What does the underscore mean before a variable in Swiftui in an init()?

Question

What does the _ mean before momentDate? Why is it needed?

@Binding var momentDate: Date
init(momentDate: Binding<Date>) {
    UIDatePicker.appearance().backgroundColor = UIColor.white
    self._momentDate = momentDate
}

Answer 1

The underscored variable name refers to the underlying storage for the Binding struct. This is part of a language feature called Property Wrappers.

Given one variable declaration, @Binding var momentDate: Date, you can access three variables:

• self._momentDate is the Binding<Date> struct itself.
• self.momentDate, equivalent to self._momentDate.wrappedValue, is a Date. You would use this when rendering the date in the view's body.
• self.$momentDate, equivalent to self._momentDate.projectedValue, is also the Binding<Date>. You would pass this down to child views if they need to be able to change the date.

For Binding, the "projected value" ($) is just self, and the difference between _ and $ is only in the access level. However, other property wrappers may project a different type of value (see the @SmallNumber example in the language guide).