apex/up

// Package inject provides script and style injection.
package inject

import (
    "bytes"
    "io"
    "net/http"
    "strconv"
    "strings"

    "github.com/apex/up"
    "github.com/apex/up/internal/inject"
)

// response wrapper.
type response struct {
    http.ResponseWriter
    rules  inject.Rules
    body   bytes.Buffer
    header bool
    ignore bool
    code   int
}

// Write implementation.
func (r *response) Write(b []byte) (int, error) {
    if !r.header {
        r.WriteHeader(200)
        return r.Write(b)
    }

    return r.body.Write(b)
}

// WriteHeader implementation.
func (r *response) WriteHeader(code int) {
    r.header = true
    w := r.ResponseWriter
    kind := w.Header().Get("Content-Type")
    r.ignore = !strings.HasPrefix(kind, "text/html") || code >= 300
    r.code = code
}

// end injects if necessary.
func (r *response) end() {
    w := r.ResponseWriter

    if r.ignore {
        w.WriteHeader(r.code)
        r.body.WriteTo(w)
        return
    }

    body := r.rules.Apply(r.body.String())
    w.Header().Set("Content-Length", strconv.Itoa(len(body)))
    io.WriteString(w, body)
}

// New inject handler.
func New(c *up.Config, next http.Handler) (http.Handler, error) {
    if len(c.Inject) == 0 {
        return next, nil
    }

    h := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        res := &response{ResponseWriter: w, rules: c.Inject}
        next.ServeHTTP(res, r)
        res.end()
    })

    return h, nil
}