LiveData extension function that prevents emissions from receiver until after trigger emits a value that satisfies predicate.

allowAfter.kt

/**
 * Prevent emissions from receiver until after [trigger] emits a value that satisfies [predicate].
 */
private fun <T, R> LiveData<T>.allowAfter(
    trigger: LiveData<R>,
    predicate: (R?) -> Boolean
) = object : MediatorLiveData<T?>() {

    private var isReady = false
    private var hasValue = false
    private var pendingValue: T? = null

    init {
        addSource(this@allowAfter) {
            hasValue = true
            pendingValue = it
            emit()
        }

        addSource(trigger) {
            if (predicate.invoke(it)) {
                isReady = true
            }
            emit()
        }
    }

    private fun emit() {
        if (isReady && hasValue) {
            value = pendingValue
        }
    }
}