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POUsing MVP, how to create a view from another view, linked with the same model object
 

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  1. POUsing MVP, how to create a view from another view, linked with the same model object
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    <p><strong>Background</strong></p> <p>We use the Model-View-Presenter design pattern along with the abstract factory pattern and the "signal/slot" pattern in our application, to fullfill 2 main requirements </p> <ul> <li>Enhance testability (very lightweight GUI, every action can be simulated in unit tests)</li> <li>Make the "view" totally independant from the rest, so we can change the actual view implementation, without changing anything else</li> </ul> <p>In order to do so our code is divided in 4 layers :</p> <ul> <li>Core : which holds the model </li> <li>Presenter : which manages interactions between the view interfaces (see bellow) and the core</li> <li>View Interfaces : they define the signals and slots for a View, but not the implementation</li> <li>Views : the actual implementation of the views</li> </ul> <p>When the presenter creates or deals with views, it uses an abstract factory and only knows about the view interfaces. </p> <p>It does the signal/slot binding between views interfaces. It doesn't care about the actual implementation. In the "views" layer, we have a concrete factory which deals with implementations.</p> <p>The signal/slot mechanism is implemented using a custom framework built upon boost::function.</p> <p>Really, what we have is something like that : <a href="http://martinfowler.com/eaaDev/PassiveScreen.html" rel="nofollow">http://martinfowler.com/eaaDev/PassiveScreen.html</a></p> <p>Everything works fine. </p> <p><strong>The problem</strong></p> <p>However, there's a problem I don't know how to solve. </p> <p>Let's take for example a very simple drag and drop example.</p> <p>I have two ContainersViews (ContainerView1, ContainerView2). ContainerView1 has an ItemView1. I drag the ItemView1 from ContainerView1 to ContainerView2.</p> <p>ContainerView2 must create an ItemView2, of a different type, but which "points" to the same model object as ItemView1.</p> <p>So the ContainerView2 gets a callback called for the drop action with ItemView1 as a parameter. It calls ContainerPresenterB passing it ItemViewB</p> <p>In this case we are only dealing with views. In MVP-PV, views aren't supposed to know anything about the presenter nor the model, right ? </p> <p>How can I create the ItemView2 from the ItemView1, not knowing which model object is ItemView1 representing ?</p> <p>I thought about adding an "itemId" to every view, this id being the id of the core object the view represents.</p> <p>So in pseudo code, ContainerPresenter2 would do something like </p> <pre><code>itemView2=abstractWidgetFactory.createItemView2(); this.add(itemView2,itemView1.getCoreObjectId()) </code></pre> <p>I don't get too much into details. That just work. The problem I have here is that those itemIds are just like pointers. And pointers can be dangling. Imagine that by mistake, I delete itemView1, and this deletes coreObject1. The itemView2 will have a coreObjectId which represents an invalid coreObject.</p> <p>Isn't there a more elegant and "bulletproof" solution ? </p> <p>Even though I never did ObjectiveC or macOSX programming, I couldn't help but notice that our framework is very similar to Cocoa framework. How do they deal with this kind of problem ? Couldn't find more in-depth information about that on google. If someone could shed some light on this.</p> <p>I hope this question isn't too confusing ... </p>
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    1. COU sure you don't mean MVC -- Model View Controller?
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      1. USCheers and hth. - Alf
    2. CONope, in MVC the View knows about the model which is something we definitely want to avoid
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    3. COConsidering that the view presents the model, how do you avoid the view knowing about the model?
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      1. POUsing MVP, how to create a view from another view, linked with the same model object
      1. USCheers and hth. - Alf
 

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