fx_cast/IMPLEMENTATION.md

Extension Lifetime

A bridge application instance (statusBridge) is created to keep track of receivers’ statuses. This is expected to exist throughout the lifetime of the extension and will automatically reconnect if unexpectedly disconnected.

The shim/content.ts content script is registered for all pages. It creates an empty window.chrome object in the page context since some sites may expect it to exist. It also intercepts any src attribute changes on <script> elements where the cast API may be loaded directly from a chrome-extension:// URL, then sets them to the regular cast API script URL.

Shim Initialization

The background script registers a webRequest.onBeforeRequest listener that intercepts requests to Google’s Cast API library.

When a request is intercepted, the shim/contentBridge.ts script is executed in the content script context. This facilitates any message passing across content/page script isolation (the shim itself is executed in the page context, both for convenience — since it interacts substantially with page scripts — and security reasons).

Messages passed to the shim are custom events of type __castMessage. Messages passed back from the shim are custom events of type __castMessageResponse. Event listening and creation is handled by the shim/messageBridge.ts module.

The shim/content.ts script creates a message port connection (named shim) to the background script through which messages from the shim are forwarded. Messages are forwarded to the bridge and other parts of the extension via the background script.
The connection triggers the background script to spawn a bridge application instance. The tab/frame ID is stored and used identify shim instances.

The request is then redirected to the shim bundle (shim/index.ts) which creates the window.chrome.cast API object.

The shim:/initialized message is sent to the shim and the window.__onGCastApiAvailable callback is called with the availability state (bridge availability/compatibility is passed as the message data).

The cast API is now available to the web app.

The web app calls chrome.cast.initialize with an ApiConfig object containing the Chromecast receiver app ID to use. The shim sends a main:/shimInitialized message to the background script. The bridge sends shim:/serviceUp messages for any discovered devices with device info (address, port, label, etc…).

Extension Sender Apps

The initialization for built-in sender apps (media/mirroring) works slightly differently. They run in the content script context and import the shim, rather than running in the page script context.

The exported shim comes from shim/export.ts. Instead of setting the window.__onGCastApiAvailable callback, the module provides an init function which returns a promise. The shim/contentBridge.ts script is executed from that module and the initialization process is identical until the shim:/initialized message is received, at which point the promise is resolved.

User Interaction

A user will trigger casting through the web app interface and the app calls chrome.cast.requestSession. The shim sends a main:/selectReceiverBegin message to the background script to open the receiver selector.

The receiver selector is opened (popup browser window or native window depending on the receiver type). The receiver selector will handle user input and emit a selected, cancelled, or error event. Depending on the event, the background script will send a shim:/selectReceiverEnd, shim:/selectReceiverCancelled message to the shim. The shim:/selectReceiverEnd message contains info about the selected receiver and media type (ReceiverSelection).

The shim then makes a connection to the selected receiver device and establishes the session.

Shim Implementation

Cast SDK API calls are translated into Chromecast protocol messages and sent via node-castv2. Based on @GPMDP/electron-chromecast, so there are many similarities. The shim and the bridge exchange messages to implement API methods which require communication with the receiver device.

Session and Media objects have a counterpart object within the bridge. Some messages are routed directly to these objects. For Session, these are in the format bridge:/session/impl_<methodName>. For Media, it's bridge:/media/impl_<methodName>.

Messages (WIP)

No.	Subject	Origin	Destination	Description
1	shim:/initialized	background	shim	Sent once bridge has been created.
2	bridge:/initialize	shim	bridge	Starts network discovery.
3	shim:/serviceUp	bridge	shim	Sent once a receiver device has been found.
4	shim:/serviceDown	bridge	shim	Sent once a receiver device has been lost.
5	main:/openPopup	shim	background	Opens the receiver selection popup.
6	popup:/assignShim	background	popup	Provides popup with tab/frame ID for the opener shim so that it can make a direct connection.
7	shim:/popupReady	popup	shim	Sent once popup is ready to receive data.
8	popup:/populateReceiverList	shim	popup	Provides popup with current state.receiverList.
9	shim:/selectReceiver	popup	shim	Sent once a receiver has been selected.
10	popup:/close	shim	popup	Closes popup.
11	bridge:/session/initialize	shim	bridge	Initializes cast session with receiver device.
12	bridge:/session/close	shim	bridge	Closes cast session.
13	shim:/session/connected	bridge	shim	Sent once cast session has connected.
14	shim:/session/updateStatus	bridge	shim	Provides shim session with status updates.
15	bridge:/session/impl_addMessageListener	shim	bridge	Sends data to bridge for implementation of Session#addMessageListener method.
16	bridge:/session/impl_sendMessage	shim	bridge	Sends data to bridge for implementation of Session#sendMessage method.
17	bridge:/session/impl_setReceiverMuted	shim	bridge	Sends data to bridge for implementation of Session#setReceiverMuted method.
18	bridge:/session/impl_setReceiverVolumeLevel	shim	bridge	Sends data to bridge for implementation of Session#setReceiverVolumeLevel method.
19	bridge:/session/impl_stop	shim	bridge	Sends data to bridge for implementation of Session#stop method.
20	bridge:/media/initialize	shim	bridge	Initializes bridge media message handler.
21	bridge:/media/sendMediaMessage	shim	bridge	Sends media message to receiver device.
22	shim:/session/impl_addMessageListener	shim	popup	Response from bridge->shim counterpart message.
23	shim:/session/impl_sendMessage	shim	popup	Response from bridge->shim counterpart message.
24	shim:/session/impl_setReceiverMuted	shim	popup	Response from bridge->shim counterpart message.
25	shim:/session/impl_setReceiverVolumeLevel	shim	popup	Response from bridge->shim counterpart message.
26	shim:/session/impl_stop	shim	popup	Response from bridge->shim counterpart message.
27	shim:/media/sendMediaMessageResponse	shim	popup	Response from sendMediaMessage. Contains error status.
28	shim:/media/update	shim	popup	Provides shim media with status updates.
29	bridge:/startHttpServer	mediaCast	bridge	Starts HTTP server for serving local media to receiver device.
30	bridge:/stopHttpServer	mediaCast	bridge	Stops HTTP server.
31	mediaCast:/httpServerStarted	bridge	mediaCast	Sent once HTTP server has started.
32	updater:/updateData	options	updater	Sends initialization data to updater popup.