Monday, August 28, 2006

Spyce will not waste your time: controllers/handlers

Traditional view-oriented frameworks (such as PHP, or Spyce 1.3) do not handle control logic well. Today I'll show how Spyce 2 solves this problem with "active handlers," and tomorrow I'll show how this lets Spyce provide much more elegant code-reuse tools than either other view-oriented frameworks or MVC frameworks like Turbogears and Django.
Control logic is the code that decides what happens next in response to a user action. Say we have a simple form to create new to-do lists, like this:
<form>
<input type=text name=name value="">
<input type=submit>
</form>
In a traditional view-oriented framework (like Spyce 1.3), you would process this form with code that looks something like this:
[[name = request['name']
db.todo_lists.insert(name=name)
api.db.flush()
]]
That wasn't too bad. (Especially since I cheated and used the modern Spyce db api, AKA SqlSoup from SQLAlchemy.) But even with a simple, one-element form like this, a couple things are clear:
  • code like "name = request['name']" must be written for each form element. Irritating!
  • putting this chunk of code in the page with the form on it sucks (because you have to "if" it out if you're not actually processing the form POST)
  • putting this code on another page that doesn't actually do anything else sucks too (because then you have a view-that's-not-really-a-view, which is ugly -- to say nothing of existentially disturbing)
The Spyce solution is to allow you to specify Python functions, called handlers, that run on form submission. So your submit will now look like
<f:submit handler=list_new />
(note the Spyce form submit active tag; the vanilla html submit tag won't understand the handler parameter, of course), and you can define your handler as
def list_new(api, name):
    api.db.todo_lists.insert(name=name)
    api.db.flush()
This takes care of the redundant code to access form variables -- Spyce inspects the function definition and pulls the corresponding values out of the request for you -- and it also solves the problem of how to organize your code.
Earlier, I said you could write "handler=list_new." Actually, you either need to write "handler=self.list_new" and define list_new in a class chunk in the current page (as demonstrated in this example), or you can write "handler=somemodule.list_new," and put list_new in somemodule. (In the spirit of Not Wasting Your Time, Spyce will take care of importing somemodule as necessary.)
Pretty simple stuff, but I needed to explain handlers before I get into how Spyce uses them with active tags to provide unique code re-use tools. That will be our topic next time! Until then, if you missed the entry on form processing, go read it, since it touches on some of the more advanced features of handlers.







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