[eclipse-incubator-e4-dev] Re: news.eclipse.platform.rcp: Command handle

["Paul Webster" <pwebster@xxxxxxxxxxxxxxxxxxx>]

Mike Parker wrote:
> As I see it, the problem is not directly related to the handlers, but to the commands themselves, which (being typically defined in the plug-in's plugin.xml file) are instantiated by the platform only once. Thus, if two command contribution items in two workbench windows are associated with the same command ID, they are also referencing the same command object. Since this command can, in turn, only be associated with (at most) one handler at any one time, and this handler is responsible for determining the commands enabled status, it is (as Mark says) impossible to enable the command contribution item in one workbench window, whilst keeping the command contribution using the same command ID in the other workbench window disabled (or vice versa).

Yes, this is a limitation of the current system.  The command is a
singleton object, and the context it operates against is the global
application context.  Since enabled was written as "state" for the
command and the handler, all UI elements reference the command will
reflect the same enabled state, even if not appropriate.  You can also
create the problem with a command in a multi-instance view toolbar
(since multiple views can be visible at one time, the tool item state
reflects the command state based on the active view, not its local
view).

>
> However, the above-mentioned problem does NOT occur with the "Save" toolbar item in Eclipse 3.4.

Right, the actions have both behaviour, semantics, and rendering
information all tied up in one object.  That allows them to reflect a
localized state fairly easily.

> (Note: Maybe this should be bugzilla'd, so that we have a handle on this problem?)

There should be a couple of bugs, although they might have been closed
in the 3.x stream.

>
> In order to fix this issue, surely it would be necessary to (optionally) instantiate a command once per workbench window, rather than once per workbench.

This might solve the multi-window case, but not the multi-part case
(like my multi-instance view example).

Some investigation seems to indicate that we need to activate,
enabled, and execute our handlers all within the same context.  That
way the print icon in one workbench window would be asking
isEnabled(context) within its window context, and would not be
effected by a different workbench window context.


Check out http://dev.eclipse.org/mhonarc/lists/eclipse-incubator-e4-dev/msg00356.html

We're investigating a number of aspects of this problem in the 4.0 stream.

Later,
Paul Webster

-- 
Paul Webster
http://wiki.eclipse.org/Platform_Command_Framework
http://wiki.eclipse.org/Command_Core_Expressions
http://wiki.eclipse.org/Menu_Contributions
http://wiki.eclipse.org/Menus_Extension_Mapping
http://help.eclipse.org/help33/index.jsp?topic=/org.eclipse.platform.doc.isv/guide/workbench.htm