<?xml version="1.0" encoding="utf-8"?>
<resources>
 <declare-styleable name="MyTextView">
  <attr name="titleText" format="string"/>
  <attr name="titleTextColor" format="color"/>
  <attr name="titleTextSize" format="dimension"/>
 </declare-styleable>
</resources>

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:custom="http://schemas.android.com/apk/res/com.qianmo.VerificationCode"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

 <com.qianmo.VerificationCode.view.MyTextView
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:padding="100dp"
  custom:titleText="3712"
  custom:titleTextColor="#ff0000"
  android:layout_centerInParent="true"
  custom:titleTextSize="40sp"/>

</RelativeLayout>

package com.qianmo.VerificationCode.view;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.util.Log;
import android.util.TypedValue;
import android.view.View;

import com.qianmo.VerificationCode.R;

import java.util.HashSet;
import java.util.Random;
import java.util.Set;

/**
 * Created by wangjitao on 2016/10/13 0013.
 * 用于实现获取随机码
 */
public class MyTextView extends View {
 /**
  * 由于是自定义的View，首先我们要确定那些属性是用户可以自己定义的
  * 1，View里面显示的字
  * 2，显示字的大小
  * 3，显示字的颜色
  */

 private String mTitleText;
 private int mTitleTextColor;
 private int mTitleTextSize;

 /**
  * 画笔
  */
 private Paint mPaint;
 /**
  * view的矩形背景
  */
 private Rect mBound;

 public MyTextView(Context context) {
  this(context, null);
 }

 public MyTextView(Context context, AttributeSet attrs) {
  this(context, attrs, 0);
 }

 /**
  * 获得自定义的属性
  *
  * @param context
  * @param attrs
  * @param defStyleAttr
  */
 public MyTextView(Context context, AttributeSet attrs, int defStyleAttr) {
  super(context, attrs, defStyleAttr);

  /**
   * 获得我们自定义的一些属性
   */
  TypedArray a = context.getTheme().obtainStyledAttributes(attrs, R.styleable.MyTextView, defStyleAttr, 0);
  mTitleText = randomText(); //初始化显示的数字

  for (int i = 0; i < a.getIndexCount(); i++) {
   int attr = a.getIndex(i);
   switch (attr) {
    case R.styleable.MyTextView_titleText:
     mTitleText = a.getString(attr);
     break;
    case R.styleable.MyTextView_titleTextColor:
     mTitleTextColor = a.getColor(attr, Color.BLACK);
     break;
    case R.styleable.MyTextView_titleTextSize:
     //设置默认大小为16
     mTitleTextSize = a.getDimensionPixelSize(attr, (int) TypedValue.applyDimension(
       TypedValue.COMPLEX_UNIT_SP, 16, getResources().getDisplayMetrics()));
     break;
   }
  }
  //将TypedArray对象回收
  a.recycle();

  /**
   * 初始化画笔
   */
  mPaint = new Paint();
  mPaint.setAntiAlias(true);
  mPaint.setTextSize(mTitleTextSize);
  mPaint.setColor(mTitleTextColor);
  mBound = new Rect();
  mPaint.getTextBounds(mTitleText, 0, mTitleText.length(), mBound);

  /**
   * 模仿点击换验证码
   */
  this.setOnClickListener(new OnClickListener() {
   @Override
   public void onClick(View view) {
    mTitleText = randomText();
    postInvalidate();
   }
  });
 }

 /**
  * 获取四位随机数验证码
  *
  * @return
  */
 private String randomText() {
  Random random = new Random();
  Set<Integer> set = new HashSet<Integer>();
  while (set.size() < 4) {
   int randomInt = random.nextInt(10);
   set.add(randomInt);
  }
  StringBuffer sb = new StringBuffer();
  for (Integer i : set) {
   sb.append("" + i);
  }

  return sb.toString();
 }

 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

  /**
   * 处理当宽高都是wrap_content的情况
   */
  int widthMode = MeasureSpec.getMode(widthMeasureSpec);
  int widthSize = MeasureSpec.getSize(widthMeasureSpec);
  int heightMode = MeasureSpec.getMode(heightMeasureSpec);
  int heightSize = MeasureSpec.getSize(heightMeasureSpec);

  int width = 0;
  int height = 0;

  if (widthMode == MeasureSpec.EXACTLY) {
   width = widthSize;
  } else {
   mPaint.setTextSize(mTitleTextSize);
   mPaint.getTextBounds(mTitleText, 0, mTitleText.length(), mBound);
   float textWidth = mBound.width();
   int desired = (int) (getPaddingLeft() + textWidth + getPaddingRight());
   width = desired;
  }
  if (heightMode == MeasureSpec.EXACTLY) {
   height = heightSize;
  } else {
   mPaint.setTextSize(mTitleTextSize);
   mPaint.getTextBounds(mTitleText, 0, mTitleText.length(), mBound);
   float textWidth = mBound.height();
   int desired = (int) (getPaddingTop() + textWidth + getPaddingBottom());
   height = desired;
  }
  setMeasuredDimension(width, height);
 }

 @Override
 protected void onDraw(Canvas canvas) {
  /**
   * 绘制文字和矩形
   */
  mPaint.setColor(Color.YELLOW);
  canvas.drawRect(0, 0, getMeasuredWidth(), getMeasuredHeight(), mPaint);

  mPaint.setColor(mTitleTextColor);
  canvas.drawText(mTitleText, getWidth() / 2 - mBound.width() / 2, getHeight() / 2 + mBound.height() / 2, mPaint);
 }
}