Working with field masks

Field masks are a way for API callers to list the fields that a request should return or update. Using fields masks allows the API to avoid unnecessary work and improves performance. Field masks are used for both read and update methods in the Slides API.

Reading with a field mask

Presentations are big, and often you don't need every part of the presentation returned by a read request. You can limit what's returned in a Slides API response, using the fields URL parameter. For best performance, explicitly list only the fields you need in the reply.

The format of the fields parameter is the same as the JSON encoding of a FieldMask. Stated briefly, multiple different fields are comma separated and subfields are dot-separated. Field names can be specified in camelCase or separated_by_underscores. For convenience, multiple subfields from the same type can be listed within parentheses.

The following presentations.get request example uses a field mask of slides.pageElements(objectId,size,transform) to fetch only the object IDs, sizes, and transforms of all of the pageElements on slides in a presentation:

GET https://slides.googleapis.com/v1/presentations/presentationId?fields=slides.pageElements(objectId,size,transform)

The response to this method call is a Presentation object containing the components requested in the field mask:

{
  "slides": [
    {
      "pageElements": [
        {
          "objectId": "i0",
          "size": {
            "width": {
              "magnitude": 3000000,
              "unit": "EMU"
            },
            "height": {
              "magnitude": 3000000,
              "unit": "EMU"
            }
          },
          "transform": {
            "scaleX": 1,
            "scaleY": 1
            "translateX": 311708,
            "translateY": 744575,
            "unit": "EMU"
          }
        },
        {
          "objectId": "i1",
          "size": {
            "width": {
              "magnitude": 3000000,
              "unit": "EMU"
            },
            "height": {
              "magnitude": 3000000,
              "unit": "EMU"
            }
          },
          "transform": {
            "scaleX": 1,
            "scaleY": 1
            "translateX": 311700,
            "translateY": 2834125,
            "unit": "EMU"
          }
        }
     ]
    }
  ]
}

Updating with a field mask

Sometimes you need to update only certain fields in an object while leaving the other fields unchanged. Update requests inside a presentations.batchUpdate operation use field masks to tell the API which fields are being changed. The update request ignores any fields that aren't specified in the field mask, leaving them with their current values.

You can also unset a field by not specifying it in the updated message, but adding the field to the mask. This clears whatever value that field previously had.

The syntax for update field masks is the same as read field masks. Additionally, a field mask of * is treated like a wildcard and is shorthand for specifying all of the fields in a message.

The wildcard syntax can produce unwanted results if the API is updated in the future. For production applications, always list the specific fields being updated in field masks and avoid using * wildcards.

The following example uses an updateShapeProperties request to change a shape's color fill to the DARK1 theme color and unset the shape's outline:

POST https://slides.googleapis.com/v1/presentations/presentationId:batchUpdate

{
  "requests": [
    {
      "updateShapeProperties": {
        "objectId": elementId,
        "shapeProperties": {
          "shapeBackgroundFill": {
            "solidFill": {
              "color": {
                "themeColor": "DARK1"
              }
            }
          }
        },
        "fields": "shapeBackgroundFill.solidFill.color,outline"
      }
    }
  ]
}