Derived attributes use and OCL expression to calculate the presentation of another attribute.

Derived settable attributes use EAL to do the reverse, i.e. setting the source attribute from the entered value.

This opens up special handling of user input and "user shortcuts" in the UI. For example accepting only a number as a time entry.

This example uses FromTimeText, HoursText and ToTimeText for user input, but only FromTime and Hours for storing information in the database.

/Date is a derived attribute for easy access to a neighboring class' containing the date in DateTime format.

/EffectiveToTime is a derived attribute used a companion to the FromTime, again for symmetry in access the information.

Support function StringToTime()

To help the other function with conversion, a help function like this was created

if  aText.length <= 2 then
  DateTime.Parse('1900-01-01 ' + aText + ':00')
else
  DateTime.Parse('1900-01-01 ' + aText)
endif

Property inspector

Below is the property inspector setting for an derived settable attribute. Note the AttributeMode and then the Ocl and OclSet code.

/FromTimeText: String?

OCL

if self.FromTime.notNull then
  self.FromTime.formatDateTime('HH:mm')
else
  String.nullValue
endif

EAL (OclSet)

self.FromTime := self.StringToTime(vInputParameter)

/HoursText: String?

OCL

self.Hours.ToString('N2').Replace('.', ',')

EAL

self.Hours := Decimal.Parse(vInputParameter.Replace(',', '.')).Round(2)

/ToTimeText: String?

OCL

if self.FromTime.notNull then
  self.FromTime.AddHours(self.Hours.Value.ToDouble).formatDateTime('HH:mm')
else
  String.nullValue
endif

EAL

if self.FromTime.notNull then
  self.Hours := Decimal.Create(self.StringToTime(vInputParameter).Subtract(self.FromTime).TotalHours)
else
  self.FromTime := self.StringToTime(vInputParameter).AddHours(-self.Hours.ToDouble);
  0
endif