Communication channels

Simple one-time requests

• Message format: JSON
  (see https://stackoverflow.com/questions/8593896/chrome-extension-how-to-pass-arraybuffer-or-blob-from-content-script-to-the-bac; also verified manually)

Website -> background script

• Receiver (background script): Setup listener via chrome.runtime.onMessageExternal.addListener()
• Sender (website): Call chrome.runtime.sendMessage() with the extension ID as the first argument
• Limitations: websites must be whitelisted in manifest.json's externally_connectable where at least a second level domain must be specified.

Background script -> extension

• Receiver (extension): Setup listener via chrome.runtime.onMessage.addListener()
• Sender (backround script): Call chrome.runtime.sendMessage()

Event based messaging

Content script -> Background script -> extension

Step 1:

• Receiver (content script): Setup listener via window.addEventListener() (addEventListener)
• Sender (website): Call window.dispatchEvent() (dispatchEvent)
• Format: structured clone algorithm
• Note: Don't use the messaging system via Window​.post​Message() and type "message" events to avoid any kind of cross-origin communication.

Step 2:

• Receiver (extension): Setup listener via chrome.runtime.onMessage.addListener()
• Sender (backround script): Call chrome.runtime.sendMessage()

This solution requires explicit matching of request/response by ID since events don't have a back channel. IOV-Core's JsonRpcClient could be used to simplify that.