`ObservableMIDIManager` device list is not updated when devices change `isOffline` state.

[hekuli]

Please Confirm

• I have reviewed the MIDIKit Documentation which contains descriptive guides and extensive API reference
• I have searched Issues and Discussions to see if the same question has already been asked

macOS Version(s) Used to Build

macOS 15.x Sequoia

Xcode Version(s)

Xcode 16.x

Description

I expected that changes to the isOffline status for devices would automatically refresh for the Observable variation of MidiManager. This is somehow not the case.

To reproduce just create an ObservableMIDIManager. Loop over the devices in the SwiftUI view like in the example. Be sure to display the online/offline status via device.isOffline. Then turn on off any connected midi controller. The status never changes.

ForEach(mm.devices) { device in
  Text(device.name)
  Text(device.isOffline)
}

[orchetect]

Without diving back into the codebase, I have a couple theories why that may be the case. I'm busy this week but will try to have a look at this and come up with a solution next week.

[hekuli]

Thank you. And awesome library by the way. Really glad that you’re doing this.

…

On Tue, Apr 29, 2025 at 11:06 PM Steffan Andrews ***@***.***> wrote: *orchetect* left a comment (orchetect/MIDIKit#240) <#240 (comment)> Without diving back into the codebase, I have a couple theories why that may be the case. I'm busy this week but will try to have a look at this and come up with a solution next week. — Reply to this email directly, view it on GitHub <#240 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/A7XPJ7TFDPDSTXHJITKTSOL237SWLAVCNFSM6AAAAAB4BANE7KVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQNBQGI2DGNBRHA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[orchetect]

Apologies for the delay - I have been dealing with some family emergencies.

When SwiftUI sees the collection of devices change, it may only see stored properties change. There are only a handful of stored (cached) properties within MIDIDevice, including its name property.

Stored properties:
https://github.com/orchetect/MIDIKit/blob/fc908a10b315a64c739aa754ef6c9b03e7280d5b/Sources/MIDIKitIO/MIDIDevice/MIDIDevice.swift#L24-L30C30

Most properties on MIDIDevice, MIDIEntity and MIDIEndpoint are pulled, not pushed. So when you use device.isOffline it is a computed property, not a stored property of the type.

Pull-only properties:
https://github.com/orchetect/MIDIKit/blob/main/Sources/MIDIKitIO/MIDIIOObject/MIDIIOObject%20Properties.swift

The reason for this is that name is by far the most frequently accessed property so for performance reasons it makes sense for it to be asked from the system and stored (cached) within the type. But there are many dozens of properties available, and it would be somewhat inefficient to ask the system for all of them for every device or endpoint each time these collections change in the system when 99% of the time they are never even used.

There are a few ways to address this, and it would be nice to come up with a 1st-party solution that can be included in MIDIKit. Overall, the observation features/implementation in MIDIKit is sparse and fairly basic at present.

[orchetect]

For now, it may involve something like this:

struct ContentView: View {
    @State private var midiManager = ObservableMIDIManager( ... )
    @State private var deviceInfo: [MIDIDeviceInfo] = []

    var body: some View {
        ForEach(deviceInfo) { device in
            Text(device.name)
            Text(device.isOffline ? "Offline" : "Online")
        }
        .onChange(of: midiManager.devices.devices, initial: true) { _, newValue in
            updateDeviceInfo(from: newValue)
        }
    }

    private func updateDeviceInfo(from devices: [MIDIDevice]) {
        deviceInfo = devices.map {
            MIDIDeviceInfo(id: $0. uniqueID, name: $0.name, isOffline: $0.isOffline)
        }
    }
}

enum MIDIDeviceInfo: Equatable, Hashable, Identifiable, Sendable {
    var id: MIDIIdentifier
    var name: String
    var isOffline: Bool
}

[hekuli]

Thanks I will try.