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Illustrative Examples

This chapter presents several examples that showcase Stopify's features.

A Blocking sleep Function

The browser does not have a blocking sleep function. However, we can use window.setTimeout and Stopify to simulate a blocking sleep operation:

function sleep(duration) {
  asyncRun.pauseImmediate(() => {
    window.setTimeout(() => asyncRun.continueImmediate({ type: 'normal', value: undefined }), duration);

In the code above, asyncRun is an instance of AsyncRun (:ref:`asyncrun`). Note that this function should be stopified itself and needs to be declared as an external. A complete example of a page that uses sleep is shown below.

.. literalinclude:: examples/sleep.html
  :language: html

This program runs forever and prints a period each second.

A Blocking prompt Function

The prompt and alert functions that are built-in to browsers are not the ideal way to receive input from the user. First, modal dialog boxes are unattractive; second, a user can dismiss them; and finally, if a page displays too many modal dialog boxes, the browser can give the user to suppress all of them.

.. literalinclude:: examples/prompt.html
  :language: html

This program prompts the user for two inputs without modal dialog boxes.

External Events

.. literalinclude:: examples/clicks.html
  :language: html

The onClick function in the code below is an example of an event handler that receives a stopified callback named callback. However, onClick cannot apply callback directly. To support suspending execution, callback must execute in the context of Stopify's runtime system.

Instead, onClick invokes Stopify's processEvent method, passing it a thunk that calls callback. If the program is paused, processEvent queues the event until the program is resumed. Therefore, if the user pauses the program, clicks several times, and then resumes, several clicks will clicks will occur immediately after resumption. It is straightforward to discard clicks that occur while paused by testing the paused variable before invoking processEvent.