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 } 
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2 Answers
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._momentDateis theBinding<Date>struct itself.self.momentDate, equivalent toself._momentDate.wrappedValue, is aDate. You would use this when rendering the date in the view's body.self.$momentDate, equivalent toself._momentDate.projectedValue, is also theBinding<Date>. You would pass this down to child views if they need to be able to change the date.
For Binding, the "projected value" ($) 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).
3 Comments
Chris
This comment is very helpful but I cannot find an explanation specifically mentioning the underscore syntax (e.g.
_somePropery) anywhere in the documentation. Your note "self._momentDate is the Binding<Date> struct itself." is the only information I can find on this. I am curious how this works internally. Is the underscore the only syntax available ? nothing similar to someProperty.wrapperItself ?
jtbandes
The underscores are mentioned (albeit a bit indirectly) in this documentation: docs.swift.org/swift-book/LanguageGuide/Properties.html#ID617
Anthony
This answer is not helpful, so I'm not sure why the other comments say as much. Without a deep understanding of the concepts, this solution speaks at a level which is inaccessible to the reader. It's circular because you need to already know what is going on to understand the answer.
A Binding is one of various Property Wrappers.
When you use a @PropertyWrapper on a var, it synthesizes convenience code.
That includes automatically creating a private var named _yourPropVar, whose type is YourPropertyWrapper (e.g. Binding).
When you prefix that with underscore, you're referencing the synthesized private variable - the Property Wrapper itself.
When you reference yourPropVar instead, without the underscore, it's accessing the synthesized wrapped value getter instead.
