FrameLayout rootView = findViewById(android.R.id.content); 
RelativeLayout relativeLayout = (LinearLayout) rootView.getChildAt(0);//獲取Activity的根部局

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  int width = 0;//最終確定的寬度
  int height = 0;//最終確定的高度
  //1、首先測量自身
  super.onMeasure(widthMeasureSpec, heightMeasureSpec);
  //2、為每個子view計算測量的限制信息Mode/Size
  int widthMeasureSpecMode = MeasureSpec.getMode(widthMeasureSpec);
  int widthMeasureSpecSize = MeasureSpec.getSize(widthMeasureSpec);
  int heightMeasureSpecMode = MeasureSpec.getMode(heightMeasureSpec);
  int heightMeasureSpecSize = MeasureSpec.getSize(heightMeasureSpec);
  //3、測量子View；把上一步確定的限制信息，傳遞給每一個子View，然后子View開始measure自己的尺寸
  int childCount = getChildCount();
  for(int i=0;i<childCount;i++){
    View child = getChildAt(i);
    measureChild(child,widthMeasureSpec,heightMeasureSpec);//這個方法就是確定子view的測量大小
  }
  //4、根據(jù)子View的測量尺寸以及自身的SpecMode計算自己的尺寸
  switch (widthMeasureSpecMode) {
    case MeasureSpec.EXACTLY://如果是確定值，則使用確定值
      width = widthMeasureSpecSize;
    case MeasureSpec.AT_MOST://如果是根據(jù)內(nèi)容定的大小
    case MeasureSpec.UNSPECIFIED://一般可以不用單獨處理
      for(int i=0;i<childCount;i++){
        View child = getChildAt(i);
        int childWidth = child.getMeasuredWidth();//這一步只有當(dāng)measureChild方法執(zhí)行完之后才能拿到
        width = Math.max(childWidth,width);
      }
    default:break;
  }
  switch (heightMeasureSpecMode) {
    case MeasureSpec.EXACTLY://如果是確定值，則使用確定值
      height = heightMeasureSpecSize;
    case MeasureSpec.AT_MOST://如果是根據(jù)內(nèi)容定的大小
    case MeasureSpec.UNSPECIFIED:
      for(int i=0;i<childCount;i++){
        View child = getChildAt(i);
        int childHeight = child.getMeasuredHeight();//這一步只有當(dāng)measureChild方法執(zhí)行完之后才能拿到
        height+=childHeight;
      }
    default:break;
  }
  //保存自身測量后的寬和高
  setMeasuredDimension(width,height);
}

protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);
    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result;
}

package com.example.materialdesign.selfView;
import android.content.Context;
import android.os.Build;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewGroup;
import androidx.annotation.RequiresApi;
public class FlowLayout extends ViewGroup {
  public FlowLayout(Context context) {
    super(context);
  }
  public FlowLayout(Context context, AttributeSet attrs) {
    super(context, attrs);
  }
  public FlowLayout(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
  }
  @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
  public FlowLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    super(context, attrs, defStyleAttr, defStyleRes);
  }
  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int lineWidth = 0;//記錄每一行的寬度，最終的寬度是由所有行中的最大值
    int lineHeight = 0;//記錄每一行的高度，取決于每一行中最高的那個組件
    int resH = 0;//最終的高度
    int resW = 0;//最終的寬度
    //1、首先測量自身
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    //2、為每個子view計算測量的限制信息Mode/Size
    int widthMeasureSpecMode = MeasureSpec.getMode(widthMeasureSpec);
    int widthMeasureSpecSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightMeasureSpecMode = MeasureSpec.getMode(heightMeasureSpec);
    int heightMeasureSpecSize = MeasureSpec.getSize(heightMeasureSpec);
    //3、測量每個子view的寬度和高度
    int childCount = getChildCount();
    for (int i = 0; i < childCount; i++) {
      View child = getChildAt(i);
      measureChild(child, widthMeasureSpec, heightMeasureSpec);
      MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
      int childMeasuredWidth = child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin;
      int childMeasuredHeight = child.getMeasuredHeight() + lp.bottomMargin + lp.topMargin;

      if (lineWidth + childMeasuredWidth > widthMeasureSpecSize) {//當(dāng)前行的的寬度已經(jīng)加上當(dāng)前view的寬度已經(jīng)大于建議值寬度了
        //需要換行
        resW = Math.max(resW, lineWidth);
        resH += lineHeight;
        //重新賦值
        lineWidth = childMeasuredWidth;
        lineHeight = childMeasuredHeight;
      } else {//不需要換行則累加
        lineWidth += childMeasuredWidth;
        lineHeight = Math.max(lineHeight,childMeasuredHeight);//取最高的那個
      }
      if (i == childCount - 1) {//別忘了單獨處理最后一行的最后一個元素的情況
        resH += lineHeight;
        resW = Math.max(resW, lineWidth);
      }
    }
    setMeasuredDimension((widthMeasureSpecMode==MeasureSpec.EXACTLY)?widthMeasureSpecSize:resW,
    (heightMeasureSpecMode==MeasureSpec.EXACTLY)?heightMeasureSpecSize:resH);
  }
  @Override
  protected void onLayout(boolean changed, int l, int t, int r, int b) {
    int count = getChildCount();
    int lineWidth = 0;//累加當(dāng)前行的行寬
    int lineHeight = 0;//累加當(dāng)前行的行高
    int top = 0, left = 0;//當(dāng)前控件的left坐標和top坐標
    for (int i = 0; i < count; i++) {
      View child = getChildAt(i);
      MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
      int childMeasuredWidth = child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin;
      int childMeasuredHeight = child.getMeasuredHeight() + lp.bottomMargin + lp.topMargin;
      //根據(jù)是否要換行，來計算當(dāng)前控件的top坐標和Left坐標，是否換行是需要考慮margin的
      if (childMeasuredWidth + lineWidth > getMeasuredWidth()) {
        top += lineHeight;
        left = 0;
        lineHeight = childMeasuredHeight;
        lineWidth = childMeasuredWidth;
      } else {
        lineWidth += childMeasuredWidth;
        lineHeight = Math.max(lineHeight, childMeasuredHeight);
      }
      //在已知left和top情況下計算當(dāng)前View的上下左右坐標，在真正給當(dāng)前View定位置時候需要考慮margin的
      int lc = left + lp.leftMargin;
      int tc = top + lp.topMargin;
      int rc = lc + child.getMeasuredWidth();//注意在layout的時候沒有算上margin
      int bc = tc + child.getMeasuredHeight();
      child.layout(lc, tc, rc, bc);
      left += childMeasuredWidth;//下一起點算上margin
    }
  }
  @Override
  protected LayoutParams generateLayoutParams(LayoutParams p) {
    return new MarginLayoutParams(p);
  }
  @Override
  public LayoutParams generateLayoutParams(AttributeSet attrs) {
    return new MarginLayoutParams(getContext(),attrs);
  }
  @Override
  protected LayoutParams generateDefaultLayoutParams() {
    return new MarginLayoutParams(LayoutParams.MATCH_PARENT,LayoutParams.MATCH_PARENT);
  }
}