draw method

@override void draw(Canvas canvas, Size size, bool deeper)

绘制核心方法 canvas 画布对象 size 画布尺寸 deeper 当前是否为底层绘制（true为历史记录层，false为实时绘制层）

Core drawing method canvas Canvas object size Canvas size deeper Whether this is deep layer drawing (true for history layer, false for real-time layer)

Implementation

@override
void draw(Canvas canvas, Size size, bool deeper) {
  if (startPoint == null || endPoint == null) {
    return;
  }

  canvas.drawRect(Rect.fromPoints(startPoint!, endPoint!), paint);
}