Development Overview

The codebase is split into two projects: BGMDriver, a userspace Core Audio plugin that publishes the virtual audio device, and BGMApp, which handles the UI, passing audio from the virtual device to the real output device and a few other things. The virtual device is usually referred to as "BGMDevice" in the code. Currently the only code shared between the two projects is BGM_Types.h.

Real-time Constraints

One slightly tricky part of this project is the code that runs with real-time constraints. When the HAL calls certain functions of ours--the IO functions in BGMDriver and the IOProcs in BGMApp--they have to return within a certain amount of time, every time. So they can't do things that aren't guaranteed to be fast, even if they almost always are.

For example, they can't dynamically allocate memory and they can't use an algorithm with a fast enough average-case run time if its worst-case is too slow. In the BGM_Device class, they can't lock the state mutex because a non-real-time function might be holding it and there's no guarantee it'll be released in time.

If you're interested, have a look at Real-time Audio Programming 101: Time Waits for Nothing.

BGMDriver

The BGMDriver project is an audio driver for a virtual audio device called Background Music Device, which we use to intercept the audio playing on the user's system. The driver processes the audio data to apply per-app volumes, see if the music player is playing, etc. and then writes the audio to BGMDevice's input stream. It's essentially a loopback device with a few extra features.

There are quite a few other open-source projects with drivers that do the same thing--Soundflower is probably the most well known--but as far as I know all of those drivers were either written as kernel extensions or using AudioHardwarePlugIn, which is now deprecated because of issues with the OS X sandbox. The Apple sample code we started from gives us most of the same functionality and uses the latest Core Audio APIs. The other projects are still definitely worth reading, though. There's a list in the README with a few of them.

BGMDriver is an AudioServerPlugin (see CoreAudio/AudioServerPlugIn.h) based on Apple's SimpleAudio example. An AudioServerPlugIn is a Core Audio driver that runs in userspace and is "hosted" by the HAL, which is nice because the HAL handles a lot of things for us. Only running in userspace means our bugs shouldn't be able to cause a kernel panic (though we can definitely crash the audio system) and users don't have to restart after installation. It also makes debugging much less painful and is less insecure. In addition to running in userspace, the plugin also runs in a fairly restrictive sandbox.

BGM_PlugInInterface.cpp is where you'll find the entry point functions that the HAL calls, so it's a good place to start. Those functions are our implementation of the interface defined by AudioServerPlugIn.h. They're mostly boilerplate and error handling code, and largely unchanged from the sample code. They call BGM_PlugIn or BGM_Device (and probably other subclasses of BGM_Object in future) to do the actual work.

BGM_Device is by far the largest class and should really be split up, but most of its code is very straightforward. The simple parts mostly handle audio object properties, which are often static. An audio object is a plugin, device, stream, control (e.g. volume), etc. and is part of the HAL's object model. Each object is identified by an AudioObjectID. When the HAL asks us for the value of a property (or the size of the value, whether the value can be set, etc.) BGM_Device handles properties relating to our virtual device, its controls and its streams.

The rest of BGM_Device mostly handles IO. During each IO cycle, the HAL calls us for each phase of the cycle we support--reading input, writing output, etc. When the user has our device set as default, we receive the system's audio during the read-input phase and store it in our ring buffer. Then in the write-output phase (technically, the ProcessOutput and WriteMix phases) we process the data and write from our ring buffer to our output stream. By "process" I just mean that we apply the per-app volume, keep track of whether the audio is audible or not, and other things like that.

BGMDriver's IO functions have to be real-time safe, which means any functions they call do as well. Some other functions need to be real-time safe as well because they access data shared with the IO functions and have to do so on a real-time thread to avoid priority inversion.

Building and Debugging

To test your changes, build Background Music Device.driver, either inside Xcode (set the active scheme to "BGMDevice", go Product > Build For > Running and look in the products folder) or with something like

xcodebuild -project BGMDriver/BGMDriver.xcodeproj -configuration Debug

And then run BGMDriver/BGMDriver/quick_install.sh to install. Or if you'd rather install manually, copy Background Music Device.driver to /Library/Audio/Plug-Ins/HAL and restart coreaudiod.

Before you build, Xcode might show incorrect warnings on the #pragma clang assume_nonnull lines for some reason. They go away after you build and don't seem to cause any problems.

The following debug instructions stopped working in El Capitan. System Integrity Protection stops LLDB from attaching to coreaudiod. I don't know of a workaround except to disable SIP.

To debug in Xcode,

edit the BGMDevice scheme and
- set the build configuration to Debug,
- set "Executable" to coreaudiod in /usr/sbin (you can use shift+cmd+G to get there),
- check "Debug Executable",
- set the "debug as" user to root,
- and select "wait for executable to be launched",
set BGMDevice as the active scheme,
build and install,

stop coreaudiod

sudo launchctl unload /System/Library/LaunchDaemons/com.apple.audio.coreaudiod.plist

run in Xcode,

start coreaudiod

sudo launchctl load /System/Library/LaunchDaemons/com.apple.audio.coreaudiod.plist

Xcode should attach to the coreaudiod process when it starts running.

Debug logging is to syslog by default. Console.app is probably the most convenient way to read it.

HALLab

Apple's HALLab tool can be useful for inspecting the driver's properties, notifications, etc. It's in the Audio Tools for Xcode package, which you can find in the Apple developer downloads.

BGMApp

BGMApp is a fairly standard Cocoa status-bar app for the most part. The UI is simple and mostly built in Interface Builder.

At launch, BGMApp sets BGMDevice as the system's default device and starts playing the audio from BGMDevice through the actual output device. Usually that's the device that BGMDevice replaced as default. BGMApp sets that device back as the default on exit.

There's a small amount of state data persisted by BGMApp itself. I think currently it's just whether "Auto-pause iTunes" is checked. Most state (app volumes, which music player to pause, etc.) is stored by BGMDriver.

BGMApp communicates with BGMDriver through HAL notifications. That is, except for the audio data, which is sent through BGMDevice's output stream. For example, when an app other than the music player starts playing audio, BGMDriver sends out a notification saying that BGMDevice's kAudioDeviceCustomPropertyDeviceAudibleState property has changed. BGMApp receives the notification from the HAL and decides whether it should pause the music player.

Our custom notifications are defined/documented in BGM_Types.h, which is shared between the two projects.

BGMApp and BGMDriver could also communicate through XPC or shared memory (though I haven't tried the latter) but so far notifications seem to be all we need.

BGMApp also keeps the output device in sync with BGMDevice. For example, since BGMDevice is set as the default device, when the user changes their system volume only BGMDevice's volume will change. So BGMApp listens for changes to BGMDevice's volume and sets the output device's volume to match.

The only code in BGMApp that has to be real-time safe is in BGMPlayThrough's IOProcs, InputDeviceIOProc and OutputDeviceIOProc, which don't do very much. The most complicated part of BGMApp is probably pausing/reducing IO when no other processes are playing audio, which is also handled in BGMPlayThrough.

Building

Build and run Background Music.app either inside Xcode or with something like

xcodebuild -project BGMApp/BGMApp.xcodeproj -configuration Debug
open "BGMApp/build/Debug/Background Music.app"

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