The art of Engineering (3): do not rely on each other between functions of code robustness

My code has three files , One is responsible for producing SVG character string , One is responsible for storing the string , One responsible for drawing
The code is roughly as follows , And three separate documents

class Producer {
	void createSvgString();
	...
};
class Storage {
	void setSvgStying();
	...
};
class Draw {
	void drawSvgFigure();
	...
};

Now there is a need , I need to judge this SVG Is the pattern a closed pattern .
At first, in order to avoid modifying too much logic , I'm building SVG String time , For closed containers , At the end of the string, a Z ending . Then I just need to draw classes , Judging it can solve the problem .
There is a big problem with this method , It's easy to bury a hole for subsequent developers . The method of determining whether it is a container depends heavily on generating SVG String function , If you ignore this later , When developing other functions for containers , When something goes wrong , It needs to be in another file , Solve in an irrelevant class .
So the practice here should be to create a function , Used to judge whether it is a container , Then pass the result to the drawing class .
When I work again , Pay attention to one function and concentrate on one job , Don't intersperse the work that other functions should do .