Improve customisability of headers when using simplemail

fromaddress.go

@@ -22,7 +22,6 @@ func (f *FromAddress) UnmarshalText(b []byte) error {
 	return nil
 }
 
-func (f FromAddress) ToMailAddress() *mail.Address {
-	m := mail.Address(f)
-	return &m
+func (f *FromAddress) ToMailAddress() *mail.Address {
+	return (*mail.Address)(f)
 }

mail.go

@@ -1,14 +1,12 @@
 package simplemail
 
 import (
-	"bytes"
 	"context"
 	"github.com/emersion/go-message/mail"
 	"github.com/emersion/go-sasl"
 	"github.com/emersion/go-smtp"
 	"io"
 	"net"
-	"time"
 )
 
 type Mail struct {
@@ -69,51 +67,17 @@ func isLocalhost(host string) bool {
 	return true
 }
 
-func (m *Mail) SendMail(subject string, to []*mail.Address, htmlBody, textBody io.Reader) error {
-	// generate the email in this template
-	buf := new(bytes.Buffer)
-
-	// setup mail headers
-	var h mail.Header
-	h.SetDate(time.Now())
-	h.SetSubject(subject)
-	h.SetAddressList("From", []*mail.Address{m.From.ToMailAddress()})
-	h.SetAddressList("To", to)
-	h.Set("Content-Type", "multipart/alternative")
-
-	// setup html and text alternative headers
-	var hHtml, hTxt mail.InlineHeader
-	hHtml.Set("Content-Type", "text/html; charset=utf-8")
-	hTxt.Set("Content-Type", "text/plain; charset=utf-8")
-
-	createWriter, err := mail.CreateWriter(buf, h)
-	if err != nil {
-		return err
-	}
-	inline, err := createWriter.CreateInline()
-	if err != nil {
-		return err
-	}
-	partHtml, err := inline.CreatePart(hHtml)
-	if err != nil {
-		return err
-	}
-	if _, err := io.Copy(partHtml, htmlBody); err != nil {
-		return err
-	}
-	partTxt, err := inline.CreatePart(hTxt)
-	if err != nil {
-		return err
-	}
-	if _, err := io.Copy(partTxt, textBody); err != nil {
-		return err
-	}
-
+// SendMail sends a mail message to the provided mail addresses.
+//
+// The reader should follow the format of an RFC 822-style email.
+//
+// See github.com/emersion/go-smtp.SendMail
+func (m *Mail) SendMail(to []*mail.Address, mailMessage io.Reader) error {
 	// convert all to addresses to strings
 	toStr := make([]string, len(to))
 	for i := range toStr {
 		toStr[i] = to[i].String()
 	}
 
-	return m.mailCall(toStr, buf)
+	return m.mailCall(toStr, mailMessage)
 }

preparedmail.go

@@ -0,0 +1,62 @@
+package simplemail
+
+import (
+	"bytes"
+	"github.com/emersion/go-message/mail"
+	"io"
+)
+
+type PreparedMail struct {
+	simpleMail   *SimpleMail
+	templateName string
+	Header       mail.Header
+	rcpt         []*mail.Address
+	data         map[string]any
+}
+
+func (p *PreparedMail) SendMail() error {
+	buf := new(bytes.Buffer)
+
+	var bufHtml, bufTxt bytes.Buffer
+	p.simpleMail.render(&bufHtml, &bufTxt, p.templateName, p.data)
+
+	// setup html and text alternative headers
+	var hHtml, hTxt mail.InlineHeader
+	hHtml.Set("Content-Type", "text/html; charset=utf-8")
+	hTxt.Set("Content-Type", "text/plain; charset=utf-8")
+
+	createWriter, err := mail.CreateWriter(buf, p.Header)
+	if err != nil {
+		return err
+	}
+	inline, err := createWriter.CreateInline()
+	if err != nil {
+		return err
+	}
+	err = copyToPart(inline, hHtml, &bufHtml)
+	if err != nil {
+		return err
+	}
+	err = copyToPart(inline, hTxt, &bufTxt)
+	if err != nil {
+		return err
+	}
+	err = inline.Close()
+	if err != nil {
+		return err
+	}
+
+	return p.simpleMail.mailSender.SendMail(p.rcpt, buf)
+}
+
+func copyToPart(inline *mail.InlineWriter, header mail.InlineHeader, body io.Reader) error {
+	part, err := inline.CreatePart(header)
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(part, body)
+	if err != nil {
+		return err
+	}
+	return part.Close()
+}

simplemail.go

@@ -1,13 +1,13 @@
 package simplemail
 
 import (
-	"bytes"
 	"github.com/emersion/go-message/mail"
 	htmlTemplate "html/template"
 	"io"
 	"io/fs"
 	"log"
 	textTemplate "text/template"
+	"time"
 )
 
 type SimpleMail struct {
@@ -40,8 +40,29 @@ func (m *SimpleMail) render(wrHtml, wrTxt io.Writer, name string, data any) {
 	}
 }
 
-func (m *SimpleMail) Send(templateName, subject string, to *mail.Address, data map[string]any) error {
-	var bufHtml, bufTxt bytes.Buffer
-	m.render(&bufHtml, &bufTxt, templateName, data)
-	return m.mailSender.SendMail(subject, []*mail.Address{to}, &bufHtml, &bufTxt)
+// PrepareSingle constructs the headers for sending an email to the provided mail address.
+func (m *SimpleMail) PrepareSingle(templateName, subject string, to *mail.Address, data map[string]any) *PreparedMail {
+	return m.PrepareMany(templateName, subject, []*mail.Address{to}, data)
+}
+
+// PrepareMany constructs the headers for sending an email to the provided mail addresses.
+func (m *SimpleMail) PrepareMany(templateName, subject string, to []*mail.Address, data map[string]any) *PreparedMail {
+	p := &PreparedMail{
+		simpleMail:   m,
+		templateName: templateName,
+		rcpt:         to,
+		data:         data,
+	}
+	p.Header.SetDate(time.Now())
+	p.Header.SetSubject(subject)
+	p.Header.SetAddressList("From", []*mail.Address{m.mailSender.From.ToMailAddress()})
+	p.Header.SetAddressList("To", to)
+	p.Header.Set("Content-Type", "multipart/alternative")
+	return p
+}
+
+// Send is a simplified version of PrepareSingle which sends the mail without allowing header modifications.
+func (m *SimpleMail) Send(templateName, subject string, to *mail.Address, data map[string]any) error {
+	p := m.PrepareSingle(templateName, subject, to, data)
+	return p.SendMail()
 }