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# Using Signals

Because much of MathJax operates asynchronously, it is important for MathJax to be able to indicate to other components operating on the page that certain actions have been taken. For example, as MathJax is starting up, it loads external files such as its configuration files and the various input and output jax that are used on the page. This means that MathJax may not be ready to run until well after the `<script>` tag that loads `MathJax.js` has executed. If another component on the page needs to call MathJax to process some mathematics, it will need to know when MathJax is ready to do that. Thus MathJax needs a way to signal other components that it is initialized and ready to process mathematics. Other events that might need to be signaled include the appearance of newly processed mathematics on the web page, the loading of a new extension, and so on.

The mechanism provided by MathJax for handling this type of communication is the Callback Signal. The Callback Signal object provides a standardized mechanism for sending and receiving messages between MathJax and other code on the page. A signal acts like a mailbox where MathJax places messages for others to read. Those interested in seeing the messages can register an interest in receiving a given signal, and when MathJax posts a message on that signal, all the interested parties will be notified. No new posts to the signal will be allowed until everyone who is listening to the signal has had a chance to receive the first one. If a signal causes a listener to begin an asynchronous operation (such as loading a file), the listener can indicate that its reply to the signal is going to be delayed, and MathJax will wait until the asynchronous action is complete before allowing additional messages to be posted to this signal. In this way, posting a signal may itself be an asynchronous action.

The posts to a signal are cached so that if a new listener expresses an interest in the signal, it will receive all the past posts as well as any future ones. For example, if a component on the page needs to know when MathJax is set up, it can express an interest in the startup signal’s `End` message. If MathJax is not yet set up, the component will be signaled when MathJax is ready to begin, but if MathJax is already set up, the component will receive the `End` message immediately, since that message was cached and is available to any new listeners. In this way, signals can be used to pass messages without worrying about the timing of when the signaler and listener are ready to send or receive signals: a listener will receive messages even if it starts listening after they were sent.

One way that MathJax makes use of this feature is in configuring its various extensions. The extension may not be loaded when the user’s configuration code runs, so the configuration code can’t modify the extension because it isn’t there yet. Fortunately, most extensions signal when they are loaded and initialized via an `Extension [name] Ready` message, so the configuration code can implement a listener for that message, and have the listener perform the configuration when the message arrives. But even if the extension has already been loaded, this will still work, because the listener will receive the ready signal even if it has already been posted. In this way, listening for signals is a robust method of synchronizing code components no matter when they are loaded and run.

In some cases, it may be inappropriate for a new listener to receive past messages that were sent to a signal object. There are two ways to handle this: first, a new listener can indicate that it doesn’t want to hear old messages when it attaches itself to a signal object. The sender can also indicate that past messages are not appropriate for new listeners. It does this by clearing the message history so that new listeners have no old posts to hear.

The actual message passed along by the signal can be anything, but is frequently a string constant indicating the message value. It could also be a JavaScript array containing data, or an object containing key:value pairs. All the listeners receive the data as part of the message, and can act on it in whatever ways they see fit.

## Creating a Listener

MathJax maintains two separate signal channels: the startup signal and the processing signal (or the hub signal). The startup signal is where the messages about different components starting up and becoming ready appear. The processing signal is where the messages are sent about processing mathematics, like the `New Math` messages for when newly typeset mathematics appears on the page. The latter is cleared when a new processing pass is started (so messages from past processing runs are not kept).

The easiest way to create a listener is to use either `MathJax.Hub.Register.StartupHook()` or `MathJax.Hub.Register.MessageHook()`. The first sets a listener on the startup signal, and the latter on the hub processing signal. You specify the message you want to listen for, and a callback to be called when it arrives. For example

    MathJax.Hub.Register.StartupHook("TeX Jax Ready ",function () {
      alert("The TeX input jax is loaded and ready!");
    });

See the MathJax Startup Sequence page for details of the messages sent during startup. See also the `test/sample-signals.html` file (and its source) for examples of using signals. This example lists all the signals that occur while MathJax is processing that page, so it gives useful information about the details of the signals produced by various components.

In this example, the listener starts loading an extra configuration file (from the same directory as the web page). Since it returns the callback from that request, the signal processing will wait until that file is completely loaded before it continues; that is, the configuration process is suspended until the extra configuration file has loaded.

    MathJax.Hub.Register.StartupHook("Begin Config",
      function () {return MathJax.Ajax.Require("myConfig.js")}
    );

Here is an example that produces an alert each time new mathematics is typeset on the page. The message includes the DOM id of the element on the page that contains the newly typeset mathematics as its second element, so this listener locates the `<script>` tag for the math, and displays the original source mathematics for it.

    MathJax.Hub.Register.MessageHook("New Math", function (message) {
      var script = MathJax.Hub.getJaxFor(message[1]).SourceElement();
      alert(message.join(" ")+": '"+script.text+"'");
    })

## Listening for All Messages

If you want to process every message that passes through a signal channel, you can do that by registering an interest in the signal rather than registering a message hook. You do this by calling the signal’s `Interest()` method, as in the following example.

    MathJax.Hub.Startup.signal.Interest(
      function (message) {alert("Startup: "+message)}
    );
    MathJax.Hub.signal.Interest(
      function (message) {alert("Hub: "+message)}
    );

This will cause an alert for every signal that MathJax produces. You probably don’t want to try this out, since it will produce a lot of them; instead, use the `test/sample-signals.html` file, which displays them in the web page.

See the Signal Object reference page for details on the structure and methods of the signal object.

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