayList<>();
    //  記錄所有的行高
    private List<Integer> lineHeights = new ArrayList<>();

    /**
     *      new FlowLayout(context) 的時(shí)候用
     * @param context
     */
    public FlowLayout(Context context) {
        super(context);
    }

    /**
     *  xml是序列化格式，里面都是鍵值對(duì)；所有的都在LayoutInflater解析
     *反射
     *
     * @param context
     * @param attrs
     */
    public FlowLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    /**
     *      主題style
     * @param context
     * @param attrs
     * @param defStyleAttr
     */
    public FlowLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    /**
     *      自定義屬性
     * @param context
     * @param attrs
     * @param defStyleAttr
     * @param defStyleRes
     */
    public FlowLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }


    /**
     *      onMeasure 可能會(huì)被調(diào)用多次
     */
    private void clearMeasureParams() {
        //  不斷創(chuàng)建回收會(huì)造成內(nèi)存抖動(dòng)，clear即可
        allLines.clear();
        lineHeights.clear();
    }

    /**
     *      度量---大部分是先測(cè)量孩子再測(cè)量自己。孩子的大小可能是一直在變的，父布局隨之改變
     *      只有ViewPager是先測(cè)量自己再測(cè)量孩子
     *      spec 是一個(gè)參考值，不是一個(gè)具體的值
     * @param widthMeasureSpec      父布局給的。這是個(gè)遞歸的過程
     * @param heightMeasureSpec     父布局給的
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

        clearMeasureParams();

        //  先測(cè)量孩子
        int childCount = getChildCount();

        int parentTop = getPaddingTop();
        int parentLeft = getPaddingLeft();
        int parentRight = getPaddingRight();
        int parentBottom = getPaddingBottom();

        //  爺爺給的參考值
        int selfWidth = MeasureSpec.getSize(widthMeasureSpec);
        int selfHeight = MeasureSpec.getSize(heightMeasureSpec);

        //  保存一行所有的 view
        List<View> lineViews = new ArrayList<>();
        //  記錄這行已使用多寬 size
        int lineWidthUsed = 0;
        //  一行的高
        int lineHeight = 0;

        //  measure過程中，子view要求的父布局寬高
        int parentNeedWidth = 0;
        int parentNeedHeight = 0;

        for (int i = 0; i < childCount; i++) {
            View childView = getChildAt(i);

            LayoutParams childParams = childView.getLayoutParams();
            //  將LayoutParams轉(zhuǎn)為measureSpec
            /**
             *      測(cè)量是個(gè)遞歸的過程，測(cè)量子View確定自身大小
             *      getChildMeasureSpec的三個(gè)參數(shù)，第一個(gè)是父布局傳過來的MeasureSpec，第二個(gè)參數(shù)是去除自身用掉的padding，第三個(gè)是子布局需要的寬度或高度
             */
            int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, parentLeft + parentRight, childParams.width);
            int childHeightMeasureSpec = getChildMeasureSpec(widthMeasureSpec, parentTop + parentBottom, childParams.height);

            childView.measure(childWidthMeasureSpec, childHeightMeasureSpec);

            //  獲取子View測(cè)量的寬高
            int childMeasuredWidth = childView.getMeasuredWidth();
            int childMeasuredHeight = childView.getMeasuredHeight();

            //  需要換行
            if (childMeasuredWidth + lineWidthUsed + mHorizontalSpacing > selfWidth) {

                //  換行時(shí)確定當(dāng)前需要的寬高
                parentNeedHeight = parentNeedHeight + lineHeight + mVerticalSpacing;
                parentNeedWidth = Math.max(parentNeedWidth, lineWidthUsed + mHorizontalSpacing);

                //  存儲(chǔ)每一行的數(shù)據(jù) ?。?！ 最后一行會(huì)被漏掉
                allLines.add(lineViews);
                lineHeights.add(lineHeight);

                //  數(shù)據(jù)清空
                lineViews = new ArrayList<>();
                lineWidthUsed = 0;
                lineHeight = 0;
            }

            lineViews.add(childView);
            lineWidthUsed = lineWidthUsed + childMeasuredWidth + mHorizontalSpacing;
            lineHeight = Math.max(lineHeight, childMeasuredHeight);

            //處理最后一行數(shù)據(jù)
            if (i == childCount - 1) {
                allLines.add(lineViews);
                lineHeights.add(lineHeight);
                parentNeedHeight = parentNeedHeight + lineHeight + mVerticalSpacing;
                parentNeedWidth = Math.max(parentNeedWidth, lineWidthUsed + mHorizontalSpacing);
            }

        }


        //  測(cè)量完孩子后再測(cè)量自己

        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);

        //  如果父布局給的是確切的值，測(cè)量子view則變得毫無意義
        int realWidth = (widthMode == MeasureSpec.EXACTLY) ? selfWidth : parentNeedWidth;
        int realHeight = (heightMode == MeasureSpec.EXACTLY) ? selfHeight : parentNeedHeight;
        setMeasuredDimension(realWidth, realHeight);
    }

    /**
     *      布局
     */
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {

        int currentL = getPaddingLeft();
        int currentT = getPaddingTop();

        for (int i = 0; i < allLines.size(); i++) {
            List<View> lineViews = allLines.get(i);
            int lineHeight = lineHeights.get(i);
            for (int j = 0; j < lineViews.size(); j++) {
                View view = lineViews.get(j);
                int left = currentL;
                int top = currentT;
                //  此處為什么不用 int right = view.getWidth(); getWidth是調(diào)用完onLayout才有的
                int right = left + view.getMeasuredWidth();
                int bottom = top + view.getMeasuredHeight();
                //  子view位置擺放
                view.layout(left, top, right, bottom);
                currentL = right + mHorizontalSpacing;
            }
            currentT = currentT + lineHeight + mVerticalSpacing;
            currentL = getPaddingLeft();
        }


    }

    public static int dp2px(int dp) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, Resources.getSystem().getDisplayMetrics());
    }

}

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
   if (mOrientation == VERTICAL) {
        measureVertical(widthMeasureSpec, heightMeasureSpec);
    } else {
        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
    }
}

void measureVertical(widthMeasureSpec, heightMeasureSpec){
	//	獲取子view 的 LayoutParams 
	final LayoutParams lp = (LayoutParams) child.getLayoutParams();
	...
	...
	//	開始測(cè)量
	measureChildBeforeLayout(child, i, widthMeasureSpec, 0,heightMeasureSpec, usedHeight);
}

void measureChildBeforeLayout(View child, int childIndex,int widthMeasureSpec, int totalWidth, int heightMeasureSpec,int totalHeight) {
        measureChildWithMargins(child, widthMeasureSpec, totalWidth, heightMeasureSpec, totalHeight);
    }


protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {
        
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

	//	去除自己的使用，padding、margin剩下的再給子view
    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                    + widthUsed, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                    + heightUsed, lp.height);

	//	此處子view調(diào)用其測(cè)量函數(shù)，也就是FlowLayout的測(cè)量
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}