Difference between revisions of "EvalRule Script"

Revision as of 02:15, 14 December 2008

Context: HL Kit … Kit Reference … Script Types 

Technical Details

Initial Context:	Pick
Alternate Context:	None
Fields Finalized?	No

The EvalRule script utilizes the following special symbols:

valid	Entry: Assumed the rule is not satisfied with a value of zero. Exit: Indicates whether the rule is satisfied (non-zero) or not satisfied (zero).
message	Entry: Contains the default message text if the rule is not satisfied. Exit: Contains the final message text to display if the rule is not satisfied.
summary	Entry: Contains the default summary text if the rule is not satisfied. Exit: Contains the final summary text to display if the rule is not satisfied.

Description

The EvalRule script is very similar to an Eval script, with the primary distinction being that the EvalRule script verifies a rule is observed instead of applying changes. This script is scheduled during the evaluation cycle, so it allows you to check the state of an actor at intermediate points in addition to at the end of evaluation. Intermediate checks make it possible to verify values before they are further modified by other aspects of the character.

An EvalRule script shares all the same behaviors of an Eval script, plus it adds the rule to be validated.

If the validation test is not satisfied, the message and summary for the rule are displayed within the validation report and validation summary for the actor, respectively. Since the message and summary are typically static requirements (e.g. an attribute must be a value of X or more), you will rarely need to modify them. However, there will be cases where you want to tailor the message and/or summary to provide more detailed information based on dynamic information. When this situations arise, simply set the "message" and/or "summary" special values to the desired contents and they will be used.

Since an EvalRule script is equivalent to an Eval script, it is possible to apply changes throughout the structural hierarchy via the script. However, it is generally a bad idea to do that, since the purpose of an EvalRule script is to validate a rule - not effect change. We therefore recommend that you avoid applying changes within EvalRule scripts.

the primary workhorse throughout any set of data files. This script is scheduled within the evaluation cycle and where you will orchestrate the virtually all of the effects for the game system. If a special ability confers a bonus to certain skills, you'll use an Eval script to apply them. If attributes confer adjustments to abilities, attacks, damage, or anything else, you'll use Eval scripts to apply them. Calculated abilities, such as attack ratings or casting powers will be determined through Eval scripts. The list is endless.

Every Eval script is associated with a particular thing. If an Eval script is defined for component, then it is inherited by the things which derive from that component. When a thing is added to a container, it becomes a pick, and new instances of every Eval script for that thing are created and managed by HL. You associate all of the primary behaviors with each pick via the Eval scripts defined for the underlying thing.

When invoked, an Eval script starts with its associated pick as its initial context. You are free to navigate through the hierarchy and effect changes anywhere you deem appropriate. However, all of the changes will typically either be made to the associated pick or be made to other objects based on facets of the associated pick.

As a general rule, all of the dynamic effects that are applied throughout your data files should be handled via Eval scripts. Because the timing of all Eval scripts is controlled, you can ensure that all of the different effects of different scripts are applied in a carefully defined sequence. For example, in the d20 System, adjustments to attributes (e.g. from magic items) must be applied before the bonuses conferred by attributes are calculated and applied.

Eval scripts provide a single, generalized mechanism through which you can accomplish just about anything. As a result, the actual behaviors of an Eval script will vary more widely than with any other type of script. If there is one type of script to get comfortable with first, it is definitely the Eval script.

Example

Due to the wide range of behaviors that can be applied via Eval scripts, there is no one good example. The XML below shows three separate Eval scripts that are defined as part of the basic handling of equipment within the Sample data files. Each script is scheduled to be performed at a different time during the overall evaluation cycle, with the timing ensuring that each behavior is properly interleaved with other behaviors triggered by other objects.

<!-- All melee weapons get the appropriate tag -->
<eval index="1" phase="Setup" priority="5000"><![CDATA[
  perform assign[Armory.Melee]
  ]]></eval>

<!-- Calculate the net attack roll for the weapon -->
<eval index="2" phase="Final" priority="7000" name="Calc wpNetAtk"><![CDATA[
  field[wpNetAtk].value = #trait[skMelee] + field[wpBonus].value + field[wpPenalty].value
  ]]></eval>

<!-- Prepend any derived special notes to the appropriate field -->
<eval index="3" phase="Render" priority="2000"><![CDATA[
  var special as string

  ~assign any appropriate special notes to the "special" variable here

  ~prepend any existing special details with the notes for this weapon
  if (empty(special) = 0) then
    if (field[wpNotes].isempty = 0) then
      special &= ", "
      endif
    field[wpNotes].text = special & field[wpNotes].text
    endif
  ]]></eval>