How to update the list of inputs and outputs

[FDelporte]

I'm using this to get the list of MIDI inputs and outputs:

private val access: MidiAccess = getMidiAccessApi("JVM") 

private fun getMidiAccessApi(api: String?) = when (api) {
    "EMPTY" -> EmptyMidiAccess()
    "JVM" -> JvmMidiAccess()
    else -> JvmMidiAccess()
}
       
access.inputs.iterator().asSequence().toList()

When I (dis)connect a device while the app is running, it doesn't get added to this list, and I need to restart the application. How can I "force" the access to refresh it list of devices?

[atsushieno]

JvmMidiAccess.inputs invokes MidiSystem.getMidiDeviceInfo() and MidiDevice.getTransmitters(). I believe they both return a cold snapshot list. It invokes MidiDevice.getTransmitters() for each MidiDevice, and each invocation would happen only when the previous results from getTransmitter() are handled, so it would feel like partially cold though.

If you get JvmMidiAccess.inputs in another list, that would give you the updated snapshot list at the accessed time.

[FDelporte]

Sorry, don't understand exactly what I need to call to refresh the list...

[atsushieno]

Oh, okay let me describe in other words: MidiAccess.inputs is a Kotlin property, which would be mapped to Java getInputs() method. With the code example below:

val access = JvmMidiAccess()
val inputs1 = access.inputs // this creates a snapshot of the list when this code is run.
val list1 = access.inputs.iterator().asSequence().toList() // or simply, access.inputs.toList()

// connect your MIDI device to your PC here.
...

val inputs2 = access.inputs // this creates a snapshot of the list when this code is run.
val list2 = access.inputs.iterator().asSequence().toList()

list1 "frozen" when (Sequence<T>).toList() is invoked (more specifically, when iterator() is invoked), where "frozen" means that the list of the iterated devices is fixed at the state when you invoked it.

When you try to get (Jvm)MidiAccess.inputs property after you connected your MIDI device, this getInputs() calls MidiSystem.getMidiDeviceInfo() (in javax.sound.midi API), which returns the latest list of devices to you, and our MidiAccess.inputs translates the resulting ports (in the devices in the device list).

There is no caching at ktmidi side, so if you still do not see the new device in the updated list, you are likely seeing outdated list (in this case, list1). This kind of problem could occur on Jetpack Compose where your code isn't actually run.

That's what I meant by get JvmMidiAccess.inputs in another list (input2 in the example above).

[FDelporte]

I rewrote my code to this, to always use a new JvmMidiAccess at each request of the inputs, but I still don't get new connections until the app restarts...

    fun getInputs(): List<MidiPortDetails> {
        logger.info(
            "Test input list: {}",
            JvmMidiAccess().inputs.toList().stream().map { i -> i.name }.collect(Collectors.joining(","))
        )
        return JvmMidiAccess().inputs.toList()
    }

[atsushieno]

It does not matter whether you create a new JvmMidiAccess or not - inputs from the same JvmMidiAccess instance would result in the same. You need to call your getInputs() and get a refresh (up to date) list again after you connected or disconnected your MIDI device.

I'm sure you will have the same problem if you use javax.sound.midi directly.

[FDelporte]

Yes, that's what I do... but still get the old list...

[atsushieno]

We figure it out onsite (kind of!)

[atsushieno]

We figured out that it is rather an upstream javax.sound.midi issue. https://stackoverflow.com/questions/41123620/update-list-of-midi-devices-in-java