MVC with Python

Georg Altmann george at george-net.de
Tue Sep 16 13:26:59 CEST 2008


Marco Bizzarri schrieb:
> On Mon, Sep 15, 2008 at 9:37 PM, Georg Altmann <george at george-net.de> wrote:
>> Hello,
>>
>> I need some advice on how to implement model-view-controller. I am trying to
>> develop a GUI application with PyQt, yet the problem rather applies to mvc
>> in general, not just GUI applications.
>>
>> Let's say the data is a list of objects with a common base class. The views
>> are either a graphical representation of the objects or some form of textual
>> input. The views shall change the model by using command objects (for undo,
>> e.g. QUndoCommand).
>>
>> My current approach is to implement the model as a class with a list-like
>> interface, with methods insert(), remove(), __getitem__(), __setitem__(),...
>> and a signal to notify the views. The objects in the list have methods to
>> change their state as well.
>>
>> My problem is, how do the commands interact with the model?
>> Let's say I have a command that modifies an object o in the list.
>>
>> 1) If list[key_to_o] returns a reference to the object, the command can
>> modify the object by using this reference, i.e. list[key_to_o].setX(). So
>> there is no way for the list to know when the object changed - how can it
>> emit a singal then?
>>
>> 2) If list[key_to_o] returns a deep copy of the object,
>> copy_of_o = list[key_to_o], the command mofifies the copy and the updates
>> the list: list[key_to_o] = copy_of_o. Now the list can emit a signal in
>> __setitem__().
>> While this may work, it seems awkward to copy around objects just to perform
>> a possibly simple operation on them. Additionally it might not be feasible
>> once objects get complex.
>>
>> 3) The interface of the classes of the objects could be reflected in the
>>  list class, i.e. list.set_x_on_obj(key_to_obj,x). This would probably make
>> the list class interface very bloated.
>>
>> Of course the problem is not really limited to Python, my apologies if I'm
>> totally off-topic here.
>>
>> Regards
>> Georg
>>
> 
> Well, I think one of the assumptions of the MVC is that the view can
> be notified about the changes in the model. According to your
> interface, the View can just be notified on operations about the
> *whole* model, like adding or removing elements from it.
> 
> But the elements of your list-like class should be a part of your
> model, and therefore should be able to notify the Views by
> themselfves.

Ok, consider this: say the model needs to compute some sort of value by 
iterating over all objects in the list (for example some sort of hash). 
Some kind of view then displays this value.
I could implement this by connecting a signal in each object to a slot 
in the list class which in turn emits a signal which is connected to the 
view.

But this implies all the objects in the list have to be instances of 
QObject. I wonder if this is isn't a weird design, because for example 
all value type classes in Qt don't derive from QObject, e.g. QString. 
My list objects are not that simple (in terms of the data structure). So 
is it ok to make them QObjects?

At least this seems to be better than making a copy of the object each 
time it is accessed.

Regards
Georg




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