spotify-rcd

On a standard installation of macOS, pressing the playback control keys on an Apple keyboard opens iTunes if no other media applications are open. The purpose of this project is to patch this behavior such that Spotify is opened instead.

⚠️ Compatibility

This is the README for the legacy version of the project. The legacy version is known to work on a few older versions of macOS, but the exact list of supported versions is not known.

Based on issues and commit dates, it seems reasonable to expect the legacy version to support macOS Mavericks (10.9) through Sierra (10.12). It's possible that some older versions of macOS could be supported as well. Compatibility with High Sierra (10.13) and newer is known to be broken. (Adding support for a newer version of macOS is being tracked by #3.)

This tweak has been tested primarily using the internal keyboard of a MacBook Pro, but the tweak should also work with external keyboards. Compatibility with the MacBook Pro's Touch Bar is unknown.

How it works

spotify-rcd works by injecting itself into the com.apple.rcd system launch agent, after which it uses method swizzling to alter the agent's behavior.

(I'd recommend reading this section so that you know what the patch is actually doing to your system and what risks it comes with, but you can skip to the installation section if you're not interested in the technical details.)

Injection

In order to load spotify-rcd into the system agent, we take advantage of DYLD_INSERT_LIBRARIES, an environment variable that allows us to load custom images into other processes during launch. (At the moment, this injection vector works fine for the needs of this project. However, additional restrictions are placed on DYLD injection as part of each release of macOS, so it's likely only a matter of time before a new injection vector will be needed.)

For DYLD_INSERT_LIBRARIES to work, we have to find some way of setting the environment variable in the target process before it is launched. Conveniently, macOS allows you to persistently unload system launch agents and replace them with your own patched versions as long as the labels are different. As such, all we need to do for this to work is copy the system launch agent configuration, alter the label to something unique, add our environment variable, and then use launchctl to persistently disable the system agent and enable our patched version.

Swizzling

Now that spotify-rcd has been injected into the system agent, we actually need to alter the agent's behavior to do what we want. This is where method swizzling comes in. By intercepting all AppleScript execution, we can watch for any requests to launch iTunes and replace those with requests to launch Spotify instead.

Caveats

As you can imagine, all of this can be a somewhat fragile process. It's possible that any release of macOS can break the injection or the swizzling. Even so, this patch tends to be relatively safe.

Unlike other versions of this tweak, installation doesn't require modifying any system files. The tweak can also be reverted by simply disabling the patched agent and enabling the unpatched agent. Given that com.apple.rcd isn't a critical system service, this makes the liklihood of serious side-effects very unlikely.

Installation

Automated Installation: This project contains an install.sh script that will executes all of the necessary commands to build, install, and activate the tweak. Simply run the script in Terminal to use it:
```
$ git clone https://github.com/rpendleton/spotify-rcd.git
$ cd spotify-rcd
$ ./install.sh
```
Manual Installation: If you prefer to install the plugin manually, it is possible to do so.
- Compile the tweak and copy the resulting bundle to /Library/Application Support/Tweaks
- Copy /System/Library/LaunchAgents/com.apple.rcd.plist to /Library/LaunchAgents/com.apple.rcd.patched.plist
- In the copied plist, change the Label to com.apple.rcd.patched
- Add the following section to the copied plist:
```
<key>Disabled</key>
<true/>
<key>EnvironmentVariables</key>
<dict>
        <key>DYLD_INSERT_LIBRARIES</key>
        <string>/Library/Application Support/Tweaks/SpotifyRCD.bundle/Contents/MacOS/SpotifyRCD</string>
</dict>
```
- Unload the system LaunchAgent, and load the modified one:
```
$ launchctl unload -w /System/Library/LaunchAgents/com.apple.rcd.plist
$ launchctl load -w /Library/LaunchAgents/com.apple.rcd.patched.plist
```

Uninstallation

To disable the tweak, simply unload the modified plist and load the original:

$ launchctl unload -w /Library/LaunchAgents/com.apple.rcd.patched.plist
$ launchctl load -w /System/Library/LaunchAgents/com.apple.rcd.plist

After the tweak has been disabled, you can completely uninstally it by simply deleting the tweak bundle:

$ rm -rf "/Library/Application Support/Tweaks/SpotifyRCD.bundle"

rpendleton/spotify-rcd