feat: onset hook

README.md

@@ -290,11 +290,51 @@ func main() {
 }
 ```
 
+
+### On set hooks
+
+You might want to listen to value sets and, for example, log something or do some other kind of logic.
+You can do this by passing a `OnSet` option:
+
+```go
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/caarlos0/env/v6"
+)
+
+type Config struct {
+	Username string `env:"USERNAME" envDefault:"admin"`
+	Password string `env:"PASSWORD"`
+}
+
+func main() {
+	cfg := &Config{}
+	opts := &env.Options{
+		OnSet: func(tag string, value interface{}, isDefault bool) {
+			fmt.Printf("Set %s to %v (default? %v)\n", tag, value, isDefault)
+		},
+	}
+
+	// Load env vars.
+	if err := env.Parse(cfg, opts); err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the loaded data.
+	fmt.Printf("%+v\n", cfg.envData)
+}
+```
+
 ## Making all fields to required
 
 You can make all fields that don't have a default value be required by setting the `RequiredIfNoDef: true` in the `Options`.
 
 For example
+
 ```go
 package main
 
@@ -306,8 +346,8 @@ import (
 )
 
 type Config struct {
-	Username string `env:"USERNAME" envDefault:"admin"` // not required
-	Password string `env:"PASSWORD"`                    // required
+	Username string `env:"USERNAME" envDefault:"admin"`
+	Password string `env:"PASSWORD"`
 }
 
 func main() {

env.go

@@ -95,6 +95,8 @@ var (
 // ParserFunc defines the signature of a function that can be used within `CustomParsers`.
 type ParserFunc func(v string) (interface{}, error)
 
+type OnSetFn func(tag string, value interface{}, isDefault bool)
+
 // Options for the parser.
 type Options struct {
 	// Environment keys and values that will be accessible for the service.
@@ -104,6 +106,8 @@ type Options struct {
 	// RequiredIfNoDef automatically sets all env as required if they do not declare 'envDefault'
 	RequiredIfNoDef bool
 
+	OnSet OnSetFn
+
 	// Sets to true if we have already configured once.
 	configured bool
 }
@@ -132,12 +136,19 @@ func configure(opts []Options) []Options {
 		if item.TagName != "" {
 			opt.TagName = item.TagName
 		}
+		if item.OnSet != nil {
+			opt.OnSet = item.OnSet
+		}
 		opt.RequiredIfNoDef = item.RequiredIfNoDef
 	}
 
 	return []Options{opt}
 }
 
+func getOnSetFn(opts []Options) OnSetFn {
+	return opts[0].OnSet
+}
+
 // getTagName returns the tag name.
 func getTagName(opts []Options) string {
 	return opts[0].TagName
@@ -219,9 +230,11 @@ func doParse(ref reflect.Value, funcMap map[reflect.Type]ParserFunc, opts []Opti
 
 func get(field reflect.StructField, opts []Options) (val string, err error) {
 	var exists bool
+	var isDefault bool
 	var loadFile bool
 	var unset bool
 	var notEmpty bool
+
 	required := opts[0].RequiredIfNoDef
 	expand := strings.EqualFold(field.Tag.Get("envExpand"), "true")
 
@@ -244,8 +257,9 @@ func get(field reflect.StructField, opts []Options) (val string, err error) {
 		}
 	}
 
+	expand := strings.EqualFold(field.Tag.Get("envExpand"), "true")
 	defaultValue, defExists := field.Tag.Lookup("envDefault")
-	val, exists = getOr(key, defaultValue, defExists, getEnvironment(opts))
+	val, exists, isDefault = getOr(key, defaultValue, defExists, getEnvironment(opts))
 
 	if expand {
 		val = os.ExpandEnv(val)
@@ -271,6 +285,10 @@ func get(field reflect.StructField, opts []Options) (val string, err error) {
 		}
 	}
 
+	onSetFn := getOnSetFn(opts)
+	if onSetFn != nil {
+		onSetFn(key, val, isDefault)
+	}
 	return val, err
 }
 
@@ -285,16 +303,16 @@ func getFromFile(filename string) (value string, err error) {
 	return string(b), err
 }
 
-func getOr(key, defaultValue string, defExists bool, envs map[string]string) (value string, exists bool) {
-	value, exists = envs[key]
+func getOr(key, defaultValue string, defExists bool, envs map[string]string) (string, bool, bool) {
+	value, exists := envs[key]
 	switch {
 	case (!exists || key == "") && defExists:
-		return defaultValue, true
+		return defaultValue, true, true
 	case !exists:
-		return "", false
+		return "", false, false
 	}
 
-	return value, true
+	return value, true, false
 }
 
 func set(field reflect.Value, sf reflect.StructField, value string, funcMap map[reflect.Type]ParserFunc) error {

env_test.go

@@ -662,6 +662,39 @@ func TestNoErrorRequiredSet(t *testing.T) {
 	is.Equal("", cfg.IsRequired)
 }
 
+func TestHook(t *testing.T) {
+	is := is.New(t)
+
+	type config struct {
+		Something string `env:"SOMETHING" envDefault:"important"`
+		Another   string `env:"ANOTHER"`
+	}
+
+	cfg := &config{}
+
+	os.Setenv("ANOTHER", "1")
+	defer os.Clearenv()
+
+	type onSetArgs struct {
+		tag       string
+		key       interface{}
+		isDefault bool
+	}
+
+	var onSetCalled []onSetArgs
+
+	is.NoErr(Parse(cfg, Options{
+		OnSet: func(tag string, value interface{}, isDefault bool) {
+			onSetCalled = append(onSetCalled, onSetArgs{tag, value, isDefault})
+		},
+	}))
+	is.Equal("important", cfg.Something)
+	is.Equal("1", cfg.Another)
+	is.Equal(2, len(onSetCalled))
+	is.Equal(onSetArgs{"SOMETHING", "important", true}, onSetCalled[0])
+	is.Equal(onSetArgs{"ANOTHER", "1", false}, onSetCalled[1])
+}
+
 func TestErrorRequiredWithDefault(t *testing.T) {
 	is := is.New(t)
 
@@ -1103,6 +1136,28 @@ func ExampleParse() {
 	// Output: {Home:/tmp/fakehome Port:3000 IsProduction:false Inner:{Foo:foobar}}
 }
 
+func ExampleParse_onSet() {
+	type config struct {
+		Home         string `env:"HOME,required"`
+		Port         int    `env:"PORT" envDefault:"3000"`
+		IsProduction bool   `env:"PRODUCTION"`
+	}
+	os.Setenv("HOME", "/tmp/fakehome")
+	var cfg config
+	if err := Parse(&cfg, Options{
+		OnSet: func(tag string, value interface{}, isDefault bool) {
+			fmt.Printf("Set %s to %v (default? %v)\n", tag, value, isDefault)
+		},
+	}); err != nil {
+		fmt.Println("failed:", err)
+	}
+	fmt.Printf("%+v", cfg)
+	// Output: Set HOME to /tmp/fakehome (default? false)
+	// Set PORT to 3000 (default? true)
+	// Set PRODUCTION to  (default? false)
+	// {Home:/tmp/fakehome Port:3000 IsProduction:false}
+}
+
 func TestIgnoresUnexported(t *testing.T) {
 	is := is.New(t)