구성 요소 간 양방향 메시징

Welcome! In this lesson, you'll master two-way messaging, a powerful way for different parts of your browser extension to communicate effectively.

Unlike one-way messages, two-way messaging allows a sender to not only send a request but also to receive a specific response back from the receiver.

Two-way communication follows a simple request-response model:

• A component (e.g., a popup) sends a message (the request).
• Another component (e.g., the background script) receives and processes it.
• The receiving component then sends a message back (the response).
• The original sender receives this response and acts on it.

To send a request and expect a response, you use chrome.runtime.sendMessage() with an optional callback function. This callback will execute once the response is received.

Try running this example from a popup or content script:

The receiving component (often your background service worker) listens for messages using chrome.runtime.onMessage.addListener(). This listener receives the request, sender, and a special sendResponse function.

To send a synchronous response, simply call sendResponse() with your data. The message channel will then close.

What if your response depends on an asynchronous operation, like fetching data from an API or local storage? If you call sendResponse() inside an async callback, the message channel might close before your response is ready.

To prevent this, return true from your onMessage.addListener callback. This tells Chrome to keep the channel open, allowing you to call sendResponse() later.

You can also establish two-way communication directly between a popup and a content script running on the active tab. For this, the popup uses chrome.tabs.sendMessage(), specifying the tab ID.

The content script then uses chrome.runtime.onMessage.addListener(), just like a background script.

Here's how the content script on the web page would receive the message from the popup and send back the currently selected text.

Remember, the content script also uses chrome.runtime.onMessage.addListener() and can use sendResponse().

Robust error handling is crucial:

• Check chrome.runtime.lastError: After sendMessage(), always check this property to catch errors like disconnected ports (e.g., if the receiving script isn't active).
• Include status in response: Design your response objects with a status field (e.g., 'success', 'error') and an optional message or error field for details.
• Handle undefined responses: The callback might be called with an undefined response if the receiver doesn't call sendResponse().

To ensure smooth and maintainable two-way messaging:

• Define clear message types: Use a type or action field in your message objects (e.g., {type: "getData"}) to easily identify intent.
• Keep messages simple: Pass only necessary, JSON-serializable data. Avoid complex objects or DOM elements.
• Centralize listener logic: If possible, use a single onMessage.addListener in your background script and route requests based on their type.
• Document message contracts: Clearly define what each message type expects and what response it returns.

Test your knowledge of two-way messaging!

You've mastered two-way messaging!

• You learned how to send requests using chrome.runtime.sendMessage() and receive responses via a callback.
• You can now use chrome.runtime.onMessage.addListener() with sendResponse() to reply to messages, handling both synchronous and asynchronous scenarios.
• You also saw how to enable communication between popups and content scripts using chrome.tabs.sendMessage().

Next, you'll explore how to persist data using the chrome.storage API, which often works hand-in-hand with messaging!