fx_cast/IMPLEMENTATION.md

# Extension Lifetime

## Shim Initialization

The background script registers a `webRequest.onBeforeRequest` handler that intercepts requests to Google's [Cast SDK library](https://developers.google.com/cast/docs/developers#chrome_sender_api_library).

When a request is intercepted, the `shim/content.js` script is executed in the content script context. This facilitates message passing across [content/page script isolation](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Sharing_objects_with_page_scripts) (the shim itself is executed in the page context since it interacts substantially with page scripts).  

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.js` script.

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

The `shim/content.js` script creates a message port connection (named `shim`) to the background script through which messages from the shim (except `popup:/` messages) are forwarded to the background script. The connection triggers the background script to spawn a bridge application instance. The tab/frame ID is stored and used to associate shim instances with popup windows.

Messages are forwarded to the bridge and other parts of the extension via the background script (except for popups which make a direct connection to the shim).

The `shim:/initialized` message is sent to the shim and the `window.__onGCastApiAvailable` API 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 the `bridge:/discover` message to the bridge and triggers network discovery. The bridge sends `shim:/serviceUp` messages for any discovered devices with device info (address, port, label, etc...) which are stored in the `state.receiverList` array.


## User Interaction

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

The popup window is created and the ID of the shim is stored as the popup opener (`popupShimId`). The popup script creates a message port connection (named `popup`) to the background script and receives a `popup:/assignShim` message containing the tab/frame ID of the opener shim.

The popup creates a message port connection to the shim, sends a `shim:/popupReady` message and receives a `popup:/populateReceiverList` message in response containing the `state.receiverList` and type of session (sender app, media, or screen mirroring).

Once the user selects a receiver device to cast to, the popup sends a `shim:/selectReceiver` message to the shim which then 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](https://github.com/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)

<img src="diagram.svg" width="770">

| 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. |