Inverse bidirectional binding for Boolean properties

[Tobias Oelgarte]

On 03.06.21 21:41, Michael Strauß wrote:
> Here's a simple thing I've come across frequently:
>
> Let's say you want to bidirectionally bind a CheckBox to a boolean
> property in a backend class (or any class that contains some kind of
> business logic).
>
> That's very easy, except if the backend property is worded in the
> "negative" (i.e. the frontend and backend values should be bound such
> that if one is true, the other is false).
>
> Since this is a very common scenario, I think it should be supported
> out-of-the-box in JavaFX by an addition to the
> `javafx.beans.binding.Bindings` class:
>
>      public static void bindInverseBidirectional(
>              Property<Boolean> property1,
>              Property<Boolean> property2);
>
> The semantics for any two bound properties should be as follows:
> 1. If one property is `true`, the other property is `false`.
> 2. If one property is `null`, the other property is also `null`.
>
> Any thoughts?

I had a similar need to create a bidirectional binding that could 
additionally convert between two types. I came up with an utility class 
that provides the following method:

     public static <A, B> void bindBidirectional(
             Property<A> a,
             Property<B> b,
             Function<A, B> convertTo,
             Function<B, A> convertFrom);

See https://pastebin.com/uxjp30VH

In your case one could simply write:

     bindBidirectional(a, b, a -> a == null ? null : !b, b -> b == null 
? null : !a);

or even simpler without null handling:

     bindBidirectional(a, b, a -> !b, b -> !a);