Swift Combine sink value is different from value of @Published property

Question

I'm running into an issue where in Combine where I have a boolean @Published property.

When I set it to true, the sink closure is run and I can look at the value being received. It's true. But when I compare it against the actual property that I am observing, they are different.

This bit of code can be simply run in a playground. I'm not sure how this works or why the values would be different

class TestModel {
    @Published var isLoading = false
}

let model = TestModel()

model.$isLoading.sink { (isLoading) in
    if isLoading != model.isLoading {
        print("values NOT same")
    }
}

model.isLoading = true

Answer 1

@Published publishes the new value before it actually modifies the stored value. This is documented:

When the property changes, publishing occurs in the property’s willSet block, meaning subscribers receive the new value before it’s actually set on the property.

If the @Published property is part of an ObservableObject, it also triggers the enclosing object's objectWillChange publisher before the new value is actually set on the property.

(This answer used to include a disassembly of part of the @Published wrapper, but that is no longer needed because Apple has now documented the behavior.)