| Handler expression evaluation time [message #489618] |
Mon, 05 October 2009 07:07  |
Eclipse User |
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Hi,
I'm providing my own handler for the delete action, which supplies an
activeWhen element and an enabledWhen element as follows:
<handler commandId="org.eclipse.ui.edit.delete"
class="foo.DeleteHandler">
<activeWhen>
<with variable="activeMenuSelection">
<iterate ifEmpty="false" operator="and">
<instanceof value="foo.A" />
</iterate>
</with>
</activeWhen>
<enabledWhen>
<with variable="activeMenuSelection">
<iterate ifEmpty="false" operator="and">
<property="foo.properties.checkPermission" value="Delete" />
</iterate>
</with>
</enabledWhen>
</handler>
And the following property tester declaration:
<extension point="org.eclipse.core.expressions.propertyTesters">
<propertyTester id="foo.PropertyTester"
type="foo.SuperOfA"
namespace="foo.properties"
properties="checkPermission"
class="foo.PropertyTester">
</propertyTester>
</extension>
This works as intended: to activate my handler when an the
activeMenuSelection is comprised entirely of instances of A, and enable
it when the property evaluates to true. However, I find that my
property tester is invoked by evaluation of the check enablement
expression whenever the active menu selection changes (restricted by the
active selection being an instance of foo.SuperOfA), regardless of the
activation of the handler. This seems unnecessary to me - I'd assumed
that the enablement was a property of the handler, which in the event
that the handler is inactive, is irrelevant. Is this an incorrect
assumption?
Is this not also a potential performance issue? The property tester in
question acts as an interface for the core expressions framework into a
generic checkPermissions method in my RCP, used to determine permissions
for all actions/commands. If I were to expand my use of this property
tester, would it not be invoked for all commands in my RCP which
reference it in an enabledWhen clause (assuming all of my command
handlers are registered for some subtype of foo.SuperOfA)? i.e. at the
moment, the property tester is invoked to check permissions for Delete
whenever the activeMenuSelection changes to any foo.SuperOfA, regardless
that it is only appropriate to check permissions for delete if the
activeMenuSelection is a foo.A.
Also, the checkPermissions operation could be time consuming. I'm
currently using the work-around of repeating my handler's activeWhen
expression in the enabledWhen clause, to prevent invocation of the
checkPermissions method when not appropriate.
Is there some better way of doing this?
This is using eclipse 3.4.2
Hope this makes sense! Any advice greatly appreciated.
Stewart Francis
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