callback () {
    const fn = this.compose(this.middleware)

    if (!this.listenerCount('error')) this.on('error', this.onerror)

    const handleRequest = (req, res) => {
      const ctx = this.createContext(req, res)
      if (!this.ctxStorage) {
        return this.handleRequest(ctx, fn)
      }
      return this.ctxStorage.run(ctx, async () => {
        return await this.handleRequest(ctx, fn)
      })
    }

    return handleRequest
  }
Return a request handler callback
for node's native http server.

@return {Function}
@api public