Download spreadsheets from Google Drive

A scope must be added to the Google Sign In configuration in the App Delegate’s didFinishLaunchingWithOptions method, to request the user permissions for accessing Google Drive files.

import GoogleAPIClientForREST
// ...
GIDSignIn.sharedInstance().scopes = [kGTLRAuthScopeDriveReadonly]

Here is the Google documentation for the Google Drive API requests that are needed.

A reference of the Google Drive service authorizer must be saved, add to the GoogleSignInDelegate a reference to the service.

private let googleDriveService = GTLRDriveService()

Then, in the implementation of didSignIn, save the authorizer.

googleDriveService.authorizer = user.authentication.fetcherAuthorizer()

Fetch all spreadsheets

A GTLRDrive_FileList will be downloaded using Google Drive’s REST API query. This can also download files from Google Photos, for instance. In this case, only Drive files are needed, so the spaces and corpora properties must be set.

let query = GTLRDriveQuery_FilesList.query()
query.spaces = "drive"
query.corpora = "user"

By default, not all the fields of the GTLRDrive_File are fetched, indicate which ones must be requested.

query.fields = "files(id,name,modifiedTime,size)"

Also, tell the query to look only for spreadsheets.

let spreadsheetOnly = "mimeType = 'application/vnd.google-apps.spreadsheet'"
query.q = "\(spreadsheetOnly)"

Then, just execute the query and wait for the result, a GTLRDrive_FileList.

googleDriveService.executeQuery(query) { [weak self] _, result, error in
    guard error == nil else { return }
    for file in (result as? GTLRDrive_FileList)?.files ?? [] {
        // ...
    }
}

This query will return all the spreadsheets linked with that account, even those that are shared with other users or those that are in the Trash. Filters can be added, for instance, to fetch only files owned by the user and not other user’s shared files.

let ownedByUser = "'\(email)' in owners"
query.q = "\(spreadsheetOnly) and \(ownedByUser)"

Fetch the content of each spreadsheet

The previous query only fetched info about the files, to get the whole file, other requests must be made with the Google Drive service fetcher.

The ID of each spreadsheet must be included in the URL, in following example, replace the %@ with it.

private func spreadsheetUrl(id: String) -> URL? {
    URL(string: String(format: "https://www.googleapis.com/drive/v3/files/%@/export?alt=media&mimeType=application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", id))
}

Use the URL to fetch the resource with Google Service.

googleDriveService.fetcherService.fetcher(with: url).beginFetch { [weak self] data, error in
    guard error == nil, let data = data else { return }
    // Handle the data
}