add the available() method on viewlets, check for by the viewlet manager

CHANGES.txt

@@ -4,8 +4,10 @@ Changes
 1.9 (unreleased)
 ----------------
 
-- Nothing changed yet.
-
+- Introduce the `available()` method on viewlet component. The viewlet
+  manager will filter out unavailable viewlet by calling this method. The
+  `available()` method is called *after* the viewlet's `update()` is called,
+  but *before* the `render()` is called.
 
 1.8 (2010-12-16)
 ----------------

src/grokcore/viewlet/components.py

@@ -73,6 +73,12 @@ def default_namespace(self):
     def namespace(self):
         return {}
 
+    def update(self):
+        super(ViewletManager, self).update()
+        # Filter out the unavailable viewlets *after* the viewlet's update()
+        # has been called.
+        self.viewlets = filter(lambda v: v.available(), self.viewlets)
+
     def render(self):
         """See zope.contentprovider.interfaces.IContentProvider"""
         # Now render the view
@@ -115,5 +121,13 @@ def namespace(self):
     def update(self):
         pass
 
+    def available(self):
+        """Return True if this viewlet is to be rendered. False otherwise.
+
+        Note that the available() method is called *after* update() but
+        *before* render() but has been called.
+        """
+        return True
+
     def render(self):
         return self.template.render(self)

src/grokcore/viewlet/ftesting.zcml

@@ -25,6 +25,7 @@
   <include package="zope.annotation" />
   <include package="zope.principalregistry" />
   <include package="zope.contentprovider" />
+  <include package="zope.publisher" />
   <include package="zope.app.appsetup" />
   <include package="zope.app.publication" />
   <include package="grokcore.viewlet" />

src/grokcore/viewlet/ftests/viewlet/viewlet_available_templates/bonesview.pt

@@ -0,0 +1 @@
+<span tal:replace="provider:bones" />