测量分两步：
第一步，获取测量模式和尺寸

// 获取测量模式和尺寸
val mode = MeasureSpec.getMode(widthMeasureSpec)
val size = MeasureSpec.getSize(widthMeasureSpec)

第二步，根据测量模式分别测量尺寸

// 根据不同测量模式分别测量
when (mode) {
    MeasureSpec.EXACTLY -> {
        result = size
    }

    MeasureSpec.AT_MOST -> {
        result = 200
        result = Math.min(result, size)
    }

    MeasureSpec.UNSPECIFIED -> {
        result = 200
    }
}

测量模式：

测量模式

完整代码：

package com.gorhaf.lab

import android.content.Context
import android.util.AttributeSet
import android.util.Log
import android.view.View

class CustomView @JvmOverloads constructor(
    context: Context,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {

    private val TAG = "CustomView"

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        setMeasuredDimension(
            measureWidth(widthMeasureSpec),
            measureHeight(heightMeasureSpec)
        )
    }

    private fun measureWidth(widthMeasureSpec: Int): Int {
        // 默认大小
        var result = 0
        // 获取测量模式和尺寸
        val mode = MeasureSpec.getMode(widthMeasureSpec)
        val size = MeasureSpec.getSize(widthMeasureSpec)
        // 根据不同测量模式分别测量
        when (mode) {
            MeasureSpec.EXACTLY -> {
                result = size
            }

            MeasureSpec.AT_MOST -> {
                result = 200
                result = Math.min(result, size)
            }

            MeasureSpec.UNSPECIFIED -> {
                result = 200
            }
        }
        return result
    }

    private fun measureHeight(heightMeasureSpec: Int): Int {
        // 默认大小
        var result = 0
        // 获取测量模式和尺寸
        val mode = MeasureSpec.getMode(heightMeasureSpec)
        val size = MeasureSpec.getSize(heightMeasureSpec)
        // 根据不同测量模式来测量
        when (mode) {
            MeasureSpec.EXACTLY -> {
                result = size
            }

            MeasureSpec.AT_MOST -> {
                result = 200
                result = Math.min(result, size)
            }

            MeasureSpec.UNSPECIFIED -> {
                result = 200
            }
        }
        // 返回大小
        return result
    }
}