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

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

[![enter image description here][1]][1]


[1]:

The underscored variable name refers to the underlying storage for the [`Binding`](

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) struct. This is part of a language feature called [Property Wrappers](

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).

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](

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). However, other property wrappers may project a different type of value (see the `@SmallNumber` example in the [language guide](

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)).

[Official Docs](

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)

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.