2.5.3 On-Change scripts

On-Change script

This kind of a script is bound to a single data field and it is executed AFTER the value of this data field has changed. The purpose of the on-change script is to react to a data field change - for example by calculating and setting a value to another data field. Note that changing a value of a data field in a script can trigger an on-validate and/or an on-change script for that field. To avoid this effect, see Disabling events in scripts.

Example :

The following script checks the DBH/height ratio of a tree and warns the user if it is out of interval <0.6; 1.6>. The script takes values of the fields "DBH_mm' and "Height_m' as its input. Since there is no way to know which of these values will be measured last, there should be two On-Change scripts, one for each of these fields. The best solution here is to write only one On-Change script common to both fields, assign it to one field and reference it in the other.

The script assigned to the "DBH_mm' field:

// ---- Trees.DBH_mm On-Change script
var
  dbh, height: variant;
  ratio: double;

begin
  dbh := Trees['DBH_mm'];
  height := Trees['Height_m'];
  if (height <> null) and (dbh <> null) then
  begin
    if (height > 0) then
    begin
      ratio := (dbh / 10) / height
      if (ratio < 0.6) or (ratio > 1.6) then
        ShowMessage (Format('Warning! Slenderness ratio (%0.2f) is out of <0.6;1.6>.', [ratio]));
    end
    else
      ShowMessage('Warning! Tree has zero height.');
  end;
end.

The same script referenced in the "Height_m" field:

// ---- Trees.Height_m On-Change script
// {$REF_ATTRIBUTE Trees.DBH_mm}