[JSON Serialization]Support Default Value

[seanliu1]

apple#861

See discussion in the ticket, this PR adds a new method in Visitor Protocol.

  func traverse<V: SwiftProtobuf.Visitor>(visitor: inout V) throws {
    let shouldIncludeDefault = visitor.shouldIncludeDefault()
    if !self.text.isEmpty || shouldIncludeDefault {
      try visitor.visitSingularStringField(value: self.text, fieldNumber: 1)
    }
    if self.action != .searchCancelEventActionInvalid || shouldIncludeDefault {
      try visitor.visitSingularEnumField(value: self.action, fieldNumber: 2)
    }
    if self.resultCount != 0  || shouldIncludeDefault {
      try visitor.visitSingularUInt32Field(value: self.resultCount, fieldNumber: 3)
    }
    try unknownFields.traverse(visitor: &visitor)
  }

[seanliu1]

I am not so sure whether we need to handle this. Looks like in proto3, only submessage uses it. Most of fields in proto2 using it. If we also want to get nil in JsonFormat, we can add another method call visitNull(fieldNumber: Int) throws into Visitor Protocol. So on the codegen side, it will become

if let v = _storage._message {
    try visitor.visitSingularMessageField(value: v, fieldNumber: 1)
} else if visitor.shouldIncludeDefault {
   try visitor.visitingNull(fieldNumber: 1)
}

[thomasvl]

I haven't looked at the other languages, do they really generate an empty list in json, that seems odd?

[seanliu1]

verified on python, it will return empty array