Item9715: add 'askListeners' to the store, to support the store askin…

…g a cache if it wants to serve a topic in preference to going back to the VC for it. Sort of experimental at the moment; comments invited.

git-svn-id: http://svn.foswiki.org/trunk@10216 0b4bb1d4-4e5a-0410-9cc4-b2b747904278

core/lib/Foswiki/Store.pm

@@ -127,6 +127,29 @@ sub tellListeners {
 
 =begin TML
 
+---++ ObjectMethod askListeners( $topicObject, $version )
+Ask listeners if they would like to provide the object specified in the
+$topicObject, at the given version. The first listener to respond with a
+non-zero revision will be assumed to have loaded the topic object.
+
+Listeners are expected to implement =loadTopic=. If they do not, they
+will not be asked.
+
+=cut
+
+sub askListeners {
+    my ($this, $meta) = @_;
+    my ($gotRev, $isLatest);
+
+    foreach my $el (@{$this->{event_listeners}}) {
+	next unless $el->can('loadTopic');
+	($gotRev, $isLatest) = $el->loadTopic($meta);
+	return ($gotRev, $isLatest) if $gotRev;
+    }
+}
+
+=begin TML
+
 ---++ StaticMethod cleanUpRevID( $rev ) -> $integer
 
 Cleans up (maps) a user-supplied revision ID and converts it to an integer

core/lib/Foswiki/Store/Interfaces/Listener.pm

@@ -21,6 +21,21 @@ a topic is moved.
 ---++ ObjectMethod remove($metaObject)
 We are removing the given object.
 
+---++ ObjectMethod loadTopic($meta, $version) -> ($gotRev, $isLatest)
+Patterned on =Foswiki::Store::readTopic=, this listener is called when
+the store's =readTopic= method is called. The first listener to return
+a $meta will be assumed to have loaded that meta object with
+the requested revision.
+
+Implementors do *not* need to provide this method; it is called only if
+it's present in the listener.
+
+Note that the listener may re-bless the $meta into a subclass
+of =Foswiki::Meta=, should it be necessary to enhance that class.
+
+Listeners can also use this callback as a means of monitoring topic loads
+from a VC store.
+
 =cut
 
 1;

core/lib/Foswiki/Store/VC/Store.pm

@@ -79,9 +79,11 @@ sub getHandler {
 sub readTopic {
     my ( $this, $topicObject, $version ) = @_;
 
-    ASSERT( $topicObject->isa('Foswiki::Meta') ) if DEBUG;
+    my ($gotRev, $isLatest) =  $this->askListeners($topicObject);
+    return ($gotRev, $isLatest) if $gotRev;
+
     my $handler = $this->getHandler($topicObject);
-    my $isLatest = 0;
+    $isLatest = 0;
 
     # check that the requested revision actually exists
     if ( defined $version ) {
@@ -96,7 +98,7 @@ sub readTopic {
     $text =~ s/\r//g;    # Remove carriage returns
     $topicObject->setEmbeddedStoreForm($text);
 
-    my $gotRev = $version;
+    $gotRev = $version;
     unless ( defined $gotRev ) {
 
         # First try the just-loaded text for the revision