Why does MutableValue not emit distinct values?

[eosobande]

val distinct = MutableValue(0)

distinct.subscribe { println("SUB: $it") }

distinct.update { 0 }

// RESULT
// SUB: 0
// SUB: 0
// prints twice, one for the initial subscription and again for the updated value

MutableValue::compareAndSet does not check if the value is changing or not, it just sets the new value and emits.

Proposed FIX in MutableValue::setValue:

private inline fun setValue(value: T, predicate: (T) -> Boolean = { true }): Boolean {
        lock.synchronized {
            if (!predicate(_value)) {
                return false
            }

            if (_value == value) return true // Don't do anything if value is not changing

            _value = value

            if (isEmitting) {
                return true
            }

            isEmitting = true
        }

        emit()

        return true
}

[arkivanov]

That's because MutableValue is similar to BehaviorSubject from RxJava. It's not necessarily a state holder, but rather an arbitrary value holder. Also, the proposed change would be a behaviour change, which is only possible with a major release (e.g. 4.0.0).

This however shouldn't not be an issue, since UI frameworks should be handling this case just fine. E.g. with Jetpack Compose, MutableValue is observed via mutableStateOf.

[eosobande]

Thanks @arkivanov that answers my question