Swift Codable枚举编码成功但解码失败问题求助

我通过嵌套Codable类型定义了GeoJSONFeatureGeometryCoordinates枚举，用于解码GeoJSON文件中的点、多边形与多多边形。解码GeoJSON文件时一切正常，但将该数据编码为二进制数据存储到Core Data后，再次解码却失败，抛出typeMismatch错误。

枚举代码 #

enum GeoJSONFeatureGeometryCoordinates: Codable {
    
    case point([Double])
    case polygon([[[Double]]])
    case multipolygon([[[[Double]]]])

    
    init(from decoder: Decoder) throws {
        
        let container = try decoder.singleValueContainer()
        
        do {
            
            let polygonVal = try container.decode([[[Double]]].self)
            self = .polygon(polygonVal)
        } catch DecodingError.typeMismatch {
            do {
                
                let multipolygonVal = try container.decode([[[[Double]]]].self)
                self = .multipolygon(multipolygonVal)
            } catch DecodingError.typeMismatch {
                
                let pointVal = try container.decode([Double].self)
                self = .point(pointVal)
            }
        }
    }
}

测试代码 #

do {
    let encoder = JSONEncoder()
    let encodedCoordinates = try encoder.encode(centroid.geometry.coordinates)
    
    let decoder = JSONDecoder()
    let decodedCoordinates = try decoder.decode(GeoJSONFeatureGeometryCoordinates.self, from: encodedCoordinates)
} catch {
    fatalError("\n\n Error archiving feature.geometry.coordinates as Data")
}

问题出在你只手动实现了解码逻辑init(from:)，但没有实现对应的编码逻辑encode(to:)。Codable自动合成的编码逻辑无法正确处理这种嵌套数组类型的枚举，导致编码后的数据格式和解码时预期的单一值容器不匹配，最终触发typeMismatch错误。

需要手动添加encode(to:)方法，根据枚举的case类型将对应的数据编码到单一值容器中：

enum GeoJSONFeatureGeometryCoordinates: Codable {
    
    case point([Double])
    case polygon([[[Double]]])
    case multipolygon([[[[Double]]]])

    
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        
        do {
            let polygonVal = try container.decode([[[Double]]].self)
            self = .polygon(polygonVal)
        } catch DecodingError.typeMismatch {
            do {
                let multipolygonVal = try container.decode([[[[Double]]]].self)
                self = .multipolygon(multipolygonVal)
            } catch DecodingError.typeMismatch {
                let pointVal = try container.decode([Double].self)
                self = .point(pointVal)
            }
        }
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        switch self {
        case .point(let value):
            try container.encode(value)
        case .polygon(let value):
            try container.encode(value)
        case .multipolygon(let value):
            try container.encode(value)
        }
    }
}

添加这个方法后，编码时会将每个枚举case对应的数据以单一数组的形式写入，和解码时的读取逻辑完全匹配，二次解码就能正常完成。

内容的提问来源于stack exchange，提问作者whaleshark