Adding the Singleton pattern

“Tag! I’m in!” Phoebe exclaimed, as she nudged Kitty out of the typing chair and sat down at the keyboard. Phoebe made a show of cracking her knuckles and popping her neck. They were in the home stretch and Phoebe knew it.

“The Singleton is pretty easy,” Tom said. “I remember it,” said Phoebe. She continued, “It seems like all I need to do is change the director class in the Builder pattern.” Phoebe located the director class in the Builders folder and opened it in her IDE:

public class WheelchairBuilderDirector
{
    private IWheelchairBuilder _builder;

Phoebe adds this line to create a field to hold the current instance. She makes it nullable because if the field is null, we need to create a new instance of this class and place it in the _instance field. The field is marked static to ensure it is unique in memory:

    private static WheelchairBuilderDirector...