# 自定义服务

服务即是被注入到处理器中的参数. 你可以映射一个服务到 **全局** 或者 **请求** 的级别.

## 全局映射

因为 Macaron 实现了 [`inject.Injector`](https://gowalker.org/github.com/go-macaron/macaron/inject#Injector) 的接口, 那么映射一个服务就变得非常简单:

```go
db := &MyDatabase{}
m := macaron.Classic()
m.Map(db) // Service will be available to all handlers as *MyDatabase
m.Get("/", func(db *MyDatabase) {
    // Operations with db.
})
m.Run()
```

## 请求级别的映射

映射在请求级别的服务可以通过 [`*macaron.Context`](https://gowalker.org/github.com/go-macaron/macaron#Context) 来完成:

```go
func MyCustomLoggerHandler(ctx *macaron.Context) {
    logger := &MyCustomLogger{ctx.Req}
    ctx.Map(logger) // mapped as *MyCustomLogger
}

func main() {
    //...
    m.Get("/", MyCustomLoggerHandler, func(logger *MyCustomLogger) {
        // Operations with logger.
    })
    m.Get("/panic", func(logger *MyCustomLogger) {
        // This will panic because no logger service maps to this request.
    })
    //...
}
```

## 映射值到接口

关于服务最强悍的地方之一就是它能够映射服务到接口. 例如说, 假设你想要覆盖 [`http.ResponseWriter`](http://gowalker.org/net/http#ResponseWriter) 成为一个对象, 那么你可以封装它并包含你自己的额外操作, 你可以如下这样来编写你的处理器:

```go
func WrapResponseWriter(ctx *macaron.Context) {
    rw := NewSpecialResponseWriter(ctx.Resp)
    // override ResponseWriter with our wrapper ResponseWriter
    ctx.MapTo(rw, (*http.ResponseWriter)(nil)) 
}
```

如此一来，您不仅可以修改自定义的实现而不对客户代码做任何修改，还可以允许对于相同类型的服务使用多种实现。


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