How to bind properties between two classes with Combine?

Question

How to make a mechanism similar to binding in the view that refers to the Published in the ObservableObject class, but between 2 classes? That is, from class A, call the class B constructor, passing the prop1 from class A. And in class B prop2 exists, and these prop1 and prop2 should mirror each other.

PS: Attached code does not work, just visually show what I want ...

PPS: prop1 in ClassA and prop2 in ClassB, must be Published by condition. It's own views are also subscribed on it.

PPPS: Important to have any binding logic inside the ClassB, for easy reuse.

class ClassA: ObservableObject {
    @Published var prop1: Bool = false //When prop1 changed, it must be reflected in prop2
    let classB: ClassB

    init() {
        classB = .init(prop2: _prop1) //Should stay that simple, no setup here, passing only property, ClassB doesn't know about ClassA.
    }
}

class ClassB: ObservableObject {
    @Published var prop2: Bool //When prop2 changed, it must be reflected in prop1

    init(prop2: Published<Bool>) {
        _prop2 = prop2
    }
}

Answer 1

A simple but generic solution could look like the following.

Add the following extension:

extension Published.Publisher where Value: Equatable {
    mutating func link(with other: inout Self) {
        removeDuplicates().assign(to: &other)
        other.removeDuplicates().assign(to: &self)
    }
}

This only works with values that conform to Equatable, because otherwise you might end up in an endless loop.

Then call the extension in the initializer of ClassA:

classB.$prop2.link(with: &$prop1)

---

Original Answer(s)

A simple solution would be to put the following code into your init of ClassA:

$prop1
    .removeDuplicates()
    .assign(to: &classB.$prop2) // mirror changes of prop1 into classB.prop2
classB.$prop2
    .removeDuplicates()
    .assign(to: &$prop1) // mirror changes of classB.prop2 into prop1

However, I'd recommend removing prop1 from ClassA and accessing this value via classB.prop2 and classB.$prop2, respectively. This makes the dependency clearer and doesn't require storing and updating the value in two places.

---

Answer to question in comments regarding putting the logic in ClassB

To do this, you could add a function in ClassB that performs the linking:

fileprivate func linkProps(to classA: ClassA) {
    $prop2
        .removeDuplicates()
        .assign(to: &classA.$prop1) // mirror changes of prop2 into classA.prop1
    classA.$prop1
        .removeDuplicates()
        .assign(to: &$prop2) // mirror changes of classA.prop1 into prop2
}

And then call this function in the init of ClassA (and wherever else you need to):

init() {
    classB = .init(prop2: _prop1)
    classB.linkProps(to: self)
}

---

Answer to question in comments regarding keeping ClassB from knowing ClassA

This could be achieved by only passing the property wrapper's projected value into the function on ClassB:

fileprivate func linkProp2(to prop1: inout Published<Bool>.Publisher) {
    $prop2
        .removeDuplicates()
        .assign(to: &prop1) // mirror changes of prop2 into prop1
    prop1
        .removeDuplicates()
        .assign(to: &$prop2) // mirror changes of prop1 into prop2
}

The call in the initializer of ClassA would then look like this:

classB.linkProp2(to: &$prop1)