external

README

External Types

External types are types that can't be modified since they are defined in an external library. Hence their fields can't be annotated with the cfg tag.

To be able to use them in the config and automatically populate the config parameter value to the config struct, two things have to be done:

1. The field has to be annotated with the cfg tag.
2. A mapping function mapping from a string to the field type has to be defined and registered.

For structs all mapping functions are called with a string as input parameter.

Annotate the Field

The field Field1 uses the external type time.Time. It is annotated with the with the cfg tag. With the tag definition the mapping function strToTime is referenced.

type ExternalTypes struct {
    Field1 time.Time `cfg:"{'name':'field-1','mapfun':'strToTime'}"`
}

Define and Register a Mapping Function

As an example the following function maps a given string to a time.Time

// strToTime maps a string to a time.Time
func strToTime(rawUntypedValue interface{}, targetType reflect.Type) (interface{}, error) {
    asStr, ok := rawUntypedValue.(string)
    if !ok {
        return nil, fmt.Errorf("Expected a string. Type '%T' is not supported", rawUntypedValue)
    }
    t, err := time.Parse("2006-01-02 15:04:05 +0000 UTC", asStr)
    if err != nil {
        return nil, fmt.Errorf("Parse %s to time failed: %s", asStr, err.Error())
    }
    return t, nil
}

As a last step that function has to be registered at the provider.

[...]
if err := provider.RegisterMappingFunc("strToTime", strToTime); err != nil {
    panic(err)
}
[...]