How to enable a command only when the selected element has certain property? - eclipse

Question

I'm building am eclipse plugin and I have a tree representation where I display the structure of an element of type A(see below).
Class A{
List<B> children;
}
Abstract Class B{
...
}
Class C extend B{
...
}
Class D extend B{
...
}
I have a command which can start only when the selected element in the three is an instance of A.
This is the expression I'm using.
<extension point="org.eclipse.core.expressions.definitions">
<definition id="SelectionOfA">
<with variable="selection">
<and>
<count value="1"></count>
<iterate ifEmpty="false" operator="or">
<instanceof value="A"></instanceof>
</iterate>
</and>
</with>
</definition>
</extension>
Now I would like to refine this condition. I would like to enable the command only when an instance of A is selected and the list children contains at least an element which is instance of C.
I tried by keeping this expression and defining a new one. Then I will enable the command only if both the expressions are true. I started from the expression reported above and I tried to replace the tag instanceof with the tags with, iterate and adapt without a good result.
How can I do it?

Answer 1

I would implement a property tester to check if the condition is met.
Something like this should do the work:
public class YourConditionTester extends PropertyTester {
public boolean test(final Object receiver, final String property, final Object[] args, final Object expectedValue) {
if (property.equals("isA_and_hasC")) {
return (true if your condition is met, otherwise false)
}
return false;
}
}
Then, instead of instanceof use test with property set (e.g.) to "isA_and_hasC".
Look here for more info about property testers.