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Don Newton379ae252019-04-01 12:17:06 -04001// Package bsoncodec provides a system for encoding values to BSON representations and decoding
2// values from BSON representations. This package considers both binary BSON and ExtendedJSON as
3// BSON representations. The types in this package enable a flexible system for handling this
4// encoding and decoding.
5//
6// The codec system is composed of two parts:
7//
8// 1) ValueEncoders and ValueDecoders that handle encoding and decoding Go values to and from BSON
9// representations.
10//
11// 2) A Registry that holds these ValueEncoders and ValueDecoders and provides methods for
12// retrieving them.
13//
14// ValueEncoders and ValueDecoders
15//
16// The ValueEncoder interface is implemented by types that can encode a provided Go type to BSON.
17// The value to encode is provided as a reflect.Value and a bsonrw.ValueWriter is used within the
18// EncodeValue method to actually create the BSON representation. For convenience, ValueEncoderFunc
19// is provided to allow use of a function with the correct signature as a ValueEncoder. An
20// EncodeContext instance is provided to allow implementations to lookup further ValueEncoders and
21// to provide configuration information.
22//
23// The ValueDecoder interface is the inverse of the ValueEncoder. Implementations should ensure that
24// the value they receive is settable. Similar to ValueEncoderFunc, ValueDecoderFunc is provided to
25// allow the use of a function with the correct signature as a ValueDecoder. A DecodeContext
26// instance is provided and serves similar functionality to the EncodeContext.
27//
28// Registry and RegistryBuilder
29//
30// A Registry is an immutable store for ValueEncoders, ValueDecoders, and a type map. For looking up
31// ValueEncoders and Decoders the Registry first attempts to find a ValueEncoder or ValueDecoder for
32// the type provided; if one cannot be found it then checks to see if a registered ValueEncoder or
33// ValueDecoder exists for an interface the type implements. Finally, the reflect.Kind of the type
34// is used to lookup a default ValueEncoder or ValueDecoder for that kind. If no ValueEncoder or
35// ValueDecoder can be found, an error is returned.
36//
37// The Registry also holds a type map. This allows users to retrieve the Go type that should be used
38// when decoding a BSON value into an empty interface. This is primarily only used for the empty
39// interface ValueDecoder.
40//
41// A RegistryBuilder is used to construct a Registry. The Register methods are used to associate
42// either a reflect.Type or a reflect.Kind with a ValueEncoder or ValueDecoder. A RegistryBuilder
43// returned from NewRegistryBuilder contains no registered ValueEncoders nor ValueDecoders and
44// contains an empty type map.
45//
46// The RegisterTypeMapEntry method handles associating a BSON type with a Go type. For example, if
47// you want to decode BSON int64 and int32 values into Go int instances, you would do the following:
48//
49// var regbuilder *RegistryBuilder = ... intType := reflect.TypeOf(int(0))
50// regbuilder.RegisterTypeMapEntry(bsontype.Int64, intType).RegisterTypeMapEntry(bsontype.Int32,
51// intType)
52//
53// DefaultValueEncoders and DefaultValueDecoders
54//
55// The DefaultValueEncoders and DefaultValueDecoders types provide a full set of ValueEncoders and
56// ValueDecoders for handling a wide range of Go types, including all of the types within the
57// primitive package. To make registering these codecs easier, a helper method on each type is
58// provided. For the DefaultValueEncoders type the method is called RegisterDefaultEncoders and for
59// the DefaultValueDecoders type the method is called RegisterDefaultDecoders, this method also
60// handles registering type map entries for each BSON type.
61package bsoncodec