protobuf.md

Protobuf

Null Values

Null values are encoded as booleans with true value, but the field number is increased by 20000.

fun a(i1 : Int?) {}

message A {
  optional sint32 i1 = 1;
  optional bool i1IsNull = 20001;
}

When decoding, the decoder first checks if there is a record with the increased number, if so returns with null. Otherwise, checks the record with the original field number (and throws an exception if not found).

Standalone Values

Standalone values (function return values and items of collections) are encoded with field number 1 or field number 20001 in case of nullable values.

Collections

WireFormat's Protobuf implementation encodes Kotlin collections the same way as instances. This is results in valid Protobuf messages, but it does not support direct, inlined Protobuf lists:

If we take this function as an example:

fun a(i1 : List<Int>) {}

Technically the list can be included in the message that describes the arguments of the function.

message A {
  repeated sint32 i1 = 1;
}

However, this approach has a few drawbacks:

• complicates the message encoders and decoders
• a large list makes finding the records by tag expensive

Thereforw, WireFormat defines two implicit Protobuf messages, one for the arguments of the function and one for the list itself:

message A {
  required AI1 i1 = 1;
}

message AI1 {
  repeated sint32 item = 1;
}