Introduction

Declarations in a .proto file can be annotated with a number of options. Options do not change the overall meaning of a declaration, but may affect the way it is handled in a particular context.

Protobuf supports different types of options, e.g. file-level options, message-level options, field-level options, etc. The list of available options types is defined in /google/protobuf/descriptor.proto.

Protobuf also allows to define and use custom options. The authors say "that this is an advanced feature which most people don’t need" and it actually requires a significant amount of work to define a custom option and generate the validation code for this option. However, ProtoData provides a fairly easy way to collect the custom options metadata and extend the generated code with the desired behaviour.

Domain

This use-case of applying ProtoData demonstrates how to enrich the model with additional semantic elements and reflect their meaning in the generated code.

For example, let's define a board for Tic-tac-toe game:

// A board for Tic-tac-toe game.
message Board {
  // The name of the board.
  BoardName name = 1;

  // The size of the board.
  int32 side_size = 2;
  
  // Board cells. It is expected to have "side_size * side_size" cells. 
  repeated Cell cell = 3;
}

message BoardName {
  string value = 1;
}

enum Mark {
  M_UNDEFINED = 0;
  CROSS = 1;
  CIRCLE = 2;
}

message Cell {
  oneof value {
    bool empty = 1;
    Mark mark = 2;
  }
}

Definitely, we can use this model but it is so easy to make some mistake when the model becomes invalid state. What can we do to avoid this?

It would be much better if we could add the following:

Mark some fields as required to state the absolute necessity to fill them with real values.
Set the minimum acceptable value for the side_size field.
Add some validation rules that verify the size of the cell collection. Indeed, a tic-tac-toe board should be square, and hence have an appropriate number of cells, being the size of the side squared.

We can easily solve the points 1 and 2 as Spine provides options to mark the fields as required and validate values of the numeric fields.

Let's add these elements to the Board definition:

// A board for Tic-tac-toe game.
message Board {
  // The name of the board.
  BoardName name = 1 [(required) = true];

  // The size of the board.
  int32 side_size = 2 [(required) = true, (min).value = "3"];

  // Board cells. The collection must not be empty.
  repeated Cell cell = 3 [(required) = true];
}
...

To solve the point 3, we can define a custom option so we could set the required size for a repeated field and use ProtoData API to extend the generated by Protobuf code with the additional logic that checks the instances of Proto messages for validness at runtime. So let's do it.

Below is definition of the size custom option:

extend google.protobuf.FieldOptions {

    // See `ArrayOfSizeOption` for details.
    //
    // The field index is chosen based in `spine/options.proto`,
    // taking the next available number in Spine's reserved range.
    //
    ArrayOfSizeOption size = 73855;
}

// A field option applicable to `repeated` fields,
// telling that their size should be equal
// to some expression, involving the values
// of other fields of the same message.
//
// The `value` field supports basic math operations,
// such as `+`, `-`, `*`, `/`.
//
message ArrayOfSizeOption {

    string value = 1 [(required) = true];
}

Now let's use this option in the Board definition:

// A board for Tic-tac-toe game.
message Board {
  // The name of the board.
  BoardName name = 1 [(required) = true];

  // The size of the board.
  //
  // The board must have `side_size` number of cell rows,
  // each having `side_size` number of columns,
  // effectively making the board "square".
  //
  int32 side_size = 2 [(required) = true, (min).value = "3"];

  // Board cells. It is required to have "side_size * side_size" cells.
  repeated Cell cell = 3 [(required) = true, (size).value = "side_size * side_size"];
}
...

The ApplySizeOptionPlugin is implemented to generate the validation code for the size option. This plugin uses ProtoData API to collect the size option metadata and to generate extensions for the message builder classes with validation methods for every field with size option applied.

Below is generated validation method for the Board.cell field.

internal fun Board.Builder.validateCellCount(): Board.Builder {
    val expected = sideSize * sideSize
    check(cellCount == expected) {
        "Invalid number of 'cell' elements: " +
            "expected $expected, but actual $cellCount."
    }
    return this
}

Also, the build() method of a message builder class is updated to call such methods.

See the codegen-plugin subproject for details.

The following configuration should be applied in the Gradle configuration of a module to use the size option plugin:

protoData {
    // Run ProtoData with the `size` option plugin enabled.
    plugins(
        "io.spine.protodata.hello.ApplySizeOptionPlugin"
    )
}

Development

Prerequisites

This example is implemented using the following technologies and tools:

Java 11
Kotlin 1.9.23
Gradle 7.6

Building locally

Clone the repository:

git clone git@github.com:spine-examples/Hello-ProtoData.git

Run the Gradle build:

./gradlew build