[VB6, Vista+] Core Audio - Monitor for disabled/active, default, and property changes

[fafalone]

Monitoring audio hardware for changes

At first glance, the IMMNotificationClient interface looks like it's simple and straightforward. And in the IDE it is, but it turns out when compiled all normal ways of accessing the passed values results in a crash. Getting around this involves putting the variables into a public structure that can only be accessed and processed by subclassing the main form and using PostMessage to send the message to process things.

Setting up the notifications is easy enough:

Private Sub Command1_Click()
Command1.Enabled = False
hFrm = Me.hwnd
Subclass2 Me.hwnd, AddressOf F1WndProc, Me.hwnd
SetNotify
End Sub

Private Sub SetNotify()
Dim hr As Long

If (mDeviceEnum Is Nothing) Then
    Set mDeviceEnum = New MMDeviceEnumerator
End If

If (cMMNotify Is Nothing) Then
    Set cMMNotify = New cMMNotificationClient
    hr = mDeviceEnum.RegisterEndpointNotificationCallback(cMMNotify)
    AddMsg "Registered cMMNotify, hr=" & Hex$(hr)
End If
End Sub

Then the notification class, cMMNotify, is set up just to assign the data and fire off a window message:

Private Sub IMMNotificationClient_OnDeviceStateChanged(ByVal pwstrDeviceId As Long, ByVal dwNewState As DEVICE_STATE)
tPVC.lpszID = pwstrDeviceId
tPVC.pid = dwNewState
PostMessage hFrm, WM_MMONDEVICESTATECHANGE, 0&, ByVal 0&
End Sub

OnPropertyChanged, which uses a PROPERTYKEY, is even weirder. See the class module comments for more details.

Our custom WM_MMx messages then get handled in a normal subclassing WndProc:

Select Case uMsg
    Case WM_MMONDEVICESTATECHANGE
        sPtr = LPWSTRtoStr(tPVC.lpszID, False)
        Form1.AddMsg "StateChange: DeviceID: " & sPtr
        Form1.AddMsg "Name=" & Form1.GetDeviceName(sPtr)
        Form1.AddMsg "New status=" & GetStatusStr(tPVC.pid)

As the code suggests, this project also shows how to convert a device ID to the friendly name.

Since all messages are handled with API posting, the project should be free of instabilities that cause a crash. I haven't had crashes in testing both for the IDE and once compiled. Always unregister the notification callback before unloading the form containing an instance of the class, or the app crashes on exit (the demo project handles this automatically in Form_Unload)

Requirements
-Windows Vista or newer
-oleexp.tlb v4.0 or higher (only needed for the IDE)
-mPKEY.bas (included in oleexp download)

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twinBASIC 64bit-compatible version can be found at: https://github.com/fafalone/CoreAudioDemos