Fwd: MVC question
Fwd: MVC question
- Subject: Fwd: MVC question
- From: "Michael A. Crawford" <email@hidden>
- Date: Sat, 29 Nov 2008 15:10:47 -0800
I've included Klaus' response for the benefit of anyone who may be
interested:
On 29.11.2008, at 08:38, Michael A. Crawford wrote:
When implementing the MVC pattern, is it inappropriate or bad-form
for views and layers to have direct read-only access to the model?
The Cocoa Fundamentals Guide discusses this topic, see Reference
Library > Guides > Cocoa > Design Guidelines > Cocoa Fundamentals
Guide > Cocoa Design Patterns > The Model-View-Controller Design
Pattern > MVC as a Compound Design Pattern.
In figure 4-4 you can see, that in the traditional version of MVC
models notify views and views get model states directly. In figure 4-5
you can see, that in the Cocoa version models and views communicate
through mediating controllers only.
So: "is it inappropriate or bad-form ... ?" – generally no and Cocoa-
wise yes, I think.
Klaus
Begin forwarded message:
From: "Michael A. Crawford" <email@hidden>
Date: November 28, 2008 11:38:12 PM PST
To: email@hidden
Subject: MVC question
When implementing the MVC pattern, is it inappropriate or bad-form
for views and layers to have direct read-only access to the model?
Currently I'm using KVO to have the different views and layers get
notification of state changes in the model. Since the drawing of
the views and layers is asynchronous to the processing of the KVO
notifications, I'm cacheing local copies of the model's state in
order to draw specific content based on said state. It seems that
it would be more efficient to only handle the KVO notification as a
trigger to redraw the view or layer using [layer setNeedsDisplay]
and allow the individual layers to query the model directly.
When the [layer drawInContext] method executes, if I have const or
read-only access to the model, I don't need to have cached state
information, instead I simply read the model state in question at
the point of drawing and draw the appropriate content.
What do the more experienced MVC/NSView/CALayer coder's think of
this approach?
More detail . . .
In order to implement it, I need to change my layer hosting view so
that instead of the view being the only one with a pointer to the
model, I provide each layer and associated sub-layers with their own
pointer to the model so that, upon observation of model state
changes, the individual layers can query the model and draw.
Currently, in the view's awakeFromNib method, I add all of my
observers, passing the appropriate keys, contexts, and layers, and
then allow the layers to handle the observation call-backs, caching
the data associated with the keys being observed and calling their
own drawInContext methods via [self setNeedsDisplay]. This requires
duplication of model data.
Is it worth duplicating this data in order to reduce coupling
between the view and the model? I suspect not since the view is
always going to be dependent on the model and any changes to the
model will require changes to the view.
-Michael
----------------------
There are two ways of constructing a software design. One way is to
make it so simple that there are obviously no deficiencies.
And the other way is to make it so complicated that there are no
obvious deficiencies.
-- C.A.R. Hoare
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-Michael
----------------------
The difference between genius and stupidity...
...is that genius has its limits."
-- Albert Einstein
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