Here'a a possibility:

It seems that if you define the declare-expression to evaluate a decision tree, and that decision tree sets its "DecisionTreeStatus" to the value "NotFound", then the actual setting of the target property will be skipped.

I determnined this by making a declare-expression that used the "evaluate decision tree" choice to set the target property .pyLabel, and then I launched the coffee cup icon.  Here's the part of that resultant java showing the actual setting of the target property.

            ClipboardProperty pz_2 = myStepPage.getProperty(".pyLabel");

. . .

                            // Expression: @evaluateDecisionTree("EvaluateReport", "")

                            String pz_3 = com.pegarules.generated.pega_rules_expressionevaluators

                                    .evaluateDecisionTree("EvaluateReport", "");

                            String decisionStatus = tools

                                    .getParamValue("DecisionTreeStatus");

                            if (!"NotFound".equalsIgnoreCase(decisionStatus)) {

                                pz_2.setValue(pz_3);

                            }

Notice how the setting of .pyLabel is skipped if the status is equal to "NotFound".

So now you just have to design your decision tree to set its "DecisionTreeStatus" to "NotFound" when the circumstances are right under which  you want the Declare Expression to exit.

Matthew,

Hi. I don't know of any way to do what you're trying to do with a Declare Expression. I might take a step back and look at your business requirement. Is there a different way to get the same result? If you don't want to recalculate unless the dirty flag is clear, then you probably don't want to do it until you pass validation and are ready to move forward in the flow. Could this be done as part of the post processing on your flow action? If it needs to be done as a declarative, but only on a "clean" item, perhaps in an activity called by a declare trigger or something would meet your needs.

Thanks,

Mike