GGGrammar

A formal JSON grammar for data visualization with a cross-platform Swift implementation.

GGSchema

GGSchema contains a collection of Codable swift value types (structs and enums) generated from the Vega schema.

It enables reading, modifying, and writing JSON in the vega-lite grammar with a type-safe Swift API.

This cross-platform library contains only the raw value types for the visualiztion grammar. To render the specifications into an image, the higher-level GGViz package can be used.

The version numbers follow the Vega Lite specification version; new versions are automatically built periodically from the Actions.

The source of the generated schema is a single ~5M (~66K SLOC) GGSchema.swift source file.

The only dependency of this project is BricBrac, which is used for representing the JSONSchema types.

The following unit test from GGSchemaTests.swift illustrates the API & featues:

import GGSchema

let specJSON = """
{
    "mark": "bar",
    "data": {
        "values": [
            { "a": "CAT1", "b": 5.6 },
            { "a": "CAT2", "b": 0.1 }
        ]
    },
    "encoding": {
        "x": { "field": "a" },
        "y": { "field": "b" }
    }
}
"""

// parse the GGSchema from the JSON literal…
let parsedSpec = try TopLevelUnitSpec.parseJSON(specJSON)

// …and also create the same spec in code
var codeSpec = TopLevelUnitSpec(data:
    TopLevelUnitSpec.DataChoice(
        DataProvider(
            DataSource(
                InlineData(values:
                            InlineDataset([
                                ["a": "CAT1", "b": 5.6],
                                ["a": "CAT2", "b": 0.1]
                            ])
                )
            )
        )
    ),
    mark: AnyMark(.bar))

// initialize the encodings with a single `x` scaled to the `a` field…
codeSpec.encoding = FacetedEncoding(x:
    FacetedEncoding.EncodingX(
        FacetedEncoding.X(
            PositionFieldDef(field: .init(FieldName("a"))))))

// …then add a `y` encoding scaled to the `b` field
codeSpec.encoding!.y = FacetedEncoding.EncodingY(
    FacetedEncoding.Y(
        PositionFieldDef(field: .init(FieldName("b")))))

#if canImport(ObjectiveC) // *sigh* Apple
let expectedJSON = """
{"data":{"values":[{"a":"CAT1","b":5.5999999999999996},{"a":"CAT2","b":0.10000000000000001}]},"encoding":{"x":{"field":"a"},"y":{"field":"b"}},"mark":"bar"}
"""
#else // Linux FTW
let expectedJSON = """
{"data":{"values":[{"a":"CAT1","b":5.6},{"a":"CAT2","b":0.1}]},"encoding":{"x":{"field":"a"},"y":{"field":"b"}},"mark":"bar"}
"""
#endif

XCTAssertEqual(expectedJSON, codeSpec.jsonDebugDescription)

XCTAssertEqual(parsedSpec, codeSpec)