Smalltalk 语言 设计模式之单例模式实战

单例模式^【1】实战：Smalltalk^【2】 语言中的单例模式实现

单例模式是一种常用的设计模式^【3】，它确保一个类只有一个实例，并提供一个全局访问点^【4】。在Smalltalk语言中，单例模式同样重要，因为它可以帮助我们管理资源，避免不必要的对象创建，以及提供全局访问点。本文将围绕Smalltalk语言，通过一个实际案例来展示单例模式的实现。

单例模式概述

单例模式的主要目的是确保一个类只有一个实例，并提供一个全局访问点。以下是一些单例模式的关键特点：

1. 全局访问点：单例类提供了一个全局访问点，用于获取类的唯一实例。
2. 私有构造函数^【5】：单例类通常有一个私有的构造函数，以防止外部直接创建实例。
3. 静态实例变量^【6】：单例类包含一个静态实例变量，用于存储类的唯一实例。
4. 静态获取方法^【7】：单例类提供一个静态方法，用于获取类的唯一实例。

Smalltalk 中的单例模式实现

在Smalltalk中，我们可以通过以下步骤来实现单例模式：

1. 创建一个类，包含一个私有的类变量来存储实例。
2. 提供一个类方法来获取实例，如果实例不存在，则创建它。
3. 确保构造函数是私有的，防止外部直接创建实例。

以下是一个Smalltalk语言中单例模式的实现示例：

```smalltalk
| instance<sup>【8】</sup> |

ClassDefinition<sup>【9】</sup> new
instance: nil.
classVariable<sup>【10】</sup>: 'instance'.
classVariable: 'instance', instance.
classVariable: 'instance', instance, isPrivate<sup>【11】</sup>.
classVariable: 'instance', instance, isSingleton<sup>【12】</sup>: true.
classVariable: 'instance', instance, isClassVariable<sup>【13】</sup>.
classVariable: 'instance', instance, isImmutable<sup>【14】</sup>.
classVariable: 'instance', instance, isConstant<sup>【15】</sup>.
classVariable: 'instance', instance, isFinal<sup>【16】</sup>.
classVariable: 'instance', instance, isStatic<sup>【17】</sup>.
classVariable: 'instance', instance, isPublic<sup>【18】</sup>.
classVariable: 'instance', instance, isAccessible<sup>【19】</sup>: true.
classVariable: 'instance', instance, isAccessible: false.
classVariable: 'instance', instance, isAccessible: true.
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