Add custom sorting function in text formatter

text_formatter.go

@@ -55,6 +55,9 @@ type TextFormatter struct {
 	// be desired.
 	DisableSorting bool
 
+	// The keys sorting function, when uninitialized it uses sort.Strings.
+	SortingFunc func([]string)
+
 	// Disables the truncation of the level text to 4 characters.
 	DisableLevelTruncation bool
 
@@ -73,7 +76,7 @@ type TextFormatter struct {
 	//         FieldKeyMsg:   "@message"}}
 	FieldMap FieldMap
 
-	sync.Once
+	terminalInitOnce sync.Once
 }
 
 func (f *TextFormatter) init(entry *Entry) {
@@ -111,8 +114,29 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 		keys = append(keys, k)
 	}
 
+	fixedKeys := make([]string, 0, 3+len(entry.Data))
+	if !f.DisableTimestamp {
+		fixedKeys = append(fixedKeys, f.FieldMap.resolve(FieldKeyTime))
+	}
+	fixedKeys = append(fixedKeys, f.FieldMap.resolve(FieldKeyLevel))
+	if entry.Message != "" {
+		fixedKeys = append(fixedKeys, f.FieldMap.resolve(FieldKeyMsg))
+	}
+
 	if !f.DisableSorting {
-		sort.Strings(keys)
+		if f.SortingFunc == nil {
+			sort.Strings(keys)
+			fixedKeys = append(fixedKeys, keys...)
+		} else {
+			if !f.isColored() {
+				fixedKeys = append(fixedKeys, keys...)
+				f.SortingFunc(fixedKeys)
+			} else {
+				f.SortingFunc(keys)
+			}
+		}
+	} else {
+		fixedKeys = append(fixedKeys, keys...)
 	}
 
 	var b *bytes.Buffer
@@ -122,7 +146,7 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 		b = &bytes.Buffer{}
 	}
 
-	f.Do(func() { f.init(entry) })
+	f.terminalInitOnce.Do(func() { f.init(entry) })
 
 	timestampFormat := f.TimestampFormat
 	if timestampFormat == "" {
@@ -131,15 +155,19 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 	if f.isColored() {
 		f.printColored(b, entry, keys, timestampFormat)
 	} else {
-		if !f.DisableTimestamp {
-			f.appendKeyValue(b, f.FieldMap.resolve(FieldKeyTime), entry.Time.Format(timestampFormat))
-		}
-		f.appendKeyValue(b, f.FieldMap.resolve(FieldKeyLevel), entry.Level.String())
-		if entry.Message != "" {
-			f.appendKeyValue(b, f.FieldMap.resolve(FieldKeyMsg), entry.Message)
-		}
-		for _, key := range keys {
-			f.appendKeyValue(b, key, entry.Data[key])
+		for _, key := range fixedKeys {
+			var value interface{}
+			switch key {
+			case f.FieldMap.resolve(FieldKeyTime):
+				value = entry.Time.Format(timestampFormat)
+			case f.FieldMap.resolve(FieldKeyLevel):
+				value = entry.Level.String()
+			case f.FieldMap.resolve(FieldKeyMsg):
+				value = entry.Message
+			default:
+				value = entry.Data[key]
+			}
+			f.appendKeyValue(b, key, value)
 		}
 	}
 

text_formatter_test.go

@@ -5,11 +5,13 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"sort"
 	"strings"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestFormatting(t *testing.T) {
@@ -446,5 +448,33 @@ func TestTextFormatterIsColored(t *testing.T) {
 	}
 }
 
-// TODO add tests for sorting etc., this requires a parser for the text
-// formatter output.
+func TestCustomSorting(t *testing.T) {
+	formatter := &TextFormatter{
+		DisableColors: true,
+		SortingFunc: func(keys []string) {
+			sort.Slice(keys, func(i, j int) bool {
+				if keys[j] == "prefix" {
+					return false
+				}
+				if keys[i] == "prefix" {
+					return true
+				}
+				return strings.Compare(keys[i], keys[j]) == -1
+			})
+		},
+	}
+
+	entry := &Entry{
+		Message: "Testing custom sort function",
+		Time:    time.Now(),
+		Level:   InfoLevel,
+		Data: Fields{
+			"test":      "testvalue",
+			"prefix":    "the application prefix",
+			"blablabla": "blablabla",
+		},
+	}
+	b, err := formatter.Format(entry)
+	require.NoError(t, err)
+	require.True(t, strings.HasPrefix(string(b), "prefix="), "format output is %q", string(b))
+}