Google

When creating a Google client, you must choose to create a client ID for iOS.

You will receive a client-id with the form 9876-zyxwvu.apps.googleusercontent.com, then use an OAuth2CodeGrant with no client-secret and a redirect-uri in the form of com.googleusercontent.apps.9876-zyxwvu:/oauth. The Google API console will show the client id and redirect uri. Don't forget to register it as an URL scheme in your app's Info.plist!

var oauth2 = OAuth2CodeGrant(settings: [
    "client_id": "9876-zyxwvu.apps.googleusercontent.com",
    "authorize_uri": "https://accounts.google.com/o/oauth2/auth",
    "token_uri": "https://www.googleapis.com/oauth2/v3/token",
    "scope": "profile",     // depends on the API you use
    "redirect_uris": ["com.googleusercontent.apps.9876-zyxwvu:/oauth"],
])

When instantiating the OAuth2DataLoader, set alsoIntercept403 = true because Google returns status code 403 (instead of 401) for protected URLs such as https://www.googleapis.com/plus/v1/people/me

let loader = OAuth2DataLoader(oauth2: oauth2)
loader.alsoIntercept403 = true

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“Other” Client

Since v4, Google is no longer accepting an Other client ID; it detects the web view user agent and returns a 400 error if you are using “Other” (thanks @stefanhp!).