When to Use the Singleton Pattern | 113In order to provide a broad but by no means exhaustive view of how the Singleton
can be employed, we will step through three examples. First, the Alert example is a
simple one that shows how a single message can be placed on the stage. It is meant
to represent those kinds of interfaces where the user gets a message, such as in a dia-
log bo xor similar feedback mechanism. Only a single instance of the feedback
should be instantiated, to avoid contradictory messages. However, the example also
shows how to connect the class to display objects on the stage. As such, it is instruc-
tive for working with display programming in ActionScript 3.0.
The second example is used for playing an MP3 file. In most situations where you
play media, whether it’s a MP3 file or a video, you want to hear or see only one
media element at a time. Playing Bach and a song by Gnarls Barkley simultaneously
may create a racket that neither Johann Sebastian nor Gnarls would want to hear. A
Singleton class helps to keep the play sequential, with no more than one playing at
the same time.
The third example uses a simple shopping cart to illustrate how a single instance can
be used to keep track of a running total. In situations where your application needs
an absolutely no-questions-about-it single instance for keeping track of financial
accumulations, a Singleton can be crucial. The example also shows how to link a
class to a stage-created movie clip and embedded text field.
A Single Alert Message
Because clear communication is the cru xof good site design, mi xed messages need
to be kept out of all web applications. By using a Singleton pattern, you can help
assure that you have a single source for messages to the user. This way, you’re less
likely to send two contradictory messages. The following application uses a Single-
ton for that purpose. It’s designed so that no matter where a message originates, it
has only this one instance to deliver it.
To get started, open a new ActionScript file, and type in the script shown in
Example 3-10.
Example 3-10. Alert.as
package
{
public class Alert
{
private var _msg:String;
private static var _instance:Alert;
public function Alert(pvt:PrivateClass) {
}
public static function getInstance( ):Alert
{
if(Alert._instance == null)