public class ClockView extends View {

  }

  /**
   * 当布局为wrap_content时设置默认长宽
   * @param widthMeasureSpec
   * @param heightMeasureSpec
   */
  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(measure(widthMeasureSpec), measure(heightMeasureSpec));
  }

  private int measure(int origin){
    int result = DEFAULT_MIN_WIDTH;
    int specMode = MeasureSpec.getMode(origin);
    int specSize = MeasureSpec.getSize(origin);
    if(specMode == MeasureSpec.EXACTLY){
      result = specSize;
    }else{
      if(specMode == MeasureSpec.AT_MOST){
        result = Math.min(result, specSize);
      }
    }
    return result;
  }

//画外圆
float borderWidth = DEFAULT_BORDER_WIDTH;
Paint paintCircle = new Paint();
paintCircle.setStyle(Paint.Style.STROKE);
paintCircle.setAntiAlias(true);
paintCircle.setStrokeWidth(borderWidth);
canvas.drawCircle(getWidth() / 2, getHeight() / 2, Math.min(getHeight() / 2, getWidth() / 2) - borderWidth / 2, paintCircle);

//画刻度线
float degreeLength = 0f;
Paint paintDegree = new Paint();
paintDegree.setAntiAlias(true);
for(int i=0;i<60;i++){
  if(i % 5 == 0){
    paintDegree.setStrokeWidth(6);
    degreeLength = DEFAULT_LONG_DEGREE_LENGTH;
  }else{
    paintDegree.setStrokeWidth(3);
    degreeLength = DEFAULT_SHORT_DEGREE_LENGTH;
  }
  canvas.drawLine(getWidth()/2, Math.abs(getWidth()/2 - getHeight()/2), getWidth()/2, Math.abs(getWidth()/2 - getHeight()/2) + degreeLength, paintDegree);
  canvas.rotate(360/60, getWidth()/2, getHeight()/2);
}

//刻度数字
int degressNumberSize = 30;
canvas.translate(getWidth() / 2, getHeight() / 2);
Paint paintDegreeNumber = new Paint();
paintDegreeNumber.setTextAlign(Paint.Align.CENTER);
paintDegreeNumber.setTextSize(degressNumberSize);
paintDegreeNumber.setFakeBoldText(true);
for(int i=0;i<12;i++){
  float[] temp = calculatePoint((i+1)*30, r - DEFAULT_LONG_DEGREE_LENGTH - degressNumberSize/2 - 15);
  canvas.drawText((i+1)+"", temp[2], temp[3] + degressNumberSize/2-6, paintDegreeNumber);
}

/**
 * 根据角度和长度计算线段的起点和终点的坐标
 * @param angle
 * @param length
 * @return
 */
private float[] calculatePoint(float angle, float length){
  float[] points = new float[4];
  if(angle <= 90f){
    points[0] = -(float) Math.sin(angle*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[1] = (float) Math.cos(angle*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[2] = (float) Math.sin(angle*Math.PI/180) * length;
    points[3] = -(float) Math.cos(angle*Math.PI/180) * length;
  }else if(angle <= 180f){
    points[0] = -(float) Math.cos((angle-90)*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[1] = -(float) Math.sin((angle-90)*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[2] = (float) Math.cos((angle-90)*Math.PI/180) * length;
    points[3] = (float) Math.sin((angle-90)*Math.PI/180) * length;
  }else if(angle <= 270f){
    points[0] = (float) Math.sin((angle-180)*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[1] = -(float) Math.cos((angle-180)*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[2] = -(float) Math.sin((angle-180)*Math.PI/180) * length;
    points[3] = (float) Math.cos((angle-180)*Math.PI/180) * length;
  }else if(angle <= 360f){
    points[0] = (float) Math.cos((angle-270)*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[1] = (float) Math.sin((angle-270)*Math.PI/180) * DEFAULT_POINT_BACK_LENGTH;
    points[2] = -(float) Math.cos((angle-270)*Math.PI/180) * length;
    points[3] = -(float) Math.sin((angle-270)*Math.PI/180) * length;
  }
  return points;
}

//画指针
Paint paintHour = new Paint();
paintHour.setAntiAlias(true);
paintHour.setStrokeWidth(15);
Paint paintMinute = new Paint();
paintMinute.setAntiAlias(true);
paintMinute.setStrokeWidth(10);
Paint paintSecond = new Paint();
paintSecond.setAntiAlias(true);
paintSecond.setStrokeWidth(5);
Calendar now = Calendar.getInstance();
float[] hourPoints = calculatePoint(now.get(Calendar.HOUR_OF_DAY)/12f*360, hourPointerLength);
canvas.drawLine(hourPoints[0], hourPoints[1], hourPoints[2], hourPoints[3], paintHour);
float[] minutePoints = calculatePoint(now.get(Calendar.MINUTE)/60f*360, minutePointerLength);
canvas.drawLine(minutePoints[0], minutePoints[1], minutePoints[2], minutePoints[3], paintMinute);
float[] secondPoints = calculatePoint(now.get(Calendar.SECOND)/60f*360, secondPointerLength);
canvas.drawLine(secondPoints[0], secondPoints[1], secondPoints[2], secondPoints[3], paintSecond);

//画圆心
Paint paintCenter = new Paint();
paintCenter.setColor(Color.WHITE);
canvas.drawCircle(0, 0, 2, paintCenter);
最后只要启动一个无限循环的线程，每隔1秒针重绘一下View就能让指针动起来了
private Thread timeThread = new Thread() {
  @Override
  public void run() {
    try {
      while(true){
        updateHandler.sendEmptyMessage(0);
        Thread.sleep(1000);
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
};

private Handler updateHandler = new Handler() {
  @Override
  public void handleMessage(Message msg) {
    invalidate();
  }
};