Dexterity
=========

    "Same, same, but different"

Dexterity is a system for building content types, both through-the-web and
as filesystem code. It is aimed at Plone, although this package should work
with plain Zope + CMF systems.

Key use cases
-------------

Dexterity wants to make some things really easy. These are:

- Create a "real" content type entirely through-the-web without having to
  know programming.
  
- As a business user, create a schema using visual or through-the-web tools,
  and augment it with adapters, event handlers, and other Python code
  written on the filesystem by a Python programmer.

- Create content types in filesystem code quickly and easily, without losing
  the ability to customise any aspect of the type and its operation later
  if required.

- Support general "behaviours" that can be enabled on a custom type in a
  declarative fashion. Behaviours can be things like title-to-id naming,
  support for locking or versioning, or sets of standard metadata with
  associated UI elements.
 
- Easily package up and distribute content types defined through-the-web,
  on the filesystem, or using a combination of the two.

Philosophy
----------

Dexterity is designed with a specific philosophy in mind. This can be 
summarised as follows:

Reuse over reinvention

   As far as possible, Dexterity should reuse components and technologies
   that already exist. More importantly, however, Dexterity should reuse
   *concepts* that exist elsewhere. It should be easy to learn Dexterity
   by analogy, and to work with Dexterity types using familiar APIs and 
   techniques.

Small over big

   Mega-frameworks be damned. Dexterity consists of a number of speciaised
   packages, each of which is independently tested and reusable. Furthermore,
   packages should has as few dependencies as possible, and should declare 
   their dependencies explicitly. This helps keep the design clean and the 
   code manageable.

Natural interaction over excessive generality

   The Dexterity design was driven by several use cases (see docs/Design.txt)
   that express the way in which we want people to work with Dexterity. The
   end goal is to make it easy to get started, but also easy to progress from
   an initial prototype to a complex set of types and associated behaviours
   through step-wise learning and natural interaction patterns. Dexterity
   aims to consider its users - be they business analysts, light integrators
   or Python developers, and be they new or experienced - and cater to them
   explicitly with obvious, well-documented, natural interaction patterns.

Real code over generated code

   Generated code is difficult to understand and difficult to debug when it
   doesn't work as expected. There is rarely, if ever, any reason to scribble
   methods or 'exec' strings of Python code.

Zope 3 over Zope 2

   Although Dexterity does not pretend to work with non-CMF systems, as
   many components as possible should work with plain Zope 3, and even where
   there are dependencies on Zope 2, CMF or Plone, they should - as far as
   is practical - follow Zope 3 techniques and best practices. Many 
   operations (e.g. managing objects in a folder, creating new objects
   or manipulating objects through a defined schema) are better designed in
   Zope 3 than they were in Zope 2. 

Zope concepts over new paradigms

   We want Dexterity to be "Zope-ish" (and really, "Zope 3-ish"). Zope is a
   mature, well-designed (well, mostly) and battle tested platform. We do 
   not want to invent brand new paradigms and techniques if we can help it.

Automated testing over wishful thinking

   "Everything" should be covered by automated tests. Dexterity necessarily
   has a lot of moving parts. Untested moving parts tend to come lose and
   fall on people's heads. Nobody likes that.

What's it all about?
--------------------

With the waffle out of the way, let's look in a bit more detail about what
makes up a "content type" in the Dexterity system.

The model

   The Dexterity "model" describes a type's schemata (most types will have
   only one) and metadata associated with those schemata. A schema is just
   a series of fields that can be used to render add/edit forms and 
   introspect an object of the given type. The metadata storage is extensible
   via the component architecture. Typical forms of metadata include UI
   hints such as specifying the type of widget to use when rendering a
   particular field, and per-field security settings.
   
   The model is typically described in XML, though at runtime it is an 
   instance of an object providing the IModel interface from 
   plone.supermodel. Schemata in the model are interfaces with zope.schema
   fields.
   
   The model can exist purely as data in the ZODB if a type is created
   through-the-web. Alternatively, it can be loaded from a file. The XML
   representation is intended to be human-readable and self-documenting.
   It is also designed with tools like ArchGenXML and Genesis in mind,
   that can generate models from a visual representation.

The schema

   All content types have at least one (unnamed) schema. A schema is
   simply an Interface with zope.schema fields. The schema can be specified
   in Python code (in which case it is simply referenced by name), or it
   can be loaded from an XML model.
   
   The unnamed schema is also known as the IContentType schema, in that the
   schema interface will provide the Zope 3 IContentType interface. This
   means that if you call queryContentType() on a Dexterity content object,
   you should get back its unnamed schema, and that schema should be
   provided by the object that was queried. Thus, the object will directly
   support the attributes promised by the schema. This makes Dexterity
   content objects "Pythonic" and easy to work with.

The class

   Of course, all content objects are instances of a particular class.
   It is easy to provide your own class, and Dexterity has convenient
   base classes for you to use. However, many types will not need a class
   at all. Instead, they will use the standard Dexterity "Item" and 
   "Container" classes.
   
   Dexterity's content factory will initialise an object of one of these
   classes with the fields in the type's content schema, and will ensure
   that objects provide the relevant interfaces, including the schema
   interface itself.
   
   The preferred way to add behaviour and logic to Dexterity content objects
   is via adapters. In this case, you will probably want a filesystem
   version of the schema interface (this can still be loaded from XML if you
   wish, but it will have an interface with a real module path) that you
   can register components against.

The factory

   Dexterity content is constructed using a standard Zope 3 IFactory
   named utility. By convention the factory utility has the same name as the
   portal_type of the content type.
   
   When a Dexterity FTI is created, an appropriate factory will be registered
   as a local utility unless one with that name already exists.
   
   The default factory is capable of initialising a generic 'Item' or
   'Container' object to exhibit a content type schema and have the
   security and other aspects specified in the type's model. You can use
   this if you wish, or provide your own factory.

Views

   Dexterity will by default create an add view (registered as a local 
   utility, since it needs to take the portal_type of the content type into
   account when determining what fields to render) and an edit view (
   registered as a generic, global view, which inspects the context's
   portal_type at runtime) for each type. There is also a default main
   view, which simply outputs the fields set on the context.
   
   To register new views, you will normally need a filesystem schema
   interface. You can then register views for this interface as you
   normally would.
   
   If you need to override the default add view, create a view for IAdding
   with a name corresponding to the portal_type of the content type.
   This will prevent Dexterity from registering a local view with the same
   name when the FTI is created.

The Factory Type Information (FTI)

   The FTI holds various information about the content type. Many operations
   performed by the Dexterity framework begin by looking up the type's
   FTI to find out some information about the type.
   
   The FTI is an object stored in portal_types in the ZMI. Most settings can
   be changed through the web. See the IDexterityFTI interface for more
   information.
   
   When a Dexterity FTI is created, an event handler will create a few
   local components, including the factory utility and add view for the
   new type. The FTI itself is also registered as a named utility, to
   make it easy to look up using syntax like:
   
       getUtility(IDexterityFTI, name=portal_type)
       
   The FTI is also fully importable and exportable using GenericSetup.
   Thus, the easiest way to create and distribute a content type is to 
   create a new FTI, set some properties (including a valid XML model, 
   which can be entered TTW if there is no file or schema interface to use),
   and export it as a GenericSetup extension profile.

Behaviours

   Behaviors are a way write make re-usable bits of functionality that can
   be toggled on or off on a per-type basis. Examples may include common
   metadata, or common functionality such as locking, tagging or ratings.
   
   Behaviors are implemented using the plone.behavior package. See its
   documentation for more details about how to write your own behaviors.
   
   In Dexterity, behaviors can "inject" fields into the standard add and edit
   forms, and may provide marker interfaces for newly created objects. See
   the example.dexterity package for an example of a behavior that provides
   form fields.
   
   In use, a behavior is essentially just an adapter that only appears to be
   registered if the behavior is enabled in the FTI of the object being
   adapted. Thus, if you have a behavior described by my.package.IMyBehavior,
   you'll typically interact with this behavior by doing::
   
       my_behavior = IMyBehavior(context, None)
       if my_behavior is not None:
           ...
   
   The enabled behaviors for a given type are kept in the FTI, as a
   list of dotted interface names.

The Dexterity Ecosystem
-----------------------

The Dexterity system comprises a number of packages, most of which are
independently re-usable. In addition, Dexterity uses many components from
Zope and CMF.

The most important packages are:

plone.alterego (Python)

   Support for dynamic modules that create objects on the fly. Dexterity
   uses this to dynamically create "real" interfaces for types that exist
   only through-the-web. This allows these types to have a proper 
   IContentType schema, and allows local adapters to be registered for
   this interface (e.g. a custom view with a template defined through the 
   web).
   
   Note that if a type uses a filesystem interface (whether written manually
   or loaded from an XML model), this module is not used.

plone.supermodel (Zope 3)

   Supports parsing and serialisation of interfaces from/to XML. The XML
   format is based directly on the interfaces that describe zope.schema type
   fields, and is thus easily extensible to new field types. This has the
   added benefit that the interface documentation in the zope.schema package
   applies to the XML format as well.
   
   Supermodel is extensible via adapters and XML namespaces. plone.dexterity
   uses this to allow security and UI hints to be embedded as metadata in the
   XML model.

plone.behavior (Zope 3)

   Supports "conditional" adapters. A product author can write and register
   a generic behaviour that works via a simple adapter. The adapter will
   appear to be registered for types that have the named behaviour
   available.
   
   Dexterity wires this up in such a way that the list of enabled behaviours
   is stored as a property in the FTI. This makes it easy to add/remove
   behaviours through the web, or using GenericSetup at install time.

plone.folder (CMF)

   This is an implementation of an ordered, BTree-backed folder, with Zope 3
   dictionary-style semantics for managing content items inside the folder.
   The standard Dexterity 'Container' type uses plone.folder as its base.

plone.autoform (CMF)

   Contains helper functions to construct forms based on tagged values stored
   on schema interfaces.
   
plone.directives.form (CMF)

   Adds convention-over-configuration support for plone.supermodel schemata
   and plone.autoform form hints.

plone.dexterity (CMF)

   This package defines the FTI and content classes, provides basic views
   (with forms based on z3c.form), handles security and so on. It also
   provides components to orchestrate the various functionality provided
   by the packages above in order to bring the Dexterity system together.

plone.directives.dexterity (CMF)

   Adds convention-over-configuration support for Dexterity content and 
   add/edit forms.
   
plone.app.dexterity (Plone)

   This package contains all Plone-specific aspects of Dexterity, including
   Ploneish UI components, behaviours and defaults.

Usage examples
--------------

Take a look at the example.dexterity package, which can be found in the 
Plone Collective (http://dev.plone.org/collective), for examples of various
ways to use Dexterity.

See also Dexterity's pages on plone.org (http://plone.org/products/dexterity)
where you will also find extensive documentation.