Add a WithMapVisitor option. (#28)

copy.go

@@ -240,8 +240,19 @@ type options struct {
 
 	// CopyListSize can control the number of elements copied from src depending on src's Value
 	CopyListSize func(src *reflect.Value) int
+
+	// MapVisitor is called for every filtered field in structToMap.
+	//
+	// It is called before copying the data from source to destination allowing custom processing.
+	// If the visitor function returns true the visited field is skipped.
+	MapVisitor mapVisitor
 }
 
+// mapVisitor is called for every filtered field in structToMap.
+type mapVisitor func(
+	filter FieldFilter, src interface{}, dst map[string]interface{},
+	srcFieldName, dstFieldName string, srcFieldValue reflect.Value) (skipToNext bool)
+
 // Option function modifies the given options.
 type Option func(*options)
 
@@ -259,6 +270,13 @@ func WithCopyListSize(f func(src *reflect.Value) int) Option {
 	}
 }
 
+// WithMapVisitor sets the fields visitor function for StructToMap.
+func WithMapVisitor(visitor mapVisitor) Option {
+	return func(o *options) {
+		o.MapVisitor = visitor
+	}
+}
+
 func newDefaultOptions() *options {
 	// set default CopyListSize is func which return src.Len()
 	return &options{CopyListSize: func(src *reflect.Value) int { return src.Len() }}
@@ -306,6 +324,9 @@ func structToMap(filter FieldFilter, src interface{}, dst map[string]interface{}
 		}
 
 		dstName := dstKey(userOptions.DstTag, srcType.Field(i))
+		if userOptions.MapVisitor != nil && userOptions.MapVisitor(filter, src, dst, fieldName, dstName, srcField) {
+			continue
+		}
 
 		switch srcField.Kind() {
 		case reflect.Ptr, reflect.Interface:

copy_test.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"reflect"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -1790,6 +1791,72 @@ func TestStructToMap_CopyIntArray_WithMaxCopyListSize(t *testing.T) {
 	}, dst)
 }
 
+func TestStructToMap_CopyStructWithPrivateFields_WithMapVisitor(t *testing.T) {
+	type A struct {
+		Time  time.Time
+		Other int
+	}
+	unixTime := time.Unix(10, 10)
+	src := &A{Time: unixTime}
+	dst := map[string]interface{}{}
+	mask := fieldmask_utils.MaskFromString("Time")
+	err := fieldmask_utils.StructToMap(mask, src, dst, fieldmask_utils.WithMapVisitor(
+		func(_ fieldmask_utils.FieldFilter, _ interface{}, dst map[string]interface{},
+			srcFieldName, dstFieldName string, srcFieldValue reflect.Value) (skipToNext bool) {
+			if srcFieldName == "Time" {
+				dst[dstFieldName] = srcFieldValue.Interface()
+				skipToNext = true
+			}
+			return
+		}))
+	require.NoError(t, err)
+	assert.Equal(t, map[string]interface{}{
+		"Time": unixTime,
+	}, dst)
+}
+
+func TestStructToMap_MapVisitorVisitsOnlyFilteredFields(t *testing.T) {
+	type A struct {
+		Field1 int
+		Field2 string
+		Field3 int
+	}
+	src := &A{Field1: 42, Field2: "hello", Field3: 44}
+	dst := map[string]interface{}{}
+	mask := fieldmask_utils.MaskFromString("Field1, Field2")
+	var visitedFields []string
+	err := fieldmask_utils.StructToMap(mask, src, dst, fieldmask_utils.WithMapVisitor(
+		func(_ fieldmask_utils.FieldFilter, _ interface{}, _ map[string]interface{},
+			srcFieldName, _ string, _ reflect.Value) (skipToNext bool) {
+			visitedFields = append(visitedFields, srcFieldName)
+			return
+		}))
+	require.NoError(t, err)
+	assert.Equal(t, visitedFields, []string{"Field1", "Field2"})
+}
+
+func TestStructToMap_WithMapVisitor_SkipsToNextField(t *testing.T) {
+	type A struct {
+		Field1 int
+		Field2 string
+		Field3 int
+	}
+	src := &A{Field1: 42, Field2: "hello", Field3: 44}
+	dst := map[string]interface{}{}
+	mask := fieldmask_utils.MaskFromString("Field1, Field2")
+	err := fieldmask_utils.StructToMap(mask, src, dst, fieldmask_utils.WithMapVisitor(
+		func(_ fieldmask_utils.FieldFilter, _ interface{}, _ map[string]interface{},
+			srcFieldName, dstFieldName string, _ reflect.Value) (skipToNext bool) {
+			if srcFieldName == "Field1" {
+				dst[dstFieldName] = 33
+				skipToNext = true
+			}
+			return
+		}))
+	require.NoError(t, err)
+	assert.Equal(t, map[string]interface{}{"Field1": 33, "Field2": "hello"}, dst)
+}
+
 func TestStructToStruct_CopySlice_WithDiffentItemKind(t *testing.T) {
 	type A struct {
 		Field1 []int