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Jifty::Manual::ObjectModel -- An overview of the Jifty object model

Jifty::Manual::ObjectModel -- An overview of the Jifty object model


Jifty::Manual::ObjectModel -- An overview of the Jifty object model


Jifty applications are generally built in a similar way. There's no reason you need to use the model we've built, but we find it a reasonably ok way to do things.

This document should serve as a roadmap to the Jifty class library, as well as an introduction to the way Jifty applications are put together.

We start with the classes in your application and move on to the bits of Jifty itself.

If you create a brand new application, let's call it MyWeblog, and create one model class called MyWeblog::Post, you'll end up with the following files and directories:
















            #some test files.

At least that's the scaffolding Jifty creates for you. Behind the scenes, Jifty is actually doing a lot more. Rather than create a bunch of little ``stub'' classes and libraries for you, Jifty generates them on the fly. It's always possible to actually create these libraries when you need to customize the default behavior, but we work really hard to make sure you don't need to.

Right now, Jifty is autocreating libraries, static web pages and web templates.

We're not 100% satisfied with how Jifty autocreates web templates and static pages and are working to redesign that.

The library you see when creating a Jifty app is:

MyWeblog::Model::Post describes the schema and business logic of your post class. It uses two namespaces, MyWeblog::Model::Post::Schema that has actual column definitions and MyWeblog::Model::Post that contains the (optional) business logic, access control and so on.

That's it. But if you look carefully at MyWeblog::Model::Post, you'll see the line:

    use base qw/MyWeblog::Record/;

How can that possibly work? There is no MyWeblog::Record class in your application. And Jifty, while it tries to be a comprehensive framework, draws the line somewhat short of including application-specific base classes for every application you might possibly contrive.

The answer lies in the Jifty::ClassLoader manpage, a utility module Jifty uses to create the boring stuff for you when you need it.

It'd certainly be possible for Jifty to create every class you might need as a file on disk when you first create your application (and indeed we may decide to do so if enough people yell at us), but when the stub classes we'd provide are just little shims that inherit from or call to the Jifty core, it doesn't make much sense to create them before you need them. You could build a Jifty application without these shims by having your model classes inherit directly from Jifty::Record, but then you'll run into trouble the second you want to add application-specific code and have to go back and retrofit each and every one of your classes to use your new base class. It's a little thing, but one that can save you a bunch of pain and suffering later on.

MyWeblog::Record is the first autogenerated class you'll run into but probably not the last. A full list of everything Jifty provides for your new application follows:

You get one each of the these:

This class is, as discussed above, a thin wrapper around the Jifty::Record manpage. You might choose to create your own MyWeblog::Record if you want to build in custom access control by overriding current_user_can in the Jifty::Record manpage or want to implement methods that every model class should have access to.

We haven't talked much about collections yet, but as their name implies, collections are bundles of the Jifty::Record manpage objects that match some set of criteria. It's relatively uncommon that you'll want to override this, but if you want the rope, it's here.

MyWeblog::Notification is an app-specific implementation of the the Jifty::Notification manpage email driver. You might want to override this class if you want to set an application-specific header or footer for all outgoing email.

MyWeblog::Dispatcher is an application-specific ``dispatcher'' class that allows you to write code that runs when a client makes a request to the server before Jifty runs actions or renders templates. See the Jifty::Dispatcher manpage for more information about the dispatcher.

Most every web application that grows past a personal hack eventually starts to provide personalization, require access control or otherwise want to know who's currently in the driver's seat. The ``current user'' for an application is a first-class object in Jifty. To get user-based authentication working out of the box, you'll have to override the default MyWeblog::CurrentUser. (Out of the box, it treats everyone as the same user.) We're working to generalize the authentication system we've used in a few Jifty apps so far to the point where it feels ``right'' as a core Jifty component, but we're not quite there just yet.

Most of what you'll need to override in MyWeblog::CurrentUser is the _init function, which needs to load up an application-specific model class that represents one of your users into its user_object accessor. To make all this work, you'll also need an application-specific MyWeblog::Action::Login and likely also a passel of user-management code.

(And yes, this is the topic of a future generalization and a future tutorial. At that point, a bunch of this documentation will be extracted to the Jifty::CurrentUser manpage.)

But wait! There's more! You also get one each of these for your default model class:

It's no fun having a weblog that only shows you one post at a time, is it? Jifty provides you with default the Jifty::Collection manpage classes for every the Jifty::Record manpage subclass in your model. You get all the standard limit, order_by, columns, paging support and so-on out of the box, but sometimes when you're going to be getting collections matching certain criteria often, it makes sense to actually create your own subclass and start dropping methods in.

MyWeblog::Action::CreatePost, MyWeblog::Action::UpdatePost, MyWeblog::Action::DeletePost
One of Jifty's strengths is that it makes it easy to build applications by tying application-specific controller functions to your model classes and intuiting what parameters they take by having a look inside the models.

For each class in your model, Jifty creates three actions, Create,Update and Delete. They're named, perhaps a bit unadventureously, MyWeblog::Action::CreatePost, MyWeblog::Action::UpdatePost, MyWeblog::Action::DeletePost and inherit directly from the Jifty::Action::Record::Create manpage, the Jifty::Action::Record::Update manpage and the Jifty::Action::Record::Delete manpage, respectively. Sometimes, it makes sense to override these default actions when you want to change the behaviour of one or more of the actions. One common use is to add or remove AJAX validation or autocompletion for an argument or to change an argument's default value for webforms. This, isn't, however the place to start inserting business logic or access control. That belongs in your model class, which these wrappers will hand things off to. By putting logic in your actions, you make your model classes less useful and run into trouble when you want to start scripting your model outside a web environment.

There's no reason you need to stick with these default ``implementations'' if they're not meeting your needs. Just create your own classes and Jifty will use your real classes instead.