In multiprocess Firefox there are (at least) two processes:
- the chrome process, also called the parent process, runs the browser UI (chrome) code and code inserted by extensions
- one or more content processes, also called child processes. These processes run web content.
Message managers are designed to enable chrome-privileged JavaScript code in one process to communicate with chrome-privileged JavaScript code in a different process.
This article describes the different types of message manager, how to access them, and at a high level, what sorts of things you can use them for.
At the top level, there are two different sorts of message managers:
- Frame message managers: these enable chrome process code to load a script into a browser frame (essentially, a single browser tab) in the content process. These scripts are called frame scripts, and as the name suggests, they are scoped to a specific browser frame. If chrome code wants to run code in the content process so it can access web content, this is usually the sort of message manager to use.
- Process message managers: these correspond to process boundaries, and enable code running in the parent (chrome) process to communicate with code running in the child (content) process. From Firefox 38 onwards, they also enable code running in the parent process to load process scripts into the child process. These are like frame scripts, except they are global to the child process. Process scripts are most likely to be useful when an extension wants to run some code only once in the content process, to access some global service: for example, to register an observer or a content policy.
Frame message managers
In multiprocess Firefox, when chrome code needs to interact with web content, it needs to:
- factor the code that needs direct access to content into separate scripts, which are called "frame scripts"
- use a frame message manager to load these frame scripts into the content process
- use the frame message manager API to communicate with the frame script
Some older articles on multiprocess Firefox and the message manager might refer to "content scripts" instead of "frame scripts". This usage is deprecated because the Add-on SDK uses "content script" to refer to a similar but different kind of script.
So fundamentally, frame message managers enable chrome code to:
- load a script into a frame (essentially, a single browser tab) in the content process. These scripts are called "frame scripts".
- communicate with frame scripts using message-passing APIs
There are various types of frame message managers, as depicted in this diagram:
This diagram shows the setup when there are 2 browser windows open, one with 2 tabs open and one with 1 tab open.
Chrome process
In the chrome process, there's a hierarchy of frame message managers: the global frame message manager, window message managers, and browser message managers.
Global frame message manager
Description |
There's a single global frame message manager in the chrome process. This operates on all frames, in all content tabs. If you load a frame script using the global frame message manager, the script gets loaded separately into every open tab: three times, in the diagram above. Similarly, if you send a message using the global frame message manager, it's received by all content tabs, and is then delivered to any frame scripts that are listening for it. Its most important functions and attributes are:
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Interfaces | |
How to access |
Access it using // chrome script let globalMM = Cc["@mozilla.org/globalmessagemanager;1"] .getService(Ci.nsIMessageListenerManager); You can also access it as the |
Window message manager
Description |
There's a window message manager for every browser window: two, in the diagram above. It operates on all content tabs in a given window. If you load a frame script using the window message manager it gets loaded separately into each tab open in that particular window. If you send a message using the window message manager, it gets sent to all content tabs in that window. Its most important functions and attributes are:
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Interfaces | |
How to access |
You can access it as a property of the browser window: // chrome script let windowMM = window.messageManager; |
Browser message manager
Note that in this context, "browser" refers to the XUL <browser> object, which is a frame that hosts a single Web document. It does not refer to the more general sense of a Web browser.
Description |
Finally, there's a browser message manager for every open content tab: three, in the diagram above. This corresponds one-to-one with a content tab. Scripts you load using a browser message manager are loaded only into that content tab, and messages you send are delivered only to that content tab. You can mix and match: so for example, you could load a script into every tab using the global message manager, but then send a message to the script instance loaded into a specific tab by using the browser message manager. Its most important functions are:
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Interfaces |
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How to access |
The browser message manager can be accessed as a property of the XUL // chrome script let browserMM = gBrowser.selectedBrowser.messageManager; |
Content process
Content frame message manager
Description |
There's a content frame message manager for every open tab. It's the content-side end of frame message manager conversations. Frame scripts are loaded into the content frame message manager scope, and messages from chrome message managers end up here. The content frame message manager provides the global object for frame scripts (but note that there is trickery to ensure that top-level variables defined by frame scripts are not shared). Frame scripts can use this object to send messages to the chrome process, and to receive messages from the chrome process. Its most important attributes and functions are:
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Interfaces | |
How to access | The content frame message manager is the global object in frame scripts. |
Process message managers
Process message managers correspond to process boundaries, and enable code running in different processes to communicate. Multiprocess Firefox has the concept of:
- a "parent process"
- "child processes" which are processes spawned by the parent process.
For practical purposes, in multiprocess Firefox the parent process is the chrome process, and child processes are content processes.
In each child process, there's a single child process message manager (CPMM). There's also an additional child-in-process message manager (CIPMM) in the parent process.
For each child process message manager, there's a parent process message manager (PPMM) in the parent process.
There's also a single global parent process message manager (GPPMM) in the parent process, that provides access to all the parent process message managers. The diagram below shows the setup that would result from having two child processes:
With the GPPMM, you can broadcast messages to the CIPMM and all CPMMs. With a PPMM, you can send a message to its corresponding CPMM. With a CPMM, you can send messages to the parent process: these messages are received first by the corresponding PPMM, then by the GPPMM.
From Firefox 38 onwards, you can also use a parent process message manager to load a script into a child process. This is the recommended way to load a script that executes just once per child process, which is something you might want to do if you are interacting with some global service (for example, adding listeners to observer notifications or registering a content policy).
Parent process
Global parent process message manager
Description |
The global parent process message manager (GPPMM) is global to the parent process.
Its most important functions and attributes are:
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Interfaces | |
How to access |
You can access the GPPMM with code like this: // parent process let ppmm = Cc["@mozilla.org/parentprocessmessagemanager;1"] .getService(Ci.nsIMessageBroadcaster); You can also access it as the |
Parent process message manager
Description |
There's one parent process message manager (PPMM) in the parent process for every child process, and its API is oriented to that one child process.
Its most important functions are:
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Interfaces |
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How to access |
You can access a PPMM using the // parent process let ppmm = Services.ppmm.getChildAt(1); |
Child process
Child process message manager
Description |
There's one child process message manager (CPMM) in each child process. Messages sent using the CPMM are sent to the corresponding PPMM and are also relayed to the GPPMM. Its most important attributes and functions are:
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Interfaces |
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How to access |
Code running in a child process can access the CPMM with code like this: // child process script let cpmm = Cc["@mozilla.org/childprocessmessagemanager;1"] .getService(Ci.nsISyncMessageSender); You can also access it as the |