Add ActionableFlag interface instead of modifying Flag interface directly

app.go

@@ -664,8 +664,10 @@ func runFlagActions(c *Context, fs []Flag) error {
 			}
 		}
 		if isSet {
-			if err := f.RunAction(c); err != nil {
-				return err
+			if af, ok := f.(ActionableFlag); ok {
+				if err := af.RunAction(c); err != nil {
+					return err
+				}
 			}
 		}
 	}

flag.go

@@ -83,6 +83,12 @@ func (f FlagsByName) Swap(i, j int) {
 	f[i], f[j] = f[j], f[i]
 }
 
+// ActionableFlag is an interface that wraps Flag interface and RunAction operation.
+type ActionableFlag interface {
+	Flag
+	RunAction(*Context) error
+}
+
 // Flag is a common interface related to parsing flags in cli.
 // For more advanced flag parsing techniques, it is recommended that
 // this interface be implemented.
@@ -92,7 +98,6 @@ type Flag interface {
 	Apply(*flag.FlagSet) error
 	Names() []string
 	IsSet() bool
-	RunAction(*Context) error
 }
 
 // RequiredFlag is an interface that allows us to mark flags as required