MVC and the communication between them. What did i miss?

MVC and the communication between them. What did i miss?

Hey all, trying to learn objective-c. It was going quite well, until I realized I was missing a link.

When building in the spirit of MVC, I understand it as you Model and View should never talk to eachother.

I have this problem I'm messing around with now.

  • A model containing a polygon class. Contains an polygon object with attributes like numberOfSides.

  • A controller, reading some UI elements and modifying the model. For example numberOfSides.

  • A view, containing a view drawing polygons.

Now, this is where the trouble arose. The view needs information about the object from the model. How do I get that? I tried some IBOutlet, but never got it to work.

I ended up creating a property for the view which could contain numberOfSides. Then I would a controller method which would call a view method back and set the numberOfSides.

I hope some of you can clarify this a bit for me and tell me how you should go ahead with this.

Thanks a bunch and goodnight! :)

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MVC in Cocoa is slightly different than traditional MVC, so I'll answer with the Cocoa way..
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You've got a controller object, which has a pointer to your Polygon object.

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Your controller should also have a pointer to your View (via an IBOutlet, most likely).

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As the polygon changes (whether numberOfSides or whatever), the controller needs to be aware of that and relay the information on to the view.

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Likewise, if the view is allowed to change the numberOfSides (say you have a slider to increase the number of sides), then the controller needs to be aware of that change and pass it to the Polygon object..
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Basically, the purpose of the controller is to pass information back and forth from the view to the model and make sure that they stay synchronized..
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Information passing in a proper MVC architecture should be done via a mediator such as an event manager.. The model would have an internal representation of everything that is needed for it to stay independent.. Now when a change occurs the model sends an event via the event manager to the view..


You can also give your view a read-only polygon instance variable (just a pointer to your polygon instance in the controller; read-only because you don't want your view to be able to change the model).

This is not necessarily a violation of MVC principles.. Or if you want a very loose coupling between model and view, try key-value observing: your model dispatches a notification saying "hey my number of sides changed to 8" and your view listens to these notifications and redraws itself as needed.. Another option is to give your view a delegate (= your controller) and then call something like [delegate numberOfSidesToDraw] from within the view class.. All three methods have pros and cons..


I understand.

My controller has an IBOutlet to the view and the polygon.. I forgot to mention that I'm using CG and drawrect to draw the polygon.. What I would like to do, and what I should do if I understand it correctly, is from the controller pass the numberOfSides to the view, asking it to draw with this number of sides.

Like:. [view drawPolygon:myPolygon.numberOfSides];. But what I have to do, because drawrect gets called automatically, is make some instance variable of the view available to contain the numberOfSides, which the controller would set.

And then call [view setNeedsDisplay];. Does this make sense? Since the view cannot access the myPolygon directly.

How would you approah this?.

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