middlewarer

Generate a middleware framework for any Go interface

Installation

Requirements:

• Go 1.20 or newer

go install github.com/DominicWuest/middlewarer@latest

Example

example.go:

//go:generate middlewarer -type=Foo
type Foo interface {
    Bar(Baz) Quz
}

foo_middleware.go

// WrapFoo wraps the methods of the provided interface
// in the provided middleware
func WrapFoo(in Foo, m FooMiddleware) Foo {
    m.wrapped = in
    return &m
}

// FooMiddleware implements Foo
type FooMiddleware struct {
    wrapped Foo

    BarMiddleware func(BarHandler) BarHandler
}

type BarHandler func(Baz) Quz

func (m *FooMiddleware) Bar(a0 Baz) Quz {
    fun := m.wrapped.Bar
    if m.BarMiddleware != nil {
        fun = m.BarMiddleware(fun)
    }
    return fun(a0)
}

Adding Middleware

This example shows how to add a logger to Foo, declared in the example section.

logger.go

func AddLoggerToFoo(f Foo) Foo {
    return WrapFoo(f, FooMiddleware{
        BarMiddleware: barLogger
    })
}

func barLogger(next BarHandler) BarHandler {
    return func(b Baz) Quz {
        log.Println("Calling Bar!")
        res := next()
        log.Println("Bar returned: ", res)
        return res
    }
}

Optional Implementation

When wrapping an instance of an interface <I> by calling Wrap<I>, the provided struct <I>Middleware is allowed to have fields evaluating to nil.

This allows adding middleware to select methods.

Take for example

//go:generate middlewarer -type=Server
type Server interface {
    Init()
    Request() Response
}

If you now want to wrap the Request method with middleware, but not the Init method, you can simply do:

// Get some server instance you want to wrap
s := getServer()

// Overwrite s with the wrapped instance
s = WrapServer(s, ServerMiddleware{
    // InitMiddleware: nil
    RequestMiddleware: someMiddlewareFunc
})

When calling Init on s, no middleware gets invoked. However, when calling Request, the passed middleware function gets called.