Custom fakeable types

README.md

@@ -168,6 +168,38 @@ fmt.Println(f.Created.String()) // 1908-12-07 04:14:25.685339029 +0000 UTC
 // Nested Struct Fields and Embedded Fields
 ```
 
+## Fakeable types
+
+It is possible to extend a struct by implementing the `Fakeable` interface
+in order to control the generation.
+
+For example, this is useful when it is not possible to modify the struct that you want to fake by adding struct tags to a field but you still need to be able to control the generation process.
+
+```go
+// Imagine a CustomTime type that is needed to support a custom JSON Marshaler
+type CustomTime time.Time
+
+func (c *CustomTime) Fake(faker *gofakeit.Faker) interface{} {
+	return CustomTime(time.Now())
+}
+
+func (c *CustomTime) MarshalJSON() ([]byte, error) {
+	//...
+}
+
+// This is the struct that we cannot modify to add struct tags
+type NotModifiable struct {
+	Token string
+	Creation *CustomTime
+}
+
+var f NotModifiable
+gofakeit.Struct(&f)
+fmt.Printf("%s", f.Token) // yvqqdH
+fmt.Printf("%s", f.Creation) // 2023-04-02 23:00:00 +0000 UTC m=+0.000000001
+
+```
+
 ## Custom Functions
 
 In a lot of situations you may need to use your own random function usage for your specific needs.

fakeable.go

@@ -0,0 +1,80 @@
+package gofakeit
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+// Fakeable is an interface that can be implemented by a type to provide a custom fake value.
+type Fakeable interface {
+	// Fake returns a fake value for the type.
+	Fake(faker *Faker) interface{}
+}
+
+func isFakeable(t reflect.Type) bool {
+	fakeableTyp := reflect.TypeOf((*Fakeable)(nil)).Elem()
+
+	return t.Implements(fakeableTyp) || reflect.PtrTo(t).Implements(fakeableTyp)
+}
+
+func callFake(faker *Faker, v reflect.Value, possibleKinds ...reflect.Kind) (interface{}, error) {
+	f, ok := v.Addr().Interface().(Fakeable)
+	if !ok {
+		return nil, errors.New("not a Fakeable type")
+	}
+
+	fakedValue := f.Fake(faker)
+	k := reflect.TypeOf(fakedValue).Kind()
+	if !containsKind(possibleKinds, k) {
+		return nil, fmt.Errorf("returned value kind %q is not amongst the valid ones: %v", k, possibleKinds)
+	}
+
+	switch k {
+	case reflect.String:
+		return reflect.ValueOf(fakedValue).String(), nil
+	case reflect.Bool:
+		return reflect.ValueOf(fakedValue).Bool(), nil
+	case reflect.Int:
+		return int(reflect.ValueOf(fakedValue).Int()), nil
+	case reflect.Int8:
+		return int8(reflect.ValueOf(fakedValue).Int()), nil
+	case reflect.Int16:
+		return int16(reflect.ValueOf(fakedValue).Int()), nil
+	case reflect.Int32:
+		return int32(reflect.ValueOf(fakedValue).Int()), nil
+	case reflect.Int64:
+		return int64(reflect.ValueOf(fakedValue).Int()), nil
+	case reflect.Uint:
+		return uint(reflect.ValueOf(fakedValue).Uint()), nil
+	case reflect.Uint8:
+		return uint8(reflect.ValueOf(fakedValue).Uint()), nil
+	case reflect.Uint16:
+		return uint16(reflect.ValueOf(fakedValue).Uint()), nil
+	case reflect.Uint32:
+		return uint32(reflect.ValueOf(fakedValue).Uint()), nil
+	case reflect.Uint64:
+		return uint64(reflect.ValueOf(fakedValue).Uint()), nil
+	case reflect.Float32:
+		return float32(reflect.ValueOf(fakedValue).Float()), nil
+	case reflect.Float64:
+		return float64(reflect.ValueOf(fakedValue).Float()), nil
+	case reflect.Slice:
+		return reflect.ValueOf(fakedValue).Interface(), nil
+	case reflect.Map:
+		return reflect.ValueOf(fakedValue).Interface(), nil
+	case reflect.Struct:
+		return reflect.ValueOf(fakedValue).Interface(), nil
+	default:
+		return nil, fmt.Errorf("unsupported type %q", k)
+	}
+}
+
+func containsKind(possibleKinds []reflect.Kind, kind reflect.Kind) bool {
+	for _, k := range possibleKinds {
+		if k == kind {
+			return true
+		}
+	}
+	return false
+}