Support fs.FS (#118)

aconfig.go

@@ -3,6 +3,7 @@ package aconfig
 import (
 	"flag"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
@@ -24,6 +25,7 @@ type Loader struct {
 	config  Config
 	dst     interface{}
 	fields  []*fieldData
+	fsys    fs.FS
 	flagSet *flag.FlagSet
 	errInit error
 }
@@ -69,6 +71,9 @@ type Config struct {
 	// FailOnFileNotFound will stop Loader on a first not found file from Files field in this structure.
 	FailOnFileNotFound bool
 
+	// FileSystem from which files will be loaded. Default is nil (OS file system).
+	FileSystem fs.FS
+
 	// MergeFiles set to true will collect all the entries from all the given files.
 	// Easy wat to cobine base.yaml with prod.yaml
 	MergeFiles bool
@@ -100,6 +105,7 @@ type Config struct {
 type FileDecoder interface {
 	Format() string
 	DecodeFile(filename string) (map[string]interface{}, error)
+	// Init(fsys fs.FS)
 }
 
 // Field of the user configuration structure.
@@ -142,12 +148,21 @@ func (l *Loader) init() {
 		l.config.FlagPrefix += l.config.FlagDelimiter
 	}
 
+	l.fsys = &fsOrOS{l.config.FileSystem}
+
 	if _, ok := l.config.FileDecoders[".json"]; !ok {
 		if l.config.FileDecoders == nil {
 			l.config.FileDecoders = map[string]FileDecoder{}
 		}
 		l.config.FileDecoders[".json"] = &jsonDecoder{}
 	}
+	for _, dec := range l.config.FileDecoders {
+		dec, ok := dec.(interface{ Init(fs.FS) })
+		if !ok {
+			continue
+		}
+		dec.Init(l.fsys)
+	}
 
 	if l.config.Args == nil {
 		l.config.Args = os.Args[1:]
@@ -280,7 +295,7 @@ func (l *Loader) loadFiles() error {
 	}
 
 	for _, file := range l.config.Files {
-		if _, err := os.Stat(file); os.IsNotExist(err) {
+		if _, err := fs.Stat(l.fsys, file); os.IsNotExist(err) {
 			if l.config.FailOnFileNotFound {
 				return err
 			}

aconfig_test.go

@@ -1,6 +1,7 @@
 package aconfig
 
 import (
+	"embed"
 	"io/ioutil"
 	"net/url"
 	"os"
@@ -242,6 +243,44 @@ func TestFile(t *testing.T) {
 	}
 }
 
+//go:embed testdata
+var configEmbed embed.FS
+
+func TestFileEmbed(t *testing.T) {
+	filepath := "testdata/config.json"
+
+	var cfg TestConfig
+	loader := LoaderFor(&cfg, Config{
+		SkipDefaults: true,
+		SkipEnv:      true,
+		SkipFlags:    true,
+		Files:        []string{filepath},
+		FileSystem:   configEmbed,
+	})
+	if err := loader.Load(); err != nil {
+		t.Fatal(err)
+	}
+
+	want := TestConfig{
+		Str:      "str-json",
+		Bytes:    []byte("Ynl0ZXMtanNvbg=="),
+		Int:      int32Ptr(101),
+		HTTPPort: 65000,
+		Sub: SubConfig{
+			Float: 999.111,
+		},
+		Anon: struct {
+			IsAnon bool `default:"true"`
+		}{
+			IsAnon: true,
+		},
+	}
+
+	if got := cfg; !reflect.DeepEqual(want, got) {
+		t.Fatalf("want %v, got %v", want, got)
+	}
+}
+
 func TestFileMerging(t *testing.T) {
 	file1 := "testdata/config1.json"
 	file2 := "testdata/config2.json"

utils.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"io/fs"
 	"os"
 	"reflect"
 	"strings"
@@ -138,7 +139,24 @@ func cut(s, sep string) (before, after string, found bool) {
 	return s, "", false
 }
 
-type jsonDecoder struct{}
+var _ fs.FS = &fsOrOS{}
+
+type fsOrOS struct{ fs.FS }
+
+func (fs *fsOrOS) Open(name string) (fs.File, error) {
+	if fs.FS == nil {
+		return os.Open(name)
+	}
+	return fs.FS.Open(name)
+}
+
+type jsonDecoder struct {
+	fsys fs.FS
+}
+
+func (d *jsonDecoder) Init(fsys fs.FS) {
+	d.fsys = fsys
+}
 
 // Format of the decoder.
 func (d *jsonDecoder) Format() string {
@@ -147,7 +165,7 @@ func (d *jsonDecoder) Format() string {
 
 // DecodeFile implements FileDecoder.
 func (d *jsonDecoder) DecodeFile(filename string) (map[string]interface{}, error) {
-	f, err := os.Open(filename)
+	f, err := d.fsys.Open(filename)
 	if err != nil {
 		return nil, err
 	}