如果android自身的控件满足不了我们的需求时，这时我们应当考虑自定义view

继承view或者是ViewGroup,实现自定义。

两者区别：
①.直接继承view，在android中所有控件的基类都是view，相当于java中的object。
②.继承ViewGroup，例如线性布局，相对布局。像这种可以包裹子孩子的view的需求可以继承ViewGroup，它的父类也是View。

实现构造方法小技巧：

★ 少参使用this调用多参。最多的参数使用super调用父类构造。
这样写的好处是：无论你是创建对象还是在布局中使用，构造函数之间会相互调用，都会执行构造中的方法。

private Paint mPaint;
  private Path mPath;

  public CanvasTestView(Context context) {
      this(context, null);          // 少参调用多参
  }

  public CanvasTestView(Context context, AttributeSet attrs) {
      this(context, attrs, 0);
  }
public CanvasTestView(Context context, AttributeSet attrs, int defStyleAttr) {
      super(context, attrs, defStyleAttr);    //最后调用父类的多参构造
      init();
  }

  private void init() {
      //初始化画笔等。
      mPaint = new Paint();
      mPaint.setColor(Color.RED);
      mPaint.setStyle(Paint.Style.STROKE); // 设置画笔的样式
      mPaint.setAntiAlias(true);  // 去除锯齿，但会加大内存，不建议
      // 初始化路径
      mPath = new Path();
  }
  
    //该类用于封装坐标
  PointF startLine1 = new PointF(240, 230);
  PointF endLine1 = new PointF(450, 230);
  PointF circlePoint1 = new PointF(350, 350);
  PointF startLine2 = new PointF(345, 230);
  PointF endLine2 = new PointF(345, 450);
  PointF endLine3 = new PointF(315, 420);
  PointF circlePoint2 = new PointF(280, 310);
  PointF circlePoint3 = new PointF(410, 310);
  PointF arcPoint = new PointF(280, 490);
  PointF arcPoint2 = new PointF(410, 490);

  @Override
  protected void onDraw(Canvas canvas) {
      super.onDraw(canvas);
      canvas.drawCircle(circlePoint1.x, circlePoint1.y, 200, mPaint);
      canvas.drawLine(startLine1.x, startLine1.y, endLine1.x, endLine1.y, mPaint);
      canvas.drawLine(startLine2.x, startLine2.y, endLine2.x, endLine2.y, mPaint);
      canvas.drawLine(endLine2.x, endLine2.y, endLine3.x, endLine3.y, mPaint);
      canvas.drawCircle(circlePoint2.x, circlePoint2.y, 50, mPaint);
      canvas.drawCircle(circlePoint3.x, circlePoint3.y, 50, mPaint);

      canvas.drawCircle(circlePoint2.x, circlePoint2.y, 10, mPaint);
      canvas.drawCircle(circlePoint3.x, circlePoint3.y, 10, mPaint);

      mPath.moveTo(arcPoint.x, arcPoint.y);
      mPath.quadTo(arcPoint2.x, arcPoint2.y,445,440);
      canvas.drawPath(mPath,mPaint);
  }

效果图