MVC best practice question

Question

Hello!

I have a question regarding MVC design, based on the Stanford iPhone lectures.

I have 3 classes;

Polygon - this holds information like number of sides and so on. This is my Model class

Controller - this responds to things like button presses in the view and then calles methods in the model to increase and decrease the number of sides etc. This is my controller (surprise!)

View - For this question the view will be a Class representing a single view, which draws the polygon to screen.

My question is what is the best way for the View class to obtain information pertaining to the Polygon model class? Although this is trivial for this example I'm hoping that the answer wil help me when building more complicated applications. Options I have;

1) Pass the instance of the Polygon class to the View so the view has a pointer to it. Then I can just call refresh at any time and the view will know what to do. This is what i would usually do but seens to break the MVC approach as the View and Model seem to be bypassing the controller, which makes me think this may not be the best way.

2) Have a redraw(...) method in the view, which takes as its args whatever new information received. This seems clean but would not scale well i would think.

Any advice would be great. As I say usually I would do option one, but would love someone to tell me something to improve the way i think about this....

Thanks!

Answer 1

your view should be doing nothing but showing the view. so if you need to show updated information in the view, you either do it from cache (which isn't really applicable here, but I'm throwing that out because its something I've been doing lots lately with web stuff), or you re-engage the controller and get the controller to call the view again with updated data about the model.

so technically, your second option is the more correct one. the view should redraw itself by calling the controller and asking it for updated information.

Answer 2

Model is data. Controller is logic. View is display.

As such, the Model be like a data clerk. The controller should be like a administrator or team leader in an office. And the view should be like a news reporter reading a teleprompt reader.

The controller can boss everyone around, and controls the data entered and the text on the teleprompter - and outsources the actual work to the model (for data) and view (for display).

As such, to answer your question. The view should just do echo $this->view->myPentagon->someAttribute. The controller fetches the myPentagon object from the model, and assigns it to the view object. The model handles the data structure and database api. The view handles the display. The controller tells the view when to display.

Answer 3

The essential thing here is coupling. If it’s too tight, the design will suffer, you will repeat yourself and the code will be hard to maintain. If it’s too loose, it makes simple things too hard to manage. If you want to draw a simple polygon based on some model, you should have the model at hand, for it would be crazy to pull all the vertices through a controller. After all, the view is specifically written to display a polygon, so that it is perfectly natural to have a pointer to its representation.

What view does not care about is the context, the “life story” of the displayed object. It might come from the network, it may change soon, it may become twice as big when you click on it. The view does not care. If you click on it, the view can report the event back to controller and does not care about it any more. If the object changes, the controller will tell the view to update.

I don’t think there is a hard rule for designing such relationships, but the point is simple: keep loose coupling without sweating the details too much. It often helps me to thing about testing and changes. Can I isolate the model for testing? Can I use a completely different view without changing the other parts? Could I write a different user interface based on the same model? How about a different “skin”? How much would I have to rewrite?