feat: allow hooks to be declared on embedded fields

Specifically, on Go embedded fields, not on fields tagged with `embed`.

Fixes #90.

README.md

@@ -307,8 +307,8 @@ func main() {
 
 ## Hooks: BeforeReset(), BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option
 
-If a node in the grammar has a `BeforeReset(...)`, `BeforeResolve
-(...)`, `BeforeApply(...) error` and/or `AfterApply(...) error` method, those
+If a node in the CLI, or any of its embedded fields, has a `BeforeReset(...) error`, `BeforeResolve
+(...) error`, `BeforeApply(...) error` and/or `AfterApply(...) error` method, those
 methods will be called before values are reset, before validation/assignment,
 and after validation/assignment, respectively.
 
@@ -341,40 +341,6 @@ func main() {
 }
 ```
 
-Another example of using hooks is load the env-file:
-
-```go
-package main
-
-import (
-  "fmt"
-  "github.com/alecthomas/kong"
-  "github.com/joho/godotenv"
-)
-
-type EnvFlag string
-
-// BeforeResolve loads env file.
-func (c EnvFlag) BeforeReset(ctx *kong.Context, trace *kong.Path) error {
-  path := string(ctx.FlagValue(trace.Flag).(EnvFlag)) // nolint
-  path = kong.ExpandPath(path)
-  if err := godotenv.Load(path); err != nil {
-    return err
-  }
-  return nil
-}
-
-var CLI struct {
-  EnvFile EnvFlag
-  Flag `env:"FLAG"`
-}
-
-func main() {
-  _ = kong.Parse(&CLI)
-  fmt.Println(CLI.Flag)
-}
-```
-
 ## Flags
 
 Any [mapped](#mapper---customising-how-the-command-line-is-mapped-to-go-values) field in the command structure _not_ tagged with `cmd` or `arg` will be a flag. Flags are optional by default.

callbacks.go

@@ -68,6 +68,33 @@ func getMethod(value reflect.Value, name string) reflect.Value {
 	return method
 }
 
+// Get methods from the given value and any embedded fields.
+func getMethods(value reflect.Value, name string) []reflect.Value {
+	// Collect all possible receivers
+	receivers := []reflect.Value{value}
+	if value.Kind() == reflect.Ptr {
+		value = value.Elem()
+	}
+	if value.Kind() == reflect.Struct {
+		t := value.Type()
+		for i := 0; i < value.NumField(); i++ {
+			field := value.Field(i)
+			fieldType := t.Field(i)
+			if fieldType.IsExported() && fieldType.Anonymous {
+				receivers = append(receivers, field)
+			}
+		}
+	}
+	// Search all receivers for methods
+	var methods []reflect.Value
+	for _, receiver := range receivers {
+		if method := getMethod(receiver, name); method.IsValid() {
+			methods = append(methods, method)
+		}
+	}
+	return methods
+}
+
 func callFunction(f reflect.Value, bindings bindings) error {
 	if f.Kind() != reflect.Func {
 		return fmt.Errorf("expected function, got %s", f.Type())

kong.go

@@ -361,16 +361,14 @@ func (k *Kong) applyHook(ctx *Context, name string) error {
 		default:
 			panic("unsupported Path")
 		}
-		method := getMethod(value, name)
-		if !method.IsValid() {
-			continue
-		}
-		binds := k.bindings.clone()
-		binds.add(ctx, trace)
-		binds.add(trace.Node().Vars().CloneWith(k.vars))
-		binds.merge(ctx.bindings)
-		if err := callFunction(method, binds); err != nil {
-			return err
+		for _, method := range getMethods(value, name) {
+			binds := k.bindings.clone()
+			binds.add(ctx, trace)
+			binds.add(trace.Node().Vars().CloneWith(k.vars))
+			binds.merge(ctx.bindings)
+			if err := callFunction(method, binds); err != nil {
+				return err
+			}
 		}
 	}
 	// Path[0] will always be the app root.
@@ -392,13 +390,11 @@ func (k *Kong) applyHookToDefaultFlags(ctx *Context, node *Node, name string) er
 			if !flag.HasDefault || ctx.values[flag.Value].IsValid() || !flag.Target.IsValid() {
 				continue
 			}
-			method := getMethod(flag.Target, name)
-			if !method.IsValid() {
-				continue
-			}
-			path := &Path{Flag: flag}
-			if err := callFunction(method, binds.clone().add(path)); err != nil {
-				return next(err)
+			for _, method := range getMethods(flag.Target, name) {
+				path := &Path{Flag: flag}
+				if err := callFunction(method, binds.clone().add(path)); err != nil {
+					return next(err)
+				}
 			}
 		}
 		return next(nil)

kong_test.go

@@ -2406,3 +2406,36 @@ func TestProviderMethods(t *testing.T) {
 	err = kctx.Run(t)
 	assert.NoError(t, err)
 }
+
+type EmbeddedCallback struct {
+	Embedded bool
+}
+
+func (e *EmbeddedCallback) AfterApply() error {
+	e.Embedded = true
+	return nil
+}
+
+type EmbeddedRoot struct {
+	EmbeddedCallback
+	Root bool
+}
+
+func (e *EmbeddedRoot) AfterApply() error {
+	e.Root = true
+	return nil
+}
+
+func TestEmbeddedCallbacks(t *testing.T) {
+	actual := &EmbeddedRoot{}
+	k := mustNew(t, actual)
+	_, err := k.Parse(nil)
+	assert.NoError(t, err)
+	expected := &EmbeddedRoot{
+		EmbeddedCallback: EmbeddedCallback{
+			Embedded: true,
+		},
+		Root: true,
+	}
+	assert.Equal(t, expected, actual)
+}