Add granular mock generation

README.md

@@ -6,6 +6,7 @@ Fake is a Go type-safe [mocking](https://en.wikipedia.org/wiki/Mock_object) gene
 
 - Type-safe mock generation
 - Support for generics
+- Granular mock generation
 - Mock cache for ultra-fast mock regeneration
 - Function call configuration, with Repeatability and Optional calls
 - Automatic call assertion
@@ -40,13 +41,16 @@ Usage:
 
 The flags are:
 
-  -input    STRING    Folder to scan for interfaces, can be invoked multiple times
-  -output   STRING    Output folder, it will follow a tree structure repeating the package path
-  -ignore   STRING    Folder to ignore, can be invoked multiple times
+  -input        STRING    Folder to scan for interfaces, can be invoked multiple times
+  -output       STRING    Output folder, it will follow a tree structure repeating the package path
+  -ignore       STRING    Folder to ignore, can be invoked multiple times
+
+  -interface    STRING    Usually used with go:generate for granular mock generation for specific interfaces
+  -mockPackage  STRING    Used with -interface. Specify the package name of the generated mock
 
 ```
 
-## Example
+## Examples
 
 A very simple example would be:
 
@@ -71,6 +75,15 @@ func (s *StubInterface[T]) Login(userID string) error
 ...
 ```
 
+Granular generation with go:gen:
+
+```go
+//go:generate fake -input FILENAME.go -interface Reader
+type Reader interface {
+	io.Reader
+}
+```
+
 ---
 
 ```go

entrypoint/fake/main.go

@@ -4,6 +4,7 @@ import (
 	"flag"
 	"fmt"
 	"os"
+	"path"
 
 	"github.com/rs/zerolog"
 	"github.com/rs/zerolog/log"
@@ -30,13 +31,32 @@ func (s *StrSlice) Set(value string) error {
 
 func main() {
 	var input, ignore StrSlice
+	var interfaceName, pkgName *string
 	flag.Var(&input, "input", "Folder to scan for .go files recursively")
 	output := flag.String("output", "mocks", "Folder to output the generated mocks")
 	flag.Var(&ignore, "ignore", "Specify which folders should be ignored")
+	interfaceName = flag.String("interface", "", "If you want to generate a single interface on the same folder, specify using this flag")
+	pkgName = flag.String("mockPackage", "", "Usable with -interface only. Provide if you want a different package from the interface being generated")
 	flag.Parse()
 	if len(input) == 0 {
 		// Defaults to $CWD
 		input = []string{"."}
 	}
+	if interfaceName != nil {
+		if len(input) == 0 {
+			log.Error().Msg("-input must be specified when using -interace")
+		}
+		if *output != "mocks" {
+			log.Error().Msgf("-output %s cannot be used when -interface is set", *output)
+			return
+		}
+		mockgen.GenerateInterface(mockgen.GenerateInterfaceConfig{
+			PackageName:   *pkgName,
+			Filename:      input[0],
+			InterfaceName: *interfaceName,
+			OutputFolder:  path.Dir(input[0]),
+		})
+		return
+	}
 	mockgen.Run(input, *output, ignore)
 }

run.go

@@ -1,13 +1,36 @@
 package fake
 
 import (
+	"fmt"
 	"path"
 	"strings"
 
 	"github.com/rs/zerolog/log"
 	"github.com/sonalys/fake/internal/caching"
+	"github.com/sonalys/fake/internal/files"
 )
 
+type GenerateInterfaceConfig struct {
+	PackageName   string
+	Filename      string
+	InterfaceName string
+	OutputFolder  string
+}
+
+func GenerateInterface(c GenerateInterfaceConfig) {
+	gen := NewGenerator(c.PackageName)
+	b := gen.GenerateFile(c.Filename, c.OutputFolder, c.InterfaceName)
+	oldFilename := strings.TrimRight(path.Base(c.Filename), path.Ext(c.Filename))
+	filename := fmt.Sprintf("%s.%s.gen.go", oldFilename, c.InterfaceName)
+	outputFilename := path.Join(c.OutputFolder, filename)
+	outputFile, err := files.CreateFileAndFolders(outputFilename)
+	if err != nil {
+		log.Fatal().Err(err).Msgf("error opening file %s", outputFilename)
+	}
+	outputFile.Write(b)
+	outputFile.Close()
+}
+
 func Run(dirs []string, output string, ignore []string) {
 	gen := NewGenerator("mocks")
 	fileHashes, err := caching.GetUncachedFiles(dirs, append(ignore, output), output)
@@ -21,8 +44,11 @@ func Run(dirs []string, output string, ignore []string) {
 		if !lockFile.Changed() {
 			continue
 		}
-		if gen.WriteFile(curFilePath, outDir) {
+		if b := gen.GenerateFile(curFilePath, outDir); len(b) > 0 {
 			counter++
+			outputFile := openOutputFile(curFilePath, output)
+			outputFile.Write(b)
+			outputFile.Close()
 		} else {
 			// Remove empty files from our new lock file.
 			delete(fileHashes, curFilePath)

testdata/stub.go

@@ -9,6 +9,7 @@ import (
 	stub "github.com/stretchr/testify/require"
 )
 
+//go:generate fake -input stub.go -interface Reader
 type Reader interface {
 	io.Reader
 }

writer.go

@@ -11,30 +11,42 @@ import (
 	"github.com/sonalys/fake/internal/files"
 )
 
-func (g *Generator) WriteFile(input, output string) bool {
+func (g *Generator) GenerateFile(input, output string, interfaceNames ...string) []byte {
 	parsedFile, err := g.ParseFile(input)
 	if err != nil {
 		log.Panic().Msgf("failed to parse file: %s", input)
 	}
 	interfaces := parsedFile.ListInterfaces()
 	if len(interfaces) == 0 {
-		return false
+		return nil
 	}
+	var selectedInterfaces []*ParsedInterface
+	if len(interfaceNames) > 0 {
+		for i := range interfaces {
+			for j := range interfaceNames {
+				if interfaces[i].Name == interfaceNames[j] {
+					selectedInterfaces = append(selectedInterfaces, interfaces[i])
+				}
+			}
+		}
+	} else {
+		selectedInterfaces = interfaces
+	}
+
 	header := bytes.NewBuffer(make([]byte, 0, parsedFile.Size))
 	body := bytes.NewBuffer(make([]byte, 0, parsedFile.Size))
+	if g.MockPackageName == "" {
+		g.MockPackageName = parsedFile.PkgName
+	}
 	writeHeader(header, g.MockPackageName)
 	// Iterate through the declarations in the file
-	for _, i := range interfaces {
+	for _, i := range selectedInterfaces {
 		i.write(body)
 	}
 	// writeImports comes after interfaces because we only add external dependencies after generating interfaces.
 	parsedFile.writeImports(header)
 	header.Write(body.Bytes())
-	b := formatCode(header.Bytes())
-	outputFile := openOutputFile(input, output)
-	defer outputFile.Close()
-	outputFile.Write(b)
-	return true
+	return formatCode(header.Bytes())
 }
 
 func writeHeader(w io.Writer, packageName string) {