System Media Controls (SMC) refers to the system across Chromium that allows playing media to show up in OS surfaces. These OS surfaces typically display what is playing and provide some controls for the user to control the playing media directly.
System Media Controls as a result has associated components across the //chrome/browser //content boundaries. This is required because SMC requires two way communication between a renderer process, the browser process and the operating system. For example, a web page may begin to play a new track which needs to be reflected in the OS information display. In the reverse direction, the user may ask to pause media from the OS, which travels to the browser process, which then must be able to pause the media on the web page.
Normally, SMC is a singleton and all Chrome tabs and windows share an instance. This works reasonably well and in most typical usages this is enough.
Since dPWAs strive to appear as standalone applications, it makes sense to make dPWAs have their own SMC instances that allows them each to look like a separate application is playing audio rather than just sharing the Chromium instance.
Instanced SMC achieves this by orchestrating an instance of SMC for each dPWA. Each instance is able to communicate to the OS independently, and looks like a separate standalone app - and is able to control the media playing for that specific dPWA.
Instanced SMCs for dPWAs has been enabled on MacOS and Win since m130.
For the browser (i.e. not dPWAs), MediaKeysListenerManagerImpl creates the singleton SMC object which is not stored inside the WebAppSystemMediaControlsManager.
This singleton tracks whatever most recently active media is playing within the browser, it will not track media playing in PWAs.
For dPWAs, WebAppSystemMediaControlsManager is the primary coordinator class who holds all the instanced system media controls. It knows if a dPWA already has an associated system media controls connection or whether it needs to be created.
This manager in turn holds a collection of objects for each dPWA which is used by that dPWA to communicate with the OS.
//components/system_media_controls contains a lot of the platform specific code to implement this functionality.
code in //content/browser/media is used to be notified of events happening in the renderer, some notable ones include:
active_media_session_controller.h is used to control instances of playing media. System Media Controls uses this to either track specific dPWAs playing media or implement the browser singleton that follows the active media playing in the browser.
system_media_controls_notifier.h receives messages from media playing and notifies the corresponding SystemMediaControls object in order to update the OS.
