How to Send Messages from Background to Injected Scripts Effectively?

Sending Messages from Background to Injected Scripts

Introduction

This article addresses the challenge of sending messages from a background script to a content script and then to an injected script. Despite following a typical approach, the message sending process stalls at the background-to-content-script stage.

Problem Analysis

The root cause lies in the injection mechanism of content scripts. Contrary to expectations, Chrome does not automatically inject content scripts into existing tabs upon extension (re)loading. As a result, when the background script attempts to send a message to the current tab, there's no listener available to receive it.

Solution Approaches

Solution 1: Conditional Injection with 'ensureSendMessage'

This method involves first pinging the tab to check if it's ready to receive messages. If not, the content script is injected programmatically, enabling it to receive and respond to messages from the background script.

Solution 2: Injection on Initialization

An alternative approach is to simply inject the content scripts indiscriminately on extension load. This is safe if the script code can handle running multiple times or after the page has fully loaded.

Solution 3: Selective Injection via Browser Action

For cases where message sending should occur on specific user actions, a browser action can be utilized. By wrapping the message-sending code in an onClicked listener, injections and communication happen only when the user explicitly triggers the browser action.

Orphaned Content Scripts and Mitigation

Finally, it's important to address a potential issue with orphaned content scripts that persist after extension reloads. These scripts may interfere with the proper execution of newly injected instances. To mitigate this, the content script can implement a heartbeat mechanism to verify its active status with the background script. Only upon receiving a positive acknowledgment from the background should the content script execute actions or process page events.

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