装饰者模式通过接口和组合动态扩展功能，适用于日志、权限等场景。定义统一接口，装饰器持有接口实例并叠加行为。Go中用结构体嵌入实现，如消息处理器可包装日志与重试逻辑，NewRetryProcessor(p, 3)层层增强。
装饰者模式在Go语言中可以通过接口和组合的方式优雅实现。它允许你在不修改原有代码的前提下，动态地给对象添加新功能。这种模式特别适合日志、权限校验、缓存、监控等横切关注点的处理。
理解装饰者模式的核心思想
装饰者模式的关键在于：
- 定义一个统一的接口，所有具体实现和装饰器都遵循这个接口
- 装饰器持有一个该接口的实例，可以在调用前后插入额外逻辑
- 通过层层包装，实现功能叠加，而不是继承扩展
在Go中，由于没有类的概念，我们依靠结构体嵌入和函数式编程技巧来实现这一模式。
基于接口的装饰者实现
假设我们要构建一个消息处理器，支持普通处理、日志记录和重试机制：
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<span style="color:blue;">type</span> MessageProcessor <span style="color:blue;">interface</span> {
Process(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span>
}
<span style="color:blue;">type</span> BasicProcessor <span style="color:blue;">struct</span>{}
<span style="color:blue;">func</span> (p *BasicProcessor) Process(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span> {
fmt.Println("Processing:", msg)
<span style="color:blue;">return</span> <span style="color:blue;">nil</span>
}
现在添加日志装饰器：
<span style="color:blue;">type</span> LoggingProcessor <span style="color:blue;">struct</span> {
processor MessageProcessor
}
<span style="color:blue;">func</span> NewLoggingProcessor(p MessageProcessor) MessageProcessor {
<span style="color:blue;">return</span> &LoggingProcessor{processor: p}
}
<span style="color:blue;">func</span> (l *LoggingProcessor) Process(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span> {
fmt.Println("Log: Start processing", msg)
err := l.processor.Process(msg)
fmt.Println("Log: Finished processing", msg)
<span style="color:blue;">return</span> err
}
再添加重试装饰器：
<span style="color:blue;">type</span> RetryProcessor <span style="color:blue;">struct</span> {
processor MessageProcessor
retries <span style="color:blue;">int</span>
}
<span style="color:blue;">func</span> NewRetryProcessor(p MessageProcessor, retries <span style="color:blue;">int</span>) MessageProcessor {
<span style="color:blue;">return</span> &RetryProcessor{processor: p, retries: retries}
}
<span style="color:blue;">func</span> (r *RetryProcessor) Process(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span> {
<span style="color:blue;">for</span> i := 0; i <= r.retries; i++ {
err := r.processor.Process(msg)
<span style="color:blue;">if</span> err == <span style="color:blue;">nil</span> {
<span style="color:blue;">return</span> <span style="color:blue;">nil</span>
}
fmt.Printf("Retry %d failed: %v\n", i+1, err)
}
<span style="color:blue;">return</span> fmt.Errorf("failed after %d retries", r.retries)
}
使用方式：
processor := &BasicProcessor{}
processor = NewLoggingProcessor(processor)
processor = NewRetryProcessor(processor, 2)
processor.Process("Hello")
函数式装饰器的简洁写法
Go中还可以用函数类型实现更灵活的装饰者模式：
<span style="color:blue;">type</span> HandlerFunc <span style="color:blue;">func</span>(<span style="color:blue;">string</span>) <span style="color:blue;">error</span>
<span style="color:blue;">func</span> (f HandlerFunc) Serve(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span> {
<span style="color:blue;">return</span> f(msg)
}
<span style="color:blue;">func</span> WithLogging(next HandlerFunc) HandlerFunc {
<span style="color:blue;">return</span> <span style="color:blue;">func</span>(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span> {
fmt.Println("Log start:", msg)
err := next(msg)
fmt.Println("Log end:", msg)
<span style="color:blue;">return</span> err
}
}
<span style="color:blue;">func</span> WithMetrics(next HandlerFunc) HandlerFunc {
<span style="color:blue;">return</span> <span style="color:blue;">func</span>(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span> {
start := time.Now()
err := next(msg)
fmt.Printf("Duration: %v\n", time.Since(start))
<span style="color:blue;">return</span> err
}
}
链式调用：
handler := HandlerFunc(<span style="color:blue;">func</span>(msg <span style="color:blue;">string</span>) <span style="color:blue;">error</span> {
fmt.Println("Handling:", msg)
<span style="color:blue;">return</span> <span style="color:blue;">nil</span>
})
handler = WithLogging(WithMetrics(handler))
handler.Serve("test message")
这种方式更轻量，适合中间件场景，如HTTP处理链。
基本上就这些。装饰者模式在Go中通过接口和组合能自然表达，关键是设计好基础行为的抽象，然后让装饰器透明地增强功能。
