Android自定义View原理(实战)

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

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  int width = 0;//最终确定的宽度
  int height = 0;//最终确定的高度
  //1、首先测量自身
  super.onMeasure(widthMeasureSpec, heightMeasureSpec);
  //2、为每个子view计算测量的限制信息Mode/Size
  int widthMeasureSpecMode = MeasureSpec.getMode(widthMeasureSpec);
  int widthMeasureSpecSize = MeasureSpec.getSize(widthMeasureSpec);
  int heightMeasureSpecMode = MeasureSpec.getMode(heightMeasureSpec);
  int heightMeasureSpecSize = MeasureSpec.getSize(heightMeasureSpec);
  //3、测量子View；把上一步确定的限制信息，传递给每一个子View，然后子View开始measure自己的尺寸
  int childCount = getChildCount();
  for(int i=0;i<childCount;i  ){
    View child = getChildAt(i);
    measureChild(child,widthMeasureSpec,heightMeasureSpec);//这个方法就是确定子view的测量大小
  }
  //4、根据子View的测量尺寸以及自身的SpecMode计算自己的尺寸
  switch (widthMeasureSpecMode) {
    case MeasureSpec.EXACTLY://如果是确定值，则使用确定值
      width = widthMeasureSpecSize;
    case MeasureSpec.AT_MOST://如果是根据内容定的大小
    case MeasureSpec.UNSPECIFIED://一般可以不用单独处理
      for(int i=0;i<childCount;i  ){
        View child = getChildAt(i);
        int childWidth = child.getMeasuredWidth();//这一步只有当measureChild方法执行完之后才能拿到
        width = Math.max(childWidth,width);
      }
    default:break;
  }
  switch (heightMeasureSpecMode) {
    case MeasureSpec.EXACTLY://如果是确定值，则使用确定值
      height = heightMeasureSpecSize;
    case MeasureSpec.AT_MOST://如果是根据内容定的大小
    case MeasureSpec.UNSPECIFIED:
      for(int i=0;i<childCount;i  ){
        View child = getChildAt(i);
        int childHeight = child.getMeasuredHeight();//这一步只有当measureChild方法执行完之后才能拿到
        height =childHeight;
      }
    default:break;
  }
  //保存自身测量后的宽和高
  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;//记录每一行的宽度，最终的宽度是由所有行中的最大值
    int lineHeight = 0;//记录每一行的高度，取决于每一行中最高的那个组件
    int resH = 0;//最终的高度
    int resW = 0;//最终的宽度
    //1、首先测量自身
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    //2、为每个子view计算测量的限制信息Mode/Size
    int widthMeasureSpecMode = MeasureSpec.getMode(widthMeasureSpec);
    int widthMeasureSpecSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightMeasureSpecMode = MeasureSpec.getMode(heightMeasureSpec);
    int heightMeasureSpecSize = MeasureSpec.getSize(heightMeasureSpec);
    //3、测量每个子view的宽度和高度
    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) {//当前行的的宽度已经加上当前view的宽度已经大于建议值宽度了
        //需要换行
        resW = Math.max(resW, lineWidth);
        resH  = lineHeight;
        //重新赋值
        lineWidth = childMeasuredWidth;
        lineHeight = childMeasuredHeight;
      } else {//不需要换行则累加
        lineWidth  = childMeasuredWidth;
        lineHeight = Math.max(lineHeight,childMeasuredHeight);//取最高的那个
      }
      if (i == childCount - 1) {//别忘了单独处理最后一行的最后一个元素的情况
        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;//累加当前行的行宽
    int lineHeight = 0;//累加当前行的行高
    int top = 0, left = 0;//当前控件的left坐标和top坐标
    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;
      //根据是否要换行，来计算当前控件的top坐标和Left坐标，是否换行是需要考虑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情况下计算当前View的上下左右坐标，在真正给当前View定位置时候需要考虑margin的
      int lc = left   lp.leftMargin;
      int tc = top   lp.topMargin;
      int rc = lc   child.getMeasuredWidth();//注意在layout的时候没有算上margin
      int bc = tc   child.getMeasuredHeight();
      child.layout(lc, tc, rc, bc);
      left  = childMeasuredWidth;//下一起点算上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);
  }
}