How to generate protobuf problem with importing - dart

Question

I have those 2 protobuf files:
user.proto:
syntax = "proto3";
import "google/protobuf/timestamp.proto";
message User{
string name = 1;
int32 age = 2;
string email = 3;
google.protobuf.Timestamp created_at = 4;
}
API.proto:
syntax = "proto3";
import "google/protobuf/empty.proto";
import "user.proto";
service ServerAPI {
rpc Init(google.protobuf.Empty) returns (google.protobuf.Empty) {}
rpc Get(User) returns (Result) {}
}
message Result {
bool result = 1;
}
Both of them are in the same dir called proto_files:
When I am trying to run protoc -I=. --dart_out=grpc:build/generated proto_files/*.proto I am getting the following error message.
google/protobuf/empty.proto: File not found.
user.proto: File not found.
proto_files/API.proto:2:1: Import "google/protobuf/empty.proto" was not found or had errors.
proto_files/API.proto:3:1: Import "user.proto" was not found or had errors.
proto_files/API.proto:6:12: "google.protobuf.Empty" is not defined.
proto_files/API.proto:6:44: "google.protobuf.Empty" is not defined.
proto_files/API.proto:7:11: "User" is not defined.
But if I am going to remove the dependencies between those 2 files, everything is going to work properly, if I will not use the import anylonger.
Can anyone help me to understand what I am doing wrong and how I can fix it?
Thx :D

Answer 1

protoc is very particular that you must fully (and consistently) reference the path(s) to your protos.
Because proto_files is used to reference your protos (i.e. proto_files/*.proto), proto_files must be in the --proto_path (-I) too.
protoc should (!?) include Google's Well-Known Types. The binary (protoc) is in the /bin folder and the Well-Known Types should be in the /include folder.
It appears that, on your machine, these aren't being automatically discovered. You can add these directly to the --proto_path too but you will need to specify the location.
Try:
# Replace with path to `protoc-${VERS}-${ARCH}` folder
# The `protoc` binary is in `${PROTOC}/bin`
# Google's Well-Known Types should be in `${PROTOC}/include`
PROTOC="..."
protoc \
--proto_path=proto_files \
--proto_path=${PROTOC}/include \
--dart_out=grpc:build/generated \
proto_files/*.proto