Mixing logic and graphics in a class with MVC pattern

Question

I am currently developing a charting application (for the iPhone, although that is largely irrelevant) using their MVC pattern.

One aspect of the application is that you can overlay a number of statistics on the charts. I am a little unsure how I am going to structure these classes.

For each statistic there will be two aspects.

1. The calculation. The function which will take the data and calculate the relevant statistical figures.

2. The display. The statistics then need to be drawn over the top of the graph.

Obviously I want the code to comply with the MVC pattern as closely as possible, but I am planning to develop possibly hundreds of these statistics.

I could create three classes. One for the graphics, one for the logic and a factory class to tie the two together. This would then fit with the pattern, but this seems to be a huge extra overhead in terms of the number of classes in the system and additional complexity which I dont feel is necessary.

So, I am very tempted to create a single class for each statistic. But that would mean each class would have logic and graphics mixed in together, which is heavily frowned upon.

Are there any other suggestions as to how I can lay these out in a structured reuseable way without adding uneccessary complexity?

EDIT

Thanks for the answers. Most useful, but has raised more questions!

MVC does fit the rest of the application perfectly. Also as its for the iPhone, I seem to be pushed along this path anyway. This is the only reason I am considering MVC for these statistics.

However, for these statistics, the user will not interact with them, they are purely for display. The statistics are painting various lines and symbols directly onto the view canvas. Each statistic paints their information in its own way. There is very little that can be shared between each one, also each piece of data can only be useful represented in one way. I can think of no other useful way that I would want to represent the information.

So it seems MVC is out for these, but I am unsure now what pattern would fit other than my newly invented "Mix logic and graphics" pattern which just feels wrong due to the Single Responsibility Principle (thanks for that link).

Answer 1

It depends on what you call complex. Most people consider methods or classes that are responsible for multiple things complex and classes or methods that are responsible for one thing simple. This is also known as the SRP When you use MVC, the view contains the display logic, the model contains the business logic (the calculation in your case), and the controller ties them together. You can use different ways to implement MVC. Complex applications need to seperate domain model and map to a viewmodel, but most simple applications can use one model.

If you define complexity on a different way, MVC might not be what you like, and you should try a different approach.

Answer 2

First of all, will the user interact with the statistics? If not, then you don't need MVC. (The Controller in MVC deals with user interaction).

You want to keep the number of classes to a minimum, which is good. Let's consider both the calculation and display separately.

How will you be displaying the statistics? Will they typically be text labels, or will there be other graph elements (error bars or things like that)? Try to figure out the different ways you will want to display your statistics.

How many different calculations will you have? Does each calculation map directly to a single graph element, or might it be drawn in a number of different ways? Try to figure out how the calculations relate to the graph elements.

As a concrete example, suppose you have a set of data points that you have plotted. You want to display the mean, the median, and the mode. You could display each of these as separate horizontal lines that cut through the chart at the appropriate Y value. The calculations are all independent, but the display logic can be shared. Or, perhaps you want to display the mean as both a line and as a text label. Here, there is only one calculation, but two different display methods.

MVC design is about separating the underlying data from its presentation. By doing this, you can re-use bits of presentation logic for many different pieces of data. Also, you can display a single piece of data in multiple ways, and they will all stay in sync.