Add AuthHubEvent For User Attribute Updated

[LikemD]

I find myself in need of a way to listen to changes to user attributes (custom attributes incl.) on my apps when changes are triggered by an external function. I realized that amplify-js supports this functionality but amplify-flutter doesn't, so i wanted to open this as a feature request.
amplify-js auth events
amplify-flutter auth events

[dgagnon]

As a workaround, we update the our data from the attributes on each signIn, but it does not pickup changes happening during a session and this has caused some issue in the past.
+1 for this feature.

[fjnoyp]

Hi @LikemD thanks for creating this issue. To clarify, you'd like to listen to a new userAttributesUpdated AuthHubEvent so that you'd know when to call Amplify.Auth.fetchUserAttributes()?

[LikemD]

@fjnoyp Yes, that's right.

[LikemD]

@fjnoyp how long do you estimate that it'll take to release this feature? As this is a blocker for my product release

[Jordan-Nelson]

@LikemD - Can you clarify what you mean by "when changes are triggered by an external function"? Do you mean a function running outside of the current device (that is, on a sever or on another client)?

[LikemD]

@Jordan-Nelson Yes, that's right. Point is, i want to be able to listen for any change to an attribute in cognito

[Jordan-Nelson]

@LikemD - I see. I believe the JS event is fired when the Auth.updateUserAttribute() API is called. I don't believe it would be triggered by a change from another client or from the server.

As far as I know, Cognito doesn't have any mechanism to subscribe to updates on a user. I think it would be possible to inspect the auth token each time it is refreshed to see if any attributes have changed, but there would be a delay between the attribute change and the auth event firing. The delay could be up as long as the auth token expiration time (between 5 minutes and 1 day).

[mehrdad-shokri]

I would also appreciate if this event becomes possible to use