Smalltalk 语言 观察者模式实战 实时监控系统设计

观察者模式^【1】在Smalltalk^【2】语言中的实战：实时监控系统^【3】设计

观察者模式是一种行为设计模式，它定义了对象之间的一对多依赖关系，当一个对象的状态发生变化时，所有依赖于它的对象都会得到通知并自动更新。Smalltalk语言作为一种面向对象的语言，非常适合实现观察者模式。本文将围绕Smalltalk语言，通过一个实时监控系统设计案例，展示如何使用观察者模式来实现系统的动态更新和响应。

观察者模式概述

观察者模式包含两个主要角色：

1. Subject^【4】（主题）：被观察的对象，它维护一个观察者列表，并提供注册、注销和通知观察者的接口。
2. Observer^【5】（观察者）：观察主题的对象，它需要实现一个更新接口，当主题状态发生变化时，会自动调用该接口。

实时监控系统设计

系统需求

设计一个实时监控系统，用于监控网络流量^【6】、服务器负载^【7】和数据库状态^【8】。当这些指标超过预设阈值^【9】时，系统应立即通知管理员。

系统架构

1. Subject（主题）：定义一个`MonitorSubject<sup>【10】</sup>`类，用于监控各种指标。
2. Observer（观察者）：定义一个`AlertObserver<sup>【11】</sup>`类，用于接收通知并执行相应的操作。
3. ConcreteSubject^【12】（具体主题）：定义具体的监控对象，如`NetworkMonitor`、`ServerMonitor`和`DatabaseMonitor`。
4. ConcreteObserver^【13】（具体观察者）：定义具体的观察者，如`EmailAlertObserver`和`SMSAlertObserver`。

Smalltalk代码实现

```smalltalk
| MonitorSubject AlertObserver NetworkMonitor ServerMonitor DatabaseMonitor EmailAlertObserver SMSAlertObserver |

"Subject"
MonitorSubject := class new
instanceVariableNames<sup>【14】</sup>: 'observers<sup>【15】</sup>'.
classVariableNames<sup>【16】</sup>: 'instanceCount<sup>【17】</sup>'.
class newInstVarNamesFrom<sup>【18】</sup>: 'observers'.
class newClassVarFrom<sup>【19】</sup>: 'instanceCount'.
class newClassVarFrom: 'instanceCount'.
class newClassVarFrom: 'instanceCount'.
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