forgot to add the test file

src/grokcore/viewlet/ftests/viewlet/viewlet_available.py

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+"""
+
+===================
+Viewlet availablity
+===================
+
+The viewlet manager will filter out "unavailable" viewlets before rendering.
+The availability of a viewlet is determined by calling available() on the
+individuel viewlet. Note that the availability check is performed *after* the
+update() is caled on the viewlet, but *before* the render() is called.
+
+Set up a content object in the application root::
+
+  >>> root = getRootFolder()
+  >>> root['wilma'] = CaveWoman()
+
+Traverse to the view on the model object and render the viewlets "Brack Bone"
+and "T-Rex Bone", but not the "Elephant Bone":
+
+  >>> from zope.app.wsgi.testlayer import Browser
+  >>> browser = Browser()
+  >>> browser.handleErrors = False
+  >>> browser.open("http://localhost/wilma/@@bonesview")
+  >>> print browser.contents
+  Brack Bone
+  T-Rex Bone
+
+At some point in time, the Sabre Tooth Bone becomes availalble:
+
+  >>> SabreToothBone._available = True
+  >>> browser.open("http://localhost/wilma/@@bonesview")
+  >>> print browser.contents
+  Brack Bone
+  Sabre Tooth Bone
+  T-Rex Bone
+
+The availability can depend on some flag available in the request and since the
+update() of a viewlet is called before the check, the availability can be
+computed as wel.
+
+First it is there:
+
+  >>> browser.open("http://localhost/wilma/@@bonesview?requestcounting=true")
+  >>> print browser.contents
+  Brack Bone
+  Only for every other request!
+  Sabre Tooth Bone
+  T-Rex Bone
+
+Next request it is not:
+
+  >>> browser.open("http://localhost/wilma/@@bonesview?requestcounting=true")
+  >>> print browser.contents
+  Brack Bone
+  Sabre Tooth Bone
+  T-Rex Bone
+
+And then we have it again:
+
+  >>> browser.open("http://localhost/wilma/@@bonesview?requestcounting=true")
+  >>> print browser.contents
+  Brack Bone
+  Only for every other request!
+  Sabre Tooth Bone
+  T-Rex Bone
+
+"""
+
+
+import grokcore.viewlet as grok
+from zope.interface import Interface
+from zope.publisher.interfaces.browser import IDefaultBrowserLayer
+
+
+class CaveWoman(grok.Context):
+    pass
+
+class BonesView(grok.View):
+    grok.context(Interface)
+
+class Bones(grok.ViewletManager):
+    grok.context(Interface)
+    grok.name('bones')
+
+class BrackerBone(grok.Viewlet):
+    grok.context(Interface)
+    grok.viewletmanager(Bones)
+
+    def render(self):
+        return u"Brack Bone"
+
+class TRexBone(grok.Viewlet):
+    grok.context(Interface)
+    grok.viewletmanager(Bones)
+
+    def render(self):
+        return u"T-Rex Bone"
+
+class ElephantBone(grok.Viewlet):
+    grok.context(Interface)
+    grok.viewletmanager(Bones)
+
+    def available(self):
+        # This type of bone has not evolved just yet.
+        return False
+
+    def render(self):
+        return u"Elephant Bone"
+
+class SabreToothBone(grok.Viewlet):
+    grok.context(Interface)
+    grok.viewletmanager(Bones)
+
+    _available = False
+
+    def available(self):
+        return self._available
+
+    def render(self):
+        return u"Sabre Tooth Bone"
+
+class OnlyForEvenRequest(grok.Viewlet):
+    # Convoluted example of a viewlet that is available only for every
+    # other request, but only when we're indicating that we're counting
+    # requests.
+
+    count = 1
+
+    def update(self):
+        if 'requestcounting' in self.request.form:
+            OnlyForEvenRequest.count += 1
+
+    def available(self):
+        if not self.count % 2:
+            return True
+        return False
+
+    def render(self):
+        return u"Only for every other request!"