build

Basic builder for a Form.

To build a basic non-composite Form (e.x. something like a text entry field), you can simply use the FormScope.currentValue property in scope of the builder and bind that to your UI:

class StringEntry(val fieldLabel: String): Form<String> by (Form.build {
    var text by currentValue

    Column {
        Text(fieldLabel)
        BasicTextField(
            value = fieldLabel,
            onValueChanged = { newText ->
                text = newText
            }
        )
    }
})

However, one of the main advantages of Forms over raw composable functions is the ability to embed sub-forms when building the entry form for a larger type without much ceremony.

Like most things in Jetpack Compose, Form works with immutable data types. We use the arrow-optics ksp plugin and the @Optics annotation to automatically generate Lenses that let us easily reference a "part" of our form. For example, given the following:

@Optics
data class User(
    val firstName: String,
    val lastName: String,
    val birthday: LocalDate
) {
    companion object
}

we can do the following to build a user entry form:

class UserEntryForm(): Form<User> by (Form.build {
    StringEntry("First name:")
        .bind(User.firstName)

    StringEntry("Last name:")
        .bind(User.lastName)

    LocalDateEntry()
        .bind(User.birthday)
})

which can then be used like any other composable in the rest of your application with .contents().