The Transform Operator

Mappie can transform source parameters using the operator transform. This is useful when we want to change the source value.

Suppose we have the data class Person

data class Person(val name: String, val dateOfBirth: LocalDate)

having a date of birth dateOfBirth and we have the data class PersonDto which has the property age

data class PersonDto(
    val name: String, 
    val age: DateTimePeriod, 
)

we can create a mapper between Person and PersonDto via

object PersonMapper : ObjectMappie<Person, PersonDto>() {
    override fun map(from: Person): PersonDto = mapping {
        PersonDto::age fromProperty Person::dateOfBirth transform { dateOfBirth ->
            Clock.todayIn(TimeZone.currentSystemDefault()).periodUntil(dateOfBirth) 
        }
    }
}

Handling Nullability #

The transform operator can also come in handy when mapping nullable values to non-nullable values.

Suppose we have the data class Dog having the property name which can be null

data class Dog(val name: String?)

and we have the data class DogDto for which the constructor parameter name is non-nullable.

data class DogDto(val name: String)

We create a mapping between Dog and DogDto via

object DogMapper : ObjectMappie<Dog, DogDto>() {
    override fun map(from: Dog): DogDto = mapping {
        DogDto::name fromProperty DogDto::name transform {
            it ?: "unknown"
        }
    }
}

which will set the target DogDto.name to "unknown" if Dog.name is null.