plone.behavior 1.0.2

Requirements

This package comes with support for registering behaviors and factories. It does not, however, implement the policy for determining what behaviors are enabled on a particular object at a particular time. That decision is deferred to an IBehaviorAssignable adapter, which you must implement.

This package also does not directly support the adding of marker interfaces to instances. To do that, you can either use an event handler to mark an object when it is created, or a dynamic __providedBy__ descriptor that does the lookup on the fly (but you probably want some caching).

The intention is that behavior assignment is generic across an application, used for multiple, optional behaviors. It probably doesn’t make much sense to use plone.behavior for a single type of object. The means to keep track of which behaviors are enabled for what types of objects will be application specific.

Usage

A behavior is written much like an adapter, except that you don’t specify the type of context being adapted directly. For example:

from zope.interface import Interface, implements

class ILockingSupport(Interface):
   """Support locking
   """

   def lock():
       """Lock an object
       """

   def unlock():
       """Unlock an object
       """

class LockingSupport(object):
    implements(ILockingSupport)

    def __init__(self, context):
        self.context = context

    def lock(self):
        # do something

    def unlock(self):
        # do something

This interface (which describes the type of behavior) and class (which describes the implementation of the behavior) then need to be registered.

The simplest way to do that is to load the meta.zcml file from this package and use ZCML:

<configure
    xmlns="http://namespaces.zope.org/zope"
    xmlns:plone="http://namespaces.plone.org/plone"
    i18n_domain="my.package">

    <include package="plone.behavior" file="meta.zcml" />

    <plone:behavior
        title="Locking support"
        description="Optional object-level locking"
        provides=".interfaces.ILockingSupport"
        factory=".locking.LockingSupport"
        />

</configure>

After this is done - and presuming an appropriate IBehaviorAssignable adapter exists for the context - you can adapt a context to ILockingSupport as normal:

locking = ILockingSupport(context, None)

if locking is not None:
    locking.lock()

You’ll get an instance of LockingSupport if context can be adapted to IBehaviorAssignable (which, recall, is application specific), and if the implementation of IBehaviorAssignable says that this context supports this particular behavior.

It is also possible to let the provided interface act as a marker interface that is to be provided directly by the instance. To achieve this, omit the ‘factory’ argument. This is useful if you need to register other adapters (including views and viewlets) for instances providing a particular behavior.

Like the IBehaviorAssignable plumbing, marker interface support needs to be enabled on a per-application basis. It can be done with a custom __providedBy__ decorator or an IObjectCreatedEvent handler for applying the marker. A sample event handler is provided with this package, but is not registered by default

Please see behavior.txt, directives.txt and annotation.txt for more details.

Usage

To use this package, you must first provide a suitable IBehaviorAssignable adapter. This is normally done by the framework. plone.dexterity, for example, will provide a suitable adapter.

Then, for each behavior:

• Write an interface describing the behavior.

• Write a factory (much like an adapter factory) that contains the logic of the behavior. This is optional if your interface is a marker interface that can be directly provided by the object.

• Register the behavior. This consists of a utility providing IBehavior and an adapter factory based on IBehaviorAdapterFactory. The <plone:behavior /> ZCML directive makes this easy. See directives.txt.

An example might be:

<plone:behavior
    title="Locking"
    description="Support object-level locking"
    provides=".interfaces.ILocking"
    factory=".locking.LockingBehaviorFactory"
    />

Once the behavior has been registered, you can use standard adaptation idioms to attempt to use it, e.g.:

locking = ILocking(context, None)
if locking is not None:
    locking.lock()

Here, ILocking is a registered behavior interface. The adaptation will only succeed if the context support behaviors (i.e. it can be adapted to IBehaviorAssignable), and if the ILocking behavior is currently enabled for this type of context.

Example

As an example, let’s create a basic behavior that’s described by the interface ILockingSupport:

>>> from zope.interface import implements
>>> from zope.interface import Interface

>>> class ILockingSupport(Interface):
...     def lock():
...         "Lock the context"
...
...     def unlock():
...         "Unlock the context"

>>> class LockingSupport(object):
...     implements(ILockingSupport)
...     def __init__(self, context):
...         self.context = context
...
...     def lock(self):
...         print 'Locked', repr(self.context)
...
...     def unlock(self):
...         print 'Unlocked', repr(self.context)

The availability of this new behavior is indicated by registering a named utility providing IBehavior. There is a default implementation of this interface that makes this easy:

>>> from plone.behavior.registration import BehaviorRegistration
>>> registration = BehaviorRegistration(
...     title=u"Locking support",
...     description=u"Provides content-level locking",
...     interface=ILockingSupport,
...     marker=None,
...     factory=LockingSupport)

>>> from zope.component import provideUtility
>>> provideUtility(registration, name=ILockingSupport.__identifier__)

NOTE: By convention, the behavior name should be the same as the identifier of its interface. This convention is maintained by the <plone:behavior /> ZCML directive.

We also need to register an adapter factory that can create an instance of an ILockingSupport for any context. This is a bit different to a standard adapter factory (which is normally just a class with a constructor that takes the context as an argument), because we want this factory to be able to adapt almost anything, but return None (and thus fail to adapt) if the behavior isn’t currently enabled for the context.

To get these semantics, we can use the BehaviorAdapterFactory helper class.

>>> from plone.behavior.factory import BehaviorAdapterFactory
>>> factory = BehaviorAdapterFactory(registration)

>>> from zope.interface import implements
>>> from zope.component import provideAdapter
>>> provideAdapter(factory=factory, adapts=(Interface,), provides=ILockingSupport)

One this is registered, it will be possible to adapt any context to ILockingSupport, if:

• The context can be adapted to IBehaviorAssignable. This is an interface that is used to determine if a particular object supports a particular behavior.

• The behavior is enabled, i.e. the IBehaviorAssignable implementation says it is.

Right now, neither of those things are true, so we’ll get a TypeError when trying to adapt:

>>> class IContextType(Interface): pass

>>> class SomeContext(object):
...     implements(IContextType)
...     def __repr__(self):
...         return "<sample context>"

>>> context = SomeContext()
>>> behavior = ILockingSupport(context) # doctest: +ELLIPSIS
Traceback (most recent call last):
...
TypeError: ('Could not adapt', ...)

Of course, we are more likely to want to code defensively:

>>> behavior = ILockingSupport(context, None)
>>> behavior is None
True

For the behavior to work, we need to define an IBehaviorAssignable adapter. For the purposes of this test, we’ll maintain a simple, global registry that maps classes to a list of enabled behavior interfaces.

>>> BEHAVIORS = {}

The adapter can thus be registered like this:

>>> from plone.behavior.interfaces import IBehavior, IBehaviorAssignable
>>> from zope.component import adapts, getUtility

>>> class TestingBehaviorAssignable(object):
...     implements(IBehaviorAssignable)
...     adapts(Interface)
...
...     def __init__(self, context):
...         self.context = context
...
...     def supports(self, behavior_interface):
...         global BEHAVIORS
...         return behavior_interface in BEHAVIORS.get(self.context.__class__, [])
...
...     def enumerateBehaviors(self):
...         global BEHAVIORS
...         for iface in BEHAVIORS.get(self.context.__class__, []):
...             yield getUtility(IBehavior, iface.__identifier__)

>>> provideAdapter(TestingBehaviorAssignable)

NOTE: Again, we are relying on the convention that the IBehavior utility name is the identifier of the behavior interface.

At this point, we know that the context support behavior assignment (since there is an adapter for it), but it’s not yet enabled, so we still can’t adapt.

>>> behavior = ILockingSupport(context, None)
>>> behavior is None
True

However, if we enable the behavior for this type…

>>> BEHAVIORS.setdefault(SomeContext, set()).add(ILockingSupport)

…then we can adapt and use the behavior adapter:

>>> behavior = ILockingSupport(context, None)
>>> behavior is None
False

>>> behavior.lock()
Locked <sample context>

Marker interfaces

Behaviors work without the aid of marker interfaces. However, it may sometimes be desirable to apply a marker interface to newly created objects that support a particular behavior, for example if you need to register specific views or viewlets that should only be available when this behavior is supported.

Note that there is no need to use marker interfaces if the desired behavior can be achieved using adapters only. For this, the standard plone.behavior adapter pattern is better, because there is no dependency on per-instance markers.

Marker interface support again requires some framework support not configured by this package. One of two possible configurations is possible:

• A custom __providedBy__ descriptor that includes the markeres of all enabled behaviors can be added to behavior-aware classes.

• An event handler can be installed that marks newly created instances with the markers of all enabled behaviors.

The first approach is better in many ways, because it can be made more robust in case a marker interface is removed or renamed, and because it is possible to turn off behavior markers without finding all objects providing the subtype and calling noLongerProvides() on them. However, it is also pretty difficult to get this right, and it cannot be generalised (you can’t make any adapter lookups in the descriptor, since you’d get infinite recursion). There’s an implementation of such a descriptor in the plone.dexterity package, which also uses some heavy caching.

An event handler is easier, and this package provides a simple one that you can use. It is not registered by default, since it may not be desirable to enable an event handler for every type of object.

For the purposes of this test, we will simulate the event handler by calling it directly.

>>> from plone.behavior.markers import applyMarkers
>>> from zope.lifecycleevent import ObjectCreatedEvent

Let us create another behavior. This time, we’ll provide a marker interface as well.

>>> from zope import schema
>>> class ITaggable(Interface):
...     pass

>>> class ITagging(Interface):
...     tags = schema.List(title=u"Tags on this object",
...                        value_type=schema.TextLine(title=u"Tag"))

>>> class Tagging(object):
...     implements(ITagging)
...     def __init__(self, context):
...         self.context = context
...
...     def get_tags(self, value):
...         return getattr(self.context, '__tags__', [])
...     def set_tags(self, value):
...         self.context.__tags__ = value
...     tags = property(get_tags, set_tags)

We will register this behavior as above, this time specifying the marker interface explicitly. In real life, of course, we’d be more likely to use the <plone:behavior /> ZCML directive with the ‘marker’ attribute. See directives.txt for more details.

>>> from plone.behavior.registration import BehaviorRegistration
>>> registration = BehaviorRegistration(
...     title=u"Tagging support",
...     description=u"",
...     interface=ITagging,
...     marker=ITaggable,
...     factory=Tagging)

>>> from zope.component import provideUtility
>>> provideUtility(registration, name=ITagging.__identifier__)
>>> factory = BehaviorAdapterFactory(registration)
>>> provideAdapter(factory=factory, adapts=(Interface,), provides=ITagging)

Let us now create a new object without the behavior being enabled. The marker interface should not be applied.

>>> context1 = SomeContext()
>>> ITagging(context1, None) is not None
False
>>> ITaggable.providedBy(context1)
False

>>> applyMarkers(context1, ObjectCreatedEvent(context1))

>>> ITaggable.providedBy(context1)
False

If we now turn on the behavior, the marker should be applied when the event is fired.

>>> BEHAVIORS.setdefault(SomeContext, set()).add(ITagging)

>>> context2 = SomeContext()
>>> ITagging(context2, None) is not None
True
>>> ITaggable.providedBy(context2)
False

>>> applyMarkers(context2, ObjectCreatedEvent(context2))

>>> ITaggable.providedBy(context2)
True

Note that since this is applied per-instance, old instances do not get the marker interface automatically:

>>> ITaggable.providedBy(context1)
False

It may be useful to mark the content with the behavior interface directly for cases where the marker is all that’s needed for the behavior to work. In these cases no factory is needed, because the object already provides the behavior directly as indicated by the marker. Note that the same interface is used as the ‘interface’ and ‘marker’:

>>> class IMarkerBehavior(Interface):
...     pass

>>> from plone.behavior.registration import BehaviorRegistration
>>> registration = BehaviorRegistration(
...     title=u"",
...     description=u"",
...     interface=IMarkerBehavior,
...     marker=IMarkerBehavior,
...     factory=None)

>>> from zope.component import provideUtility
>>> provideUtility(registration, name=IMarkerBehavior.__identifier__)
>>> factory = BehaviorAdapterFactory(registration)
>>> provideAdapter(factory=factory, adapts=(Interface,), provides=IMarkerBehavior)
>>> BEHAVIORS.setdefault(SomeContext, set()).add(IMarkerBehavior)

When we adapt an object using this behavior, we get the object itself back, since it implements our behavior interface directly:

>>> context = SomeContext()
>>> IMarkerBehavior.providedBy(context)
False
>>> applyMarkers(context, ObjectCreatedEvent(context))
>>> IMarkerBehavior.providedBy(context)
True
>>> IMarkerBehavior(context) is context
True

1.0.2 (2013-01-17)

• Remove dependence of tests on zope.app.testing. [davisagli]

1.0.1 - 2011-05-20

• Relicense under BSD license. See http://plone.org/foundation/materials/foundation-resolutions/plone-framework-components-relicensing-policy [davisagli]

1.0 - 2011-04-30

• Use stdlib doctest instead of the deprecated one in zope.testing. [davisagli]

• ‘plone:behavior’ zcml directive use now MessageID for title and description. [sylvainb]

1.0b6 - 2009-11-17

• Fix tests for Zope 2.12 [optilude]

1.0b5 - 2009-07-12

• Changed API methods and arguments to mixedCase to be more consistent with the rest of Zope. This is a non-backwards-compatible change. Our profuse apologies, but it’s now or never. :-/

  If you find that you get import errors or unknown keyword arguments in your code, please change names from foo_bar too fooBar, e.g. enumerate_behaviors() becomes enumerateBehaviors(). [optilude]

1.0b4 - 2009-06-07

• Allow a marker-interface-only behavior to be set by using the ‘provides’ attribute (previously ‘interface’) in the <plone:behavior /> directive without a ‘factory’ attribute. The ‘marker’ attribute (previously known as ‘subtype’) is now only required if there is a marker used in addition to a behavior adapter with a separate interface (‘provides’) and factory. [optilude]

• Rename the ‘interface’ attribute of <plone:behavior /> to ‘provides’ to be more consistent with the <adapter /> directive. This is a backwards incompatible change! [optilude]

• Rename the ‘subtype’ attribute of <plone:behavior /> to ‘marker’ to be more explicit about its purpose. This is a backwards incompatible change! [optilude]

1.0b3 - 2009-04-17

• Allow behaviors with no factory. [alecm]

• Provide a vocabulary of available behaviors. [davisagli]

1.0b1 - 2008-04-27

• Initial release