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2019-06-15 17:18:38 +01:00
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IMPLEMENTATION.md
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# OUTDATED
# 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/contentSetup.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` handler that intercepts requests to Google's [Cast SDK library](https://developers.google.com/cast/docs/developers#chrome_sender_api_library).
The background script registers a `webRequest.onBeforeRequest` listener that intercepts requests to Google’s Cast 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).
When a request is intercepted, the `shim/content.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.js` script.
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 request is then redirected to the shim bundle (`shim/index.js`) which creates the `window.chrome.cast` API object.
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 `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.
The request is then redirected to the shim bundle (`shim/index.ts`) which creates the `window.chrome.cast` API object.
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 `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 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.
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…).
## 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.
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 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.
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