单例模式的三种写法

Code
1.
package
{

/**
* 标准的单例可以扩展,所以不要定义成final类
* @author chancidal
*/
public class MyClass
{
//
private static var m_instance:MyClass;
//
public function MyClass()
{

}
public static function getInstance():MyClass {
if (!m_instance) {
m_instance = new MyClass();
}
return m_instance;
}
public function sayHello():void {
trace("hello,world");
}
}

}

2.
package
{

/**
*
* @author chancidal
*/
public class Ball
{
//
private static var m_instance:Ball;
//
public function Ball()
{
m_instance = this;
}
public static function getInstance():Ball {
return m_instance;
}
public function sayHello():void {
trace("hello,world");
}
}

}

3.
package
{

/**
* 利用包外类限制生成单例类的实例
* @author chancidal
*/
public class AnotherSingle
{
//
private static var m_instance:AnotherSingle;
//
public function AnotherSingle(singletonEnforcer:MyEnforcer)
{
if (!singletonEnforcer) {
throw new Error("Singleton and can only be accessed through Singleton.getInstance()");
}
}
public static function getInstance():AnotherSingle {
if (!m_instance) {
m_instance = new AnotherSingle(new MyEnforcer())
}
return m_instance;
}
public function sayHello():void {
trace("hello,world");
}
}

}
class MyEnforcer {}