Google Routes API：公交模式下指定字段掩码返回空响应

调用Google Routes API的computeRoutes接口时，设置travelMode: TRANSIT，通过字段掩码routes.legs.steps.startLocation或routes.legs.steps.endLocation请求数据，返回的steps为空对象；但使用字段掩码*时，能正常看到对应位置数据。

示例请求：

curl -X POST "https://routes.googleapis.com/directions/v2:computeRoutes" \
    -H "Content-Type: application/json" \
    -H "X-Goog-FieldMask: routes.legs.steps.endLocation" \
    -H "X-Goog-Api-Key: API_KEY" \
    -d '{
        "origin": {
            "location": {
                "latLng": {
                    "latitude": 40.748817,
                    "longitude": -73.985428
                }
            }
        },
        "destination": {
            "location": {
                "latLng": {
                    "latitude": 40.758896,
                    "longitude": -73.985130
                }
            }
        },
        "travelMode": "TRANSIT"
    }'

响应结果：

{
  "routes": [
    {
      "legs": [
        {
          "steps": [
            {},
            {},
            {},
            {},
            {},
            {},
            {}
          ]
        }
      ]
    }
  ]
}

这个问题源于TRANSIT模式下，路线步骤（steps）是多态结构（包含步行、公交换乘等不同类型），Google Routes API需要识别步骤的核心类型标识，才能正确返回嵌套的位置数据，仅单独请求位置字段会导致API无法匹配对应数据结构，返回空对象。

可通过以下两种方式解决：

1. 同时请求步骤类型与位置字段
   调整字段掩码，加入步骤的类型标识字段travelMode，示例请求：

curl -X POST "https://routes.googleapis.com/directions/v2:computeRoutes" \
    -H "Content-Type: application/json" \
    -H "X-Goog-FieldMask: routes.legs.steps.startLocation,routes.legs.steps.endLocation,routes.legs.steps.travelMode" \
    -H "X-Goog-Api-Key: API_KEY" \
    -d '{
        "origin": {
            "location": {
                "latLng": {
                    "latitude": 40.748817,
                    "longitude": -73.985428
                }
            }
        },
        "destination": {
            "location": {
                "latLng": {
                    "latitude": 40.758896,
                    "longitude": -73.985130
                }
            }
        },
        "travelMode": "TRANSIT"
    }'

2. 请求步骤下的全部核心字段
   使用更宽泛的字段掩码routes.legs.steps，直接获取步骤的所有核心数据：

curl -X POST "https://routes.googleapis.com/directions/v2:computeRoutes" \
    -H "Content-Type: application/json" \
    -H "X-Goog-FieldMask: routes.legs.steps" \
    -H "X-Goog-Api-Key: API_KEY" \
    -d '{
        "origin": {
            "location": {
                "latLng": {
                    "latitude": 40.748817,
                    "longitude": -73.985428
                }
            }
        },
        "destination": {
            "location": {
                "latLng": {
                    "latitude": 40.758896,
                    "longitude": -73.985130
                }
            }
        },
        "travelMode": "TRANSIT"
    }'

内容的提问来源于stack exchange，提问作者Joe