Added registration for conversion function to different type

[tomtwinkle]

Changed so that you can add a function that can be converted to any type.

Can freely customize the type conversion by defining the function for TypeCopier by the user.

type TypePair struct {
	SrcType reflect.Type
	DstType reflect.Type
}

type TypedCopier interface {
	// Define the conversion process
	Copy(dstValue, srcValue reflect.Value) error
	// Defines the type of the source field and the type of the destination field
	Pairs() []TypePair
}

Example

type timeToStringCopier struct{}

func (t timeToStringCopier) Pairs() []copier.TypePair {
	return []copier.TypePair{
		{
			SrcType: reflect.TypeOf(time.Time{}),
			DstType: reflect.TypeOf(""),
		},
		{
			SrcType: reflect.TypeOf(&time.Time{}),
			DstType: reflect.TypeOf(""),
		},
	}
}

func (t timeToStringCopier) Copy(dst, src reflect.Value) error {
	const timeFormat = "2006-01-02T15:04:05.999999999Z07:00"
	if src.Kind() == reflect.Ptr && src.IsNil() {
		if dst.Kind() == reflect.Ptr {
			dst.Set(reflect.Zero(reflect.TypeOf("")))
		}
		return nil
	}

	var val string
	if src.Kind() == reflect.Ptr {
		s, ok := src.Interface().(*time.Time)
		if !ok {
			return errors.New("type not match")
		}
		val = s.Format(timeFormat)
	} else {
		s, ok := src.Interface().(time.Time)
		if !ok {
			return errors.New("type not match")
		}
		val = s.Format(timeFormat)
	}
	dst.Set(reflect.ValueOf(val))
	return nil
}

func main() {
	type SrcStruct1 struct {
		Field1 time.Time
		Field2 *time.Time
	}

	type DestStruct1 struct {
		Field1 string
		Field2 string
	}

	c := copier.NewCopier()
	c.Register(&timeToStringCopier{})

	testTime := time.Date(2021, 3, 5, 1, 30, 0, 123000000, time.UTC)
	src := SrcStruct1{
		Field1: testTime,
		Field2: &testTime,
	}
	var dst DestStruct1

	if err := c.Copy(&dst, src); err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%#v", dst)
	// DestStruct1{Field1:"2021-03-05T01:30:00.123Z", Field2:"2021-03-05T01:30:00.123Z"}
}

[jinzhu]

Hello @tomtwinkle

Thank you for your PR, but maybe should pass those registered type in the Option struct? Also might be good to allow to register some default global converters.