| // Package dynamic provides an implementation for a dynamic protobuf message. |
| // |
| // The dynamic message is essentially a message descriptor along with a map of |
| // tag numbers to values. It has a broad API for interacting with the message, |
| // including inspection and modification. Generally, most operations have two |
| // forms: a regular method that panics on bad input or error and a "Try" form |
| // of the method that will instead return an error. |
| // |
| // A dynamic message can optionally be constructed with a MessageFactory. The |
| // MessageFactory has various registries that may be used by the dynamic message, |
| // such as during de-serialization. The message factory is "inherited" by any |
| // other dynamic messages created, such as nested messages that are created |
| // during de-serialization. Similarly, any dynamic message created using |
| // MessageFactory.NewMessage will be associated with that factory, which in turn |
| // will be used to create other messages or parse extension fields during |
| // de-serialization. |
| // |
| // |
| // Field Types |
| // |
| // The types of values expected by setters and returned by getters are the |
| // same as protoc generates for scalar fields. For repeated fields, there are |
| // methods for getting and setting values at a particular index or for adding |
| // an element. Similarly, for map fields, there are methods for getting and |
| // setting values for a particular key. |
| // |
| // If you use GetField for a repeated field, it will return a copy of all |
| // elements as a slice []interface{}. Similarly, using GetField for a map field |
| // will return a copy of all mappings as a map[interface{}]interface{}. You can |
| // also use SetField to supply an entire slice or map for repeated or map fields. |
| // The slice need not be []interface{} but can actually be typed according to |
| // the field's expected type. For example, a repeated uint64 field can be set |
| // using a slice of type []uint64. |
| // |
| // Descriptors for map fields describe them as repeated fields with a nested |
| // message type. The nested message type is a special generated type that |
| // represents a single mapping: key and value pair. The dynamic message has some |
| // special affordances for this representation. For example, you can use |
| // SetField to set a map field using a slice of these entry messages. Internally, |
| // the slice of entries will be converted to an actual map. Similarly, you can |
| // use AddRepeatedField with an entry message to add (or overwrite) a mapping. |
| // However, you cannot use GetRepeatedField or SetRepeatedField to modify maps, |
| // since those take numeric index arguments which are not relevant to maps |
| // (since maps in Go have no defined ordering). |
| // |
| // When setting field values in dynamic messages, the type-checking is lenient |
| // in that it accepts any named type with the right kind. So a string field can |
| // be assigned to any type that is defined as a string. Enum fields require |
| // int32 values (or any type that is defined as an int32). |
| // |
| // Unlike normal use of numeric values in Go, values will be automatically |
| // widened when assigned. So, for example, an int64 field can be set using an |
| // int32 value since it can be safely widened without truncation or loss of |
| // precision. Similar goes for uint32 values being converted to uint64 and |
| // float32 being converted to float64. Narrowing conversions are not done, |
| // however. Also, unsigned values will never be automatically converted to |
| // signed (and vice versa), and floating point values will never be |
| // automatically converted to integral values (and vice versa). Since the bit |
| // width of int and uint fields is allowed to be platform dependent, but will |
| // always be less than or equal to 64, they can only be used as values for |
| // int64 and uint64 fields, respectively. They cannot be used to set int32 or |
| // uint32 fields, which includes enums fields. |
| // |
| // Fields whose type is a nested message can have values set to either other |
| // dynamic messages or generated messages (e.g. pointers to structs generated by |
| // protoc). Getting a value for such a field will return the actual type it is |
| // set to (e.g. either a dynamic message or a generated message). If the value |
| // is not set and the message uses proto2 syntax, the default message returned |
| // will be whatever is returned by the dynamic message's MessageFactory (if the |
| // dynamic message was not created with a factory, it will use the logic of the |
| // zero value factory). In most typical cases, it will return a dynamic message, |
| // but if the factory is configured with a KnownTypeRegistry, or if the field's |
| // type is a well-known type, it will return a zero value generated message. |
| // |
| // |
| // Unrecognized Fields |
| // |
| // Unrecognized fields are preserved by the dynamic message when unmarshaling |
| // from the standard binary format. If the message's MessageFactory was |
| // configured with an ExtensionRegistry, it will be used to identify and parse |
| // extension fields for the message. |
| // |
| // Unrecognized fields can dynamically become recognized fields if the |
| // application attempts to retrieve an unrecognized field's value using a |
| // FieldDescriptor. In this case, the given FieldDescriptor is used to parse the |
| // unknown field and move the parsed value into the message's set of known |
| // fields. This behavior is most suited to the use of extensions, where an |
| // ExtensionRegistry is not setup with all known extensions ahead of time. But |
| // it can even happen for non-extension fields! Here's an example scenario where |
| // a non-extension field can initially be unknown and become known: |
| // |
| // 1. A dynamic message is created with a descriptor, A, and then |
| // de-serialized from a stream of bytes. The stream includes an |
| // unrecognized tag T. The message will include tag T in its unrecognized |
| // field set. |
| // 2. Another call site retrieves a newer descriptor, A', which includes a |
| // newly added field with tag T. |
| // 3. That other call site then uses a FieldDescriptor to access the value of |
| // the new field. This will cause the dynamic message to parse the bytes |
| // for the unknown tag T and store them as a known field. |
| // 4. Subsequent operations for tag T, including setting the field using only |
| // tag number or de-serializing a stream that includes tag T, will operate |
| // as if that tag were part of the original descriptor, A. |
| // |
| // |
| // Compatibility |
| // |
| // In addition to implementing the proto.Message interface, the included |
| // Message type also provides an XXX_MessageName() method, so it can work with |
| // proto.MessageName. And it provides a Descriptor() method that behaves just |
| // like the method of the same signature in messages generated by protoc. |
| // Because of this, it is actually compatible with proto.Message in many (though |
| // not all) contexts. In particular, it is compatible with proto.Marshal and |
| // proto.Unmarshal for serializing and de-serializing messages. |
| // |
| // The dynamic message supports binary and text marshaling, using protobuf's |
| // well-defined binary format and the same text format that protoc-generated |
| // types use. It also supports JSON serialization/de-serialization by |
| // implementing the json.Marshaler and json.Unmarshaler interfaces. And dynamic |
| // messages can safely be used with the jsonpb package for JSON serialization |
| // and de-serialization. |
| // |
| // In addition to implementing the proto.Message interface and numerous related |
| // methods, it also provides inter-op with generated messages via conversion. |
| // The ConvertTo, ConvertFrom, MergeInto, and MergeFrom methods copy message |
| // contents from a dynamic message to a generated message and vice versa. |
| // |
| // When copying from a generated message into a dynamic message, if the |
| // generated message contains fields unknown to the dynamic message (e.g. not |
| // present in the descriptor used to create the dynamic message), these fields |
| // become known to the dynamic message (as per behavior described above in |
| // "Unrecognized Fields"). If the generated message has unrecognized fields of |
| // its own, including unrecognized extensions, they are preserved in the dynamic |
| // message. It is possible that the dynamic message knows about fields that the |
| // generated message did not, like if it has a different version of the |
| // descriptor or its MessageFactory has an ExtensionRegistry that knows about |
| // different extensions than were linked into the program. In this case, these |
| // unrecognized fields in the generated message will be known fields in the |
| // dynamic message. |
| // |
| // Similarly, when copying from a dynamic message into a generated message, if |
| // the dynamic message has unrecognized fields they can be preserved in the |
| // generated message (currently only for syntax proto2 since proto3 generated |
| // messages do not preserve unrecognized fields). If the generated message knows |
| // about fields that the dynamic message does not, these unrecognized fields may |
| // become known fields in the generated message. |
| // |
| // |
| // Registries |
| // |
| // This package also contains a couple of registries, for managing known types |
| // and descriptors. |
| // |
| // The KnownTypeRegistry allows de-serialization of a dynamic message to use |
| // generated message types, instead of dynamic messages, for some kinds of |
| // nested message fields. This is particularly useful for working with proto |
| // messages that have special encodings as JSON (e.g. the well-known types), |
| // since the dynamic message does not try to handle these special cases in its |
| // JSON marshaling facilities. |
| // |
| // The ExtensionRegistry allows for recognizing and parsing extensions fields |
| // (for proto2 messages). |
| package dynamic |