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# The MathJax.Callback Class

The `MathJax.Callback` object is one of the key mechanisms used by MathJax to synchronize its actions with those that occur asynchronously, like loading files and stylesheets. A Callback object is used to tie the execution of a function to the completion of an asynchronous action. See Synchronizing with MathJax for more details, and Using Callbacks in particular for examples of how to specify and use MathJax Callback objects.

## Specifying a callback

When a method includes a callback as one of its arguments, that callback can be specified in a number of different ways, depending on the functionality that is required of the callback. The easiest case is to simply provide a function to be called, but it is also possible to include data to pass to the function when it is executed, and even the object that will be used as the javascript this object when the function is called.

Most functions that take callbacks as arguments accept a callback specification rather than an actual callback object, though you can use the `MathJax.Callback()` function to convert a callback specification into a Callback object if needed.

A callback specification is any one of the following:

> `fn`

>   A function that is to be called when the callback is executed. No additional data is passed to it (other than what it is called with at the time the callback is executed), and this will be the window object.

> `[fn]`

>   An array containing a function to be called when the callback is executed (as above).

> `[fn, data...]`

>   An array containing a function together with data to be passed to that function when the callback is executed; this is still the window object. For example,

>       [function (x,y) {return x+y}, 2, 3]

>   would specify a callback that would pass `2` and `3` to the given function, and it would return their sum, `5`, when the callback is executed.

>   `[object, fn]`

>   An array containing an object to use as this and a function to call for the callback. For example,

>       [{x:'foo', y:'bar'}, function () {this.x}]

>   would produce a callback that returns the string `"foo"` when it is called.

> `[object, fn, data...]`

>   Similar to the previous case, but with data that is passed to the function as well.

> ..describe:: [“method”, object]

> > Here, object is an object that has a method called method, and the callback will execute that method (with the object as this) when it is called. For example,

> >     ["length",[1,2,3,4]]

> > would call the length method on the array `[1,2,3,4]` when the callback is called, returning `4`.

> `["method", object, data...]`

>   Similar to the previous case, but with data that is passed to the method. E.g.,

>       ["slice",[1,2,3,4],1,3]

>   would perform the equivalent of `[1,2,3,4].slice(1,3)`, which returns the array `[2,3]` as a result.

> `{hook: fn, data: [...], object: this}`

>   Here the data for the callback are given in an associative array of key:value pairs. The value of hook is the function to call, the value of data is an array of the arguments to pass to the function, and the value of object is the object to use as this in the function call. The specification need not include all three key:value pairs; any that are missing get default values (a function that does nothing, an empty array, and the window object, respectively).

> `"string"`

>   This specifies a callback where the string is executed via an `eval()` statement. The code is run in the global context, so any variables or functions created by the string become part of the global namespace. The return value is the value of the last statement executed in the string.

### Executing a Callback Object

The Callback object is itself a function, and calling that function executes the callback. You can pass the callback additional parameters, just as you can any function, and these will be added to the callback function’s argument list following any data that was supplied at the time the callback was created. For example

    var f = function (x,y) {return x + " and " +y}
    var cb = MathJax.Callback([f, "foo"]);
    var result = cb("bar");  // sets result to "foo and bar"

Usually, the callback is not executed by the code that creates it (as it is in the example above), but by some other code that runs at a later time at the completion of some other activity (say the loading of a file), or in response to a user action. For example:

    function f(x) {alert("x contains "+x)};
    function DelayedX(time) {
        var x = "hi";
        setTimeout(MathJax.Callback([f, x], time));
    }

The `DelayedX` function arranges for the function `f` to be called at a later time, passing it the value of a local variable, `x`. Normally, this would require the use of a closure, but that is not needed when a MathJax.Callback object is used.

## Callback Object Properties

`hook`

  The function to be called when the callback is executed.

`data`

  An array containing the arguments to pass to the callback function when it is executed.

`object`

  The object to use as this during the call to the callback function.

`called`

  Set to `true` after the callback has been called, and undefined otherwise. A callback will not be executed a second time unless the callback’s `reset()` method is called first, or its `autoReset` property is set to `true`.

`autoReset`

  Set this to `true` if you want to be able to call the callback more than once. (This is the case for signal listeners, for example).

`isCallback`

  Always set to `true` (used to detect if an object is a callback or not).

## Callback Object Methods

`reset`()

  Clears the callback’s called property.

## MathJax.Callback Methods

`Delay`(time[, callback])

  Waits for the specified time (given in milliseconds) and then performs the callback. It returns the Callback object (or a blank one if none was supplied). The returned callback structure has a timeout property set to the result of the `setTimeout()` call that was used to perform the wait so that you can cancel the wait, if needed. Thus `MathJax.Callback.Delay()` can be used to start a timeout delay that executes the callback if an action doesn’t occur within the given time (and if the action does occur, the timeout can be canceled). Since `MathJax.Callback.Delay()` returns a callback structure, it can be used in a callback queue to insert a delay between queued commands.

  Parameters : | time — the amount of time to wait callback — the callback specification
  Returns : | the callback object

`executeHooks`(hooks[, data[, reset]])

  Calls each callback in the hooks array (or the single hook if it is not an array), passing it the arguments stored in the data array. If reset is `true`, then the callback’s `reset()` method will be called before each hook is executed. If any of the hooks returns a Callback object, then it collects those callbacks and returns a new callback that will execute when all the ones returned by the hooks have been completed. Otherwise, `MathJax.Callback.executeHooks()` returns `null`.

  Parameters : | hooks — array of hooks to be called, or a hook data — array of arguments to pass to each hook in turn reset — `true` if the `reset()` method should be called
  Returns : | callback that waits for all the hooks to complete, or `null`

`Queue`([callback, ...])

  Creates a MathJax.CallBack.Queue object and pushes the given callbacks into the queue. See Using Queues for more details about MathJax queues.

  Parameters : | callback — one or more callback specifications
  Returns : | the Queue object

`Signal`(name)

  Looks for a named signal, creates it if it doesn’t already exist, and returns the signal object. See Using Signals for more details.

  Parameters : | name — name of the signal to get or create
  Returns : | the Signal object

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