bitbake: bitbake-user-manual-metadata.xml: Added [eventmask] flag information.

Reported-by: Laszlo Papp <lpapp@kde.org>
(Bitbake rev: 38150bc53c6bda5be7194f9acd112d362506d2ea)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml

@@ -1308,8 +1308,8 @@
             BitBake allows installation of event handlers within
             recipe and class files.
             Events are triggered at certain points during operation,
-            such as the beginning of operation against a given
+            such as the beginning of an operation against a given recipe
-            <filename>.bb</filename>, the start of a given task,
+            (<filename>*.bb</filename> file), the start of a given task,
             task failure, task success, and so forth.
             The intent is to make it easy to do things like email
             notification on build failure.
@@ -1337,6 +1337,27 @@
             the name of the triggered event.
         </para>
 
+        <para>
+            Because you probably are only interested in a subset of events,
+            you would likely use the <filename>[eventmask]</filename> flag
+            for your event handler to be sure that only certain events
+            trigger the handler.
+            Given the previous example, suppose you only wanted the
+            <filename>bb.build.TaskFailed</filename> event to trigger that
+            event handler.
+            Use the flag as follows:
+            <literallayout class='monospaced'>
+     addhandler myclass_eventhandler
+     myclass_eventhandler[eventmask] = "bb.build.TaskFailed"
+     python myclass_eventhandler() {
+         from bb.event import getName
+         from bb import data
+         print("The name of the Event is %s" % getName(e))
+         print("The file we run for is %s" % data.getVar('FILE', e.data, True))
+     }
+            </literallayout>
+        </para>
+
         <para>
             During a standard build, the following common events might occur:
             <itemizedlist>