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Re: Hairy generics question

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Daniel Pitts wrote:
> I've just read the original Taligent, Inc. PDF on what MVP is, and it looks
> like its little more than a clarification of MVC, with some new concepts
> pulled out. Things that were inferred in the "controller" are now their own
> concepts.

Other references make a similar point, or conversely take great pains to 
distinguish MVP (most valuable player) from MVC. Such care would be 
unnecessary were the concepts actually so different.

"MVC" is a pretty loose term, which accounts for some of the controversy. In 
large terms it means no more than "model-view-controller" and the principle of 
separation of concerns imply. Really, the taxonomy dispute is by the wayside. 
Personally, I recognize "MVC" as the species, like the first "lupus" in "canis 
lupus lupus", and subspecies, like the second "lupus". Then "MVP" would be 
another subspecies, like the "familiaris" in "canis lupus familiaris".

   Life      -  Domain  - Kingdom  -    Phylum   -
Engineering - Computer - Software - Programming -

   Class      -  Order   - Family  -  Genus  - Species - Sub
Architecture - Patterns - Modular - Acyclic - MVC - MVC
Architecture - Patterns - Modular - Acyclic - MVC - MVP

So MVC is the wolf, and MVP the dog.

That's just an offhand metaphor, a "Lewnnaeus" classification, if you will. 
Perhaps it will help some of you.

> In that case, my original advice still stands. View shouldn't know about the
> Presenter, only about the Model, and event listeners. The Model shouldn't know
> about the View or Presenter, only about observers, and the Presenter is
> basically knows about everything.
>
> As such, you can't easily have a Generic Presenter. Also, as such, you needn't
> have the view have any information about the "type" of the Presenter. The View
> also only needs the interface type of the Model, not the specific type. The
> Model needs only to know only interface types as well.
>
> This allows you to completely remove the circular type dependencies, which is
> one of the main benefits of MVC and MVP architectures in the first place.

The pattern itself is acyclic, which accounts for how it accomplishes that:
"knows about":
  P |- V |- M
    |- M

This translates almost directly into type dependencies.

I don't usually make the Controller (or Presenter) generic. It works just fine 
with non-generic 'Screen' (a View artifact) and 'Handler' types sorta like 
this (pseudocode):

  public class Controller
  {
    static enum Outcome {SUCCESS, FAILURE, ERROR};
    private final Map<Screen, Handler> handlers = loadHandlerMap();
    private final Map<Screen, Map<Outcome, Screen>> navigations
        = loadNavigations();

    public void process(Request request, Response response)
    {
      Screen screen = request.getScreen();
      Handler handler = handlers.get(screen);
      Outcome outcome = handler.handle(request, response);
      Screen next = navigations.get(screen).get(outcome);
      forward(next, request, response);
    }
// etc.
  }

Of course, this is just one way to do the pattern. It's what I use for Web 
apps if I'm hand-rolling, and not dissimilar to how Struts does things; 
however, I've been doing this since before Struts was available.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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Hairy generics question sclaflin <steve@steveclaflin.com> - 2012-02-21 06:30 -0800
  Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-02-21 09:24 -0800
  Re: Hairy generics question Roedy Green <see_website@mindprod.com.invalid> - 2012-02-21 11:05 -0800
    Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-02-21 12:09 -0800
  Re: Hairy generics question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-21 18:04 -0800
    Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-02-21 18:31 -0800
    Re: Hairy generics question Steven Simpson <ss@domain.invalid> - 2012-02-22 08:31 +0000
  Re: Hairy generics question sclaflin@webucator.com - 2012-02-24 11:12 -0800
    Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-02-24 12:03 -0800
      Re: Hairy generics question sclaflin@webucator.com - 2012-02-24 14:07 -0800
        Re: Hairy generics question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-24 15:36 -0800
          Re: Hairy generics question sclaflin@webucator.com - 2012-02-25 10:27 -0800
          Re: Hairy generics question Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2012-02-26 13:13 -0400
            What is Model View Presenter. (Was: Hairy generics question) Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-26 10:22 -0800
              Re: What is Model View Presenter. (Was: Hairy generics question) Lew <noone@lewscanon.com> - 2012-02-26 13:08 -0800
                Re: What is Model View Presenter. (Was: Hairy generics question) Arne Vajhøj <arne@vajhoej.dk> - 2012-02-26 18:17 -0500
                Re: What is Model View Presenter. (Was: Hairy generics question) sclaflin@webucator.com - 2012-02-28 06:22 -0800
            Re: Hairy generics question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-28 09:46 -0800
              Re: Hairy generics question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-28 09:49 -0800
              Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-02-28 12:45 -0800
              Re: Hairy generics question Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2012-02-29 19:37 -0400
                Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-02-29 22:32 -0800
                Re: Hairy generics question Arne Vajhøj <arne@vajhoej.dk> - 2012-03-09 21:33 -0500
                Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-03-10 07:38 -0800
                Re: Hairy generics question Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2012-03-10 12:45 -0400
                Re: Hairy generics question Arne Vajhøj <arne@vajhoej.dk> - 2012-03-10 12:03 -0500
                Re: Hairy generics question Lew <noone@lewscanon.com> - 2012-03-10 09:47 -0800
                Re: Hairy generics question Arne Vajhøj <arne@vajhoej.dk> - 2012-03-10 13:26 -0500
                Re: Hairy generics question Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2012-03-11 14:06 -0300
                Re: Hairy generics question Arne Vajhøj <arne@vajhoej.dk> - 2012-03-11 18:20 -0400
              Re: Hairy generics question Arne Vajhøj <arne@vajhoej.dk> - 2012-03-09 21:43 -0500

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