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.

Example with Time (DateTime) and Decimal

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

Note the addition of :00 to the text if it is short, this help the user out if entering just the hour part.

'1900-01-01' is an ugly hack to prevent storing the current date as part of the time entry. It would work anyhow, but this is slightly better.

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 (OclSet)

Enable use of both comma and dot as decimal seperator. Round to 2 decimals to avoid fractions from the web browser double type. (JavaScript don't have Decimals, only Float)

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

/ToTimeText: String?

The to-time is all calculated and only exist when showing and entering values.

OCL

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

EAL (OclSet)

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