| Target Property Data
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Select to determine how often Pega Platform recomputes the value of the Target Property (identified in the second key part).
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| Calculate Value
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Select an option to determine which events cause this rule to run:
Value
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Description
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Whenever inputs change
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(Default). Forward-chaining recomputation. The target property is not recomputed when the property is used as a source, only when one of the expression inputs change. If the target property is not present on the page when needed or is present but has no value, the Declare Expression rule does not run.
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When used if no value present
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Compute when the value is null, blank, zero, or does not yet appear on the page using backward chaining. Assuming the computation results are not null, later requests for the target property do not cause the Declare Expression rule to run.
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When used if property is missing
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Recomputation using backward chaining when the target property is not present on the clipboard.
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Whenever used
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Just-in-time recomputation even when the property already has a value, using backward chaining.
This choice ensures that the target property value matches the computed value at the start of every activity step, by evaluating the rule upon every read access of the target property. Because this choice can be costly in terms of performance, a warning icon appears when you save the rule form.
During backward chaining computations, if the Otherwise section of a decision tree or decision table, or the Default row in a map value referenced in a Declare Expression rule can be computed, but properties needed for other parts of the form are not defined, the Otherwise value is returned as the value of the rule.
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When applied by a Rule Collection
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The target property is computed only when the defined Declare Expression rule is invoked by a collection rule. The chained expressions also are evaluated if they too are set to "When applied by a Rule Collection."
For example, if the defined rule is TotalCostAfterTax=TotalCost*(1+TaxRate) where TotalCost=ItemCost*Quantity, then this expression is run only when a collection references this rule. In this case, TotalCost is also calculated if the properties on it were set to "When applied by a Rule Collection."
This option supports self-referential expressions, for example,
MyProperty=.MyProperty+1
This option is compatible with versions earlier than PRPC 7.1.6.
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When invoked procedurally
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The target property is computed only when an independent collection rule directly invokes the target property. If the defined Declare Expression rule is chained into another expression that is set to "When invoked procedurally," the chained expression will not run.
For example, if the defined rule is TotalCostAfterTax=TotalCost*(1+TaxRate) where TotalCost=ItemCost*Quantity, then this expression is run only when a collection references this rule. In this case, TotalCost is not calculated if the properties on it were set to "When invoked procedurally."
This option supports self-referential expressions, for example,
MyProperty=.MyProperty+1
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As a best practice, select Whenever used when the expression involves values from properties on multiple pages.
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Optional. Enter property references to identify additional properties (in the same page context) to be tracked. Order is not significant. If not blank, changes to any of the properties cause this rule to execute.
This array appears only when you select Whenever inputs change for the Calculate Value field.
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| Context Execution Behavior
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| Execute this Expression
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Select one of three settings to further define which situations cause this expression to be evaluated:
Value
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Description
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Only when the top-level page is of the Applies to class
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To execute this rule only if the property is on a top-level page.
Do not select this option if the Applies To key part of this rule identifies a class derived from the Embed- base class; by definition, pages of such classes are never at the top-level.
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When the top-level page is of the Applies To class, or one of the following
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To restrict execution to contexts in which the top-level page matches the Applies To class, one of a specified list, or any descendent classes in the class hierarchy.
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Regardless of any pages it is embedded in
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To execute the rule regardless of the page context, making this rule completely "context-free".
If you select this option, you cannot use the Top or Parent keywords in the computations on the Expressions tab, either explicitly or in decision rules referenced on that tab.
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The third selection can produce high execution frequencies. Select the most restrictive value that meets your application requirements.
If this Pega Platformsystem was upgraded from Version 5.2 or earlier, execution of context-free expressions is disabled by default. Enable them through changes to the prconfig.xml file or Dynamic System Settings. See More about Declare Expression rules.
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| Allowed top-level classes
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This array appears only if you select the second choice (When the top-level page...) for the Execute this Expression field.
Enter a list of classes. Execution occurs if the top-level page has a class that exactly matches any class on this list, or is a descendent of one of the classes on this list.
Do not list the Applies To key part here; that class is included automatically.
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