config: Add RegisterNotifier

This allows to trigger a callback when a specific config key is modified
or unset, and to do some higher level actions in response to that.
This allows to remove the somewhat ugly UpdateDefaults() call from
the generic code in config.go, and to add this to RegisterSettings
instead where it makes more sense.

pkg/crc/config/config.go

@@ -14,17 +14,22 @@ const (
 	invalidType              = "Type %T for configuration property '%s' is invalid"
 )
 
+type ValueChangedFunc func(config *Config, key string, value interface{})
+
 type Config struct {
 	storage        RawStorage
 	secretStorage  RawStorage
 	settingsByName map[string]Setting
+
+	valueChangeNotifiers map[string]ValueChangedFunc
 }
 
 func New(storage, secretStorage RawStorage) *Config {
 	return &Config{
-		storage:        storage,
-		secretStorage:  secretStorage,
-		settingsByName: make(map[string]Setting),
+		storage:              storage,
+		secretStorage:        secretStorage,
+		settingsByName:       make(map[string]Setting),
+		valueChangeNotifiers: map[string]ValueChangedFunc{},
 	}
 }
 
@@ -145,6 +150,8 @@ func (c *Config) Set(key string, value interface{}) (string, error) {
 		}
 	}
 
+	c.valueChangeNotify(key, value)
+
 	return c.settingsByName[key].callbackFn(key, castValue), nil
 }
 
@@ -163,9 +170,31 @@ func (c *Config) Unset(key string) (string, error) {
 			return "", err
 		}
 	}
+
+	c.valueChangeNotify(key, nil)
+
 	return fmt.Sprintf("Successfully unset configuration property '%s'", key), nil
 }
 
+func (c *Config) RegisterNotifier(key string, changeNotifier ValueChangedFunc) error {
+	if _, hasKey := c.valueChangeNotifiers[key]; hasKey {
+		return fmt.Errorf("Config change notifier already registered for %s", key)
+	}
+
+	c.valueChangeNotifiers[key] = changeNotifier
+
+	return nil
+}
+
+func (c *Config) valueChangeNotify(key string, value interface{}) {
+	changeNotifier, hasKey := c.valueChangeNotifiers[key]
+	if !hasKey {
+		return
+	}
+
+	changeNotifier(c, key, value)
+}
+
 func (c *Config) Get(key string) SettingValue {
 	setting, ok := c.settingsByName[key]
 	if !ok {

pkg/crc/config/viper_config_test.go

@@ -304,3 +304,26 @@ func TestTwoInstancesSetAndUnsetSameConfiguration(t *testing.T) {
 	assert.Equal(t, 4, config2.Get(cpus).Value)
 	assert.Equal(t, 4, config1.Get(cpus).Value)
 }
+
+func TestNotifier(t *testing.T) {
+	dir := t.TempDir()
+	configFile := filepath.Join(dir, "crc.json")
+
+	config, err := newTestConfig(configFile, "CRC")
+	require.NoError(t, err)
+
+	var notified = false
+	err = config.RegisterNotifier(CPUs, func(config *Config, key string, value interface{}) {
+		notified = true
+		require.Equal(t, key, CPUs)
+		assert.Equal(t, SettingValue{
+			Value:     value,
+			IsDefault: false,
+		}, config.Get(cpus))
+
+	})
+	require.NoError(t, err)
+	_, err = config.Set(cpus, 5)
+	assert.NoError(t, err)
+	require.True(t, notified)
+}