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Zack Williamse940c7a2019-08-21 14:25:39 -07001// Package dynamic provides an implementation for a dynamic protobuf message.
2//
3// The dynamic message is essentially a message descriptor along with a map of
4// tag numbers to values. It has a broad API for interacting with the message,
5// including inspection and modification. Generally, most operations have two
6// forms: a regular method that panics on bad input or error and a "Try" form
7// of the method that will instead return an error.
8//
9// A dynamic message can optionally be constructed with a MessageFactory. The
10// MessageFactory has various registries that may be used by the dynamic message,
11// such as during de-serialization. The message factory is "inherited" by any
12// other dynamic messages created, such as nested messages that are created
13// during de-serialization. Similarly, any dynamic message created using
14// MessageFactory.NewMessage will be associated with that factory, which in turn
15// will be used to create other messages or parse extension fields during
16// de-serialization.
17//
18//
19// Field Types
20//
21// The types of values expected by setters and returned by getters are the
22// same as protoc generates for scalar fields. For repeated fields, there are
23// methods for getting and setting values at a particular index or for adding
24// an element. Similarly, for map fields, there are methods for getting and
25// setting values for a particular key.
26//
27// If you use GetField for a repeated field, it will return a copy of all
28// elements as a slice []interface{}. Similarly, using GetField for a map field
29// will return a copy of all mappings as a map[interface{}]interface{}. You can
30// also use SetField to supply an entire slice or map for repeated or map fields.
31// The slice need not be []interface{} but can actually be typed according to
32// the field's expected type. For example, a repeated uint64 field can be set
33// using a slice of type []uint64.
34//
35// Descriptors for map fields describe them as repeated fields with a nested
36// message type. The nested message type is a special generated type that
37// represents a single mapping: key and value pair. The dynamic message has some
38// special affordances for this representation. For example, you can use
39// SetField to set a map field using a slice of these entry messages. Internally,
40// the slice of entries will be converted to an actual map. Similarly, you can
41// use AddRepeatedField with an entry message to add (or overwrite) a mapping.
42// However, you cannot use GetRepeatedField or SetRepeatedField to modify maps,
43// since those take numeric index arguments which are not relevant to maps
44// (since maps in Go have no defined ordering).
45//
46// When setting field values in dynamic messages, the type-checking is lenient
47// in that it accepts any named type with the right kind. So a string field can
48// be assigned to any type that is defined as a string. Enum fields require
49// int32 values (or any type that is defined as an int32).
50//
51// Unlike normal use of numeric values in Go, values will be automatically
52// widened when assigned. So, for example, an int64 field can be set using an
53// int32 value since it can be safely widened without truncation or loss of
54// precision. Similar goes for uint32 values being converted to uint64 and
55// float32 being converted to float64. Narrowing conversions are not done,
56// however. Also, unsigned values will never be automatically converted to
57// signed (and vice versa), and floating point values will never be
58// automatically converted to integral values (and vice versa). Since the bit
59// width of int and uint fields is allowed to be platform dependent, but will
60// always be less than or equal to 64, they can only be used as values for
61// int64 and uint64 fields, respectively. They cannot be used to set int32 or
62// uint32 fields, which includes enums fields.
63//
64// Fields whose type is a nested message can have values set to either other
65// dynamic messages or generated messages (e.g. pointers to structs generated by
66// protoc). Getting a value for such a field will return the actual type it is
67// set to (e.g. either a dynamic message or a generated message). If the value
68// is not set and the message uses proto2 syntax, the default message returned
69// will be whatever is returned by the dynamic message's MessageFactory (if the
70// dynamic message was not created with a factory, it will use the logic of the
71// zero value factory). In most typical cases, it will return a dynamic message,
72// but if the factory is configured with a KnownTypeRegistry, or if the field's
73// type is a well-known type, it will return a zero value generated message.
74//
75//
76// Unrecognized Fields
77//
78// Unrecognized fields are preserved by the dynamic message when unmarshaling
79// from the standard binary format. If the message's MessageFactory was
80// configured with an ExtensionRegistry, it will be used to identify and parse
81// extension fields for the message.
82//
83// Unrecognized fields can dynamically become recognized fields if the
84// application attempts to retrieve an unrecognized field's value using a
85// FieldDescriptor. In this case, the given FieldDescriptor is used to parse the
86// unknown field and move the parsed value into the message's set of known
87// fields. This behavior is most suited to the use of extensions, where an
88// ExtensionRegistry is not setup with all known extensions ahead of time. But
89// it can even happen for non-extension fields! Here's an example scenario where
90// a non-extension field can initially be unknown and become known:
91//
92// 1. A dynamic message is created with a descriptor, A, and then
93// de-serialized from a stream of bytes. The stream includes an
94// unrecognized tag T. The message will include tag T in its unrecognized
95// field set.
96// 2. Another call site retrieves a newer descriptor, A', which includes a
97// newly added field with tag T.
98// 3. That other call site then uses a FieldDescriptor to access the value of
99// the new field. This will cause the dynamic message to parse the bytes
100// for the unknown tag T and store them as a known field.
101// 4. Subsequent operations for tag T, including setting the field using only
102// tag number or de-serializing a stream that includes tag T, will operate
103// as if that tag were part of the original descriptor, A.
104//
105//
106// Compatibility
107//
108// In addition to implementing the proto.Message interface, the included
109// Message type also provides an XXX_MessageName() method, so it can work with
110// proto.MessageName. And it provides a Descriptor() method that behaves just
111// like the method of the same signature in messages generated by protoc.
112// Because of this, it is actually compatible with proto.Message in many (though
113// not all) contexts. In particular, it is compatible with proto.Marshal and
114// proto.Unmarshal for serializing and de-serializing messages.
115//
116// The dynamic message supports binary and text marshaling, using protobuf's
117// well-defined binary format and the same text format that protoc-generated
118// types use. It also supports JSON serialization/de-serialization by
119// implementing the json.Marshaler and json.Unmarshaler interfaces. And dynamic
120// messages can safely be used with the jsonpb package for JSON serialization
121// and de-serialization.
122//
123// In addition to implementing the proto.Message interface and numerous related
124// methods, it also provides inter-op with generated messages via conversion.
125// The ConvertTo, ConvertFrom, MergeInto, and MergeFrom methods copy message
126// contents from a dynamic message to a generated message and vice versa.
127//
128// When copying from a generated message into a dynamic message, if the
129// generated message contains fields unknown to the dynamic message (e.g. not
130// present in the descriptor used to create the dynamic message), these fields
131// become known to the dynamic message (as per behavior described above in
132// "Unrecognized Fields"). If the generated message has unrecognized fields of
133// its own, including unrecognized extensions, they are preserved in the dynamic
134// message. It is possible that the dynamic message knows about fields that the
135// generated message did not, like if it has a different version of the
136// descriptor or its MessageFactory has an ExtensionRegistry that knows about
137// different extensions than were linked into the program. In this case, these
138// unrecognized fields in the generated message will be known fields in the
139// dynamic message.
140//
141// Similarly, when copying from a dynamic message into a generated message, if
142// the dynamic message has unrecognized fields they can be preserved in the
143// generated message (currently only for syntax proto2 since proto3 generated
144// messages do not preserve unrecognized fields). If the generated message knows
145// about fields that the dynamic message does not, these unrecognized fields may
146// become known fields in the generated message.
147//
148//
149// Registries
150//
151// This package also contains a couple of registries, for managing known types
152// and descriptors.
153//
154// The KnownTypeRegistry allows de-serialization of a dynamic message to use
155// generated message types, instead of dynamic messages, for some kinds of
156// nested message fields. This is particularly useful for working with proto
157// messages that have special encodings as JSON (e.g. the well-known types),
158// since the dynamic message does not try to handle these special cases in its
159// JSON marshaling facilities.
160//
161// The ExtensionRegistry allows for recognizing and parsing extensions fields
162// (for proto2 messages).
163package dynamic