Support customising mapstructure.DecoderConfig for Unmarshal

* Added a new `DecoderConfigOption` type allowing the user to write custom
  functions that can override the default mapstructure.DecoderConfig
  settings

* Added a new `DecodeHook` function which returns
  a `DecoderConfigOption`. This allows the user to easily set their own
  Decode hooks when Unmarshaling

* Updated Unmarshal, UnmarshalKey and defaultDecoderConfig to support variadic
  trailing `DecoderConfigOption` functions to allow for customisation of
  the default  mapstructure.DecoderConfig

* Added a test case with example usage

viper.go

@@ -113,6 +113,23 @@ func (fnfe ConfigFileNotFoundError) Error() string {
 	return fmt.Sprintf("Config File %q Not Found in %q", fnfe.name, fnfe.locations)
 }
 
+// A DecoderConfigOption can be passed to viper.Unmarshal to configure
+// mapstructure.DecoderConfig options
+type DecoderConfigOption func(*mapstructure.DecoderConfig)
+
+// DecodeHook returns a DecoderConfigOption which overrides the default
+// DecoderConfig.DecodeHook value, the default is:
+//
+//  mapstructure.ComposeDecodeHookFunc(
+//		mapstructure.StringToTimeDurationHookFunc(),
+//		mapstructure.StringToSliceHookFunc(","),
+//	)
+func DecodeHook(hook mapstructure.DecodeHookFunc) DecoderConfigOption {
+	return func(c *mapstructure.DecoderConfig) {
+		c.DecodeHook = hook
+	}
+}
+
 // Viper is a prioritized configuration registry. It
 // maintains a set of configuration sources, fetches
 // values to populate those, and provides them according
@@ -745,9 +762,11 @@ func (v *Viper) GetSizeInBytes(key string) uint {
 }
 
 // UnmarshalKey takes a single key and unmarshals it into a Struct.
-func UnmarshalKey(key string, rawVal interface{}) error { return v.UnmarshalKey(key, rawVal) }
-func (v *Viper) UnmarshalKey(key string, rawVal interface{}) error {
-	err := decode(v.Get(key), defaultDecoderConfig(rawVal))
+func UnmarshalKey(key string, rawVal interface{}, opts ...DecoderConfigOption) error {
+	return v.UnmarshalKey(key, rawVal, opts...)
+}
+func (v *Viper) UnmarshalKey(key string, rawVal interface{}, opts ...DecoderConfigOption) error {
+	err := decode(v.Get(key), defaultDecoderConfig(rawVal, opts...))
 
 	if err != nil {
 		return err
@@ -760,9 +779,11 @@ func (v *Viper) UnmarshalKey(key string, rawVal interface{}) error {
 
 // Unmarshal unmarshals the config into a Struct. Make sure that the tags
 // on the fields of the structure are properly set.
-func Unmarshal(rawVal interface{}) error { return v.Unmarshal(rawVal) }
-func (v *Viper) Unmarshal(rawVal interface{}) error {
-	err := decode(v.AllSettings(), defaultDecoderConfig(rawVal))
+func Unmarshal(rawVal interface{}, opts ...DecoderConfigOption) error {
+	return v.Unmarshal(rawVal, opts...)
+}
+func (v *Viper) Unmarshal(rawVal interface{}, opts ...DecoderConfigOption) error {
+	err := decode(v.AllSettings(), defaultDecoderConfig(rawVal, opts...))
 
 	if err != nil {
 		return err
@@ -775,8 +796,8 @@ func (v *Viper) Unmarshal(rawVal interface{}) error {
 
 // defaultDecoderConfig returns default mapsstructure.DecoderConfig with suppot
 // of time.Duration values & string slices
-func defaultDecoderConfig(output interface{}) *mapstructure.DecoderConfig {
-	return &mapstructure.DecoderConfig{
+func defaultDecoderConfig(output interface{}, opts ...DecoderConfigOption) *mapstructure.DecoderConfig {
+	c := &mapstructure.DecoderConfig{
 		Metadata:         nil,
 		Result:           output,
 		WeaklyTypedInput: true,
@@ -785,6 +806,10 @@ func defaultDecoderConfig(output interface{}) *mapstructure.DecoderConfig {
 			mapstructure.StringToSliceHookFunc(","),
 		),
 	}
+	for _, opt := range opts {
+		opt(c)
+	}
+	return c
 }
 
 // A wrapper around mapstructure.Decode that mimics the WeakDecode functionality

viper_test.go

@@ -7,6 +7,7 @@ package viper
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -18,6 +19,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/mitchellh/mapstructure"
 	"github.com/spf13/afero"
 	"github.com/spf13/cast"
 
@@ -503,6 +505,42 @@ func TestUnmarshal(t *testing.T) {
 	assert.Equal(t, &config{Name: "Steve", Port: 1234, Duration: time.Second + time.Millisecond}, &C)
 }
 
+func TestUnmarshalWithDecoderOptions(t *testing.T) {
+	Set("credentials", "{\"foo\":\"bar\"}")
+
+	opt := DecodeHook(mapstructure.ComposeDecodeHookFunc(
+		mapstructure.StringToTimeDurationHookFunc(),
+		mapstructure.StringToSliceHookFunc(","),
+		// Custom Decode Hook Function
+		func(rf reflect.Kind, rt reflect.Kind, data interface{}) (interface{}, error) {
+			if rf != reflect.String || rt != reflect.Map {
+				return data, nil
+			}
+			m := map[string]string{}
+			raw := data.(string)
+			if raw == "" {
+				return m, nil
+			}
+			return m, json.Unmarshal([]byte(raw), &m)
+		},
+	))
+
+	type config struct {
+		Credentials map[string]string
+	}
+
+	var C config
+
+	err := Unmarshal(&C, opt)
+	if err != nil {
+		t.Fatalf("unable to decode into struct, %v", err)
+	}
+
+	assert.Equal(t, &config{
+		Credentials: map[string]string{"foo": "bar"},
+	}, &C)
+}
+
 func TestBindPFlags(t *testing.T) {
 	v := New() // create independent Viper object
 	flagSet := pflag.NewFlagSet("test", pflag.ContinueOnError)