Actionscript Cookbook

1.7. Repeating a Task at Timed Intervals

Problem

You want to perform an action or actions at a specific timed interval.

Solution

Use the setInterval( ) function.

Discussion

The setInterval( ) function allows you to specify an interval (in milliseconds) at which your Flash movie will invoke a function. Use setInterval( ) to perform a particular action over time but not necessarily at the frequency of the frame rate of the movie.

// Define a function.
function myIntervalFunction (  ) {

  // Output the difference between the current timer value and its value from the
  // last time the function was called.
  trace(getTimer(  ) - lastTime);
  lastTime = getTimer(  );
}

// Set up an interval that attempts to invoke myIntervalFunction(  ) once every
// millisecond.
setInterval(myIntervalFunction, 1);

In the preceding example, even though the interval is theoretically one millisecond, in practice, its accuracy and granularity depend on computer playback performance in relation to other tasks being demanded of the processor. There are two implications to this:

• Don’t rely on intervals to be extremely precise.

• Don’t rely on intervals to be smaller than a few milliseconds.

The setInterval( ) function returns an identifier for the newly created interval. If you want to be able to stop the interval at a later time, you must store the return value, as follows:

// Set an interval such that someFunction(  ) is called approximately once per second.
// Assign setInterval(  )'s return value to the variable myIntervalID