I am looking into Ktor interceptors (from a Ktor s...

# ktor

[Jamie Craane]

I am looking into Ktor interceptors (from a Ktor server perspective) to modify the original response before it is send
to the client. A quick example of what I want to achieve:

suppose I have a route which returns some response, for example:

route {
    get("sample") {
        call.respondText("Hello {Name}")
    }
}

And I want to use a global interceptor to replace the {Name} variable in the response with some
other value. How do I access the contents of the response in an interceptor? (I know how I can send a
custom response but not yet how I can access the original response, and modify it)

[Jamie Craane]

I have worked out a potential solution by writing a custom feature (inspired by the Compression feature). A simple example of the code is shown below:

class TokenReplaceFeature : ApplicationFeature<ApplicationCallPipeline, Compression.Configuration, TokenReplacer> {
    override val key = AttributeKey<TokenReplacer>("replacer")

    override fun install(pipeline: ApplicationCallPipeline, configure: Compression.Configuration.() -> Unit): TokenReplacer {
        val feature = TokenReplacer()
        pipeline.sendPipeline.intercept(ApplicationSendPipeline.ContentEncoding) {
            feature.interceptor(this)
        }
        return feature
    }
}

class TokenReplacer {
    suspend fun interceptor(context: PipelineContext<Any, ApplicationCall>) {
        val call = context.call
        val message = context.subject
        if (message is TextContent) {
            val replaced = message.text.replace("{HOST_IP}", "10.0.0.2")
            context.proceedWith(TextContent(replaced, message.contentType, call.response.status()))
        }
    }
}

You can then simply install this feature with:

install(TokenReplaceFeature())