问题
Facebook's deprecation of the offline_access permission is coming May 2012 and the documentation isn't giving us enough information on how to handle it.
We have an iOS app and corresponding service that powers it and integrates with Facebook in a deep way to leverage a user's friend list within out app (so if your FB friends are also using the app you can more easily connect). This is like how all social apps seem to work, so nothing special here.
Client
Our app uses Facebook iOS SDK to allow user to login, which we currently ask for offline_access. The token is persisted in our iOS app, but also sent to our server where it is saved. The client acts on behalf of user to post updates to a user's newsfeed (we also ask for publish_stream permission).
Server
Our server periodically checks to see if user's FB friends are now using our app. Next time user signs in, we expose content and relationships in a certain way to promote that user's friends. The server also acts on behalf of the user to periodically connect to the graph API and get the user's current friends list. This is so we can account for changes in a user's relationships and have them reflected in our app. We do this when the user isn't currently using the app so they have the best experience the next time they do use it. To enable this, our iOS app sends the access token to our server which it uses and why we ask for offline_access.
Note: If user signs out of our app explicitly, we delete the access tokens from both client and server.
Problems
Now that there is no longer a perpetual access token we can use, I'm trying to figure out the best practice for still enabling our scenarios while leveraging facebook's new intended way of handling and extending access tokens. The documentation is unfortunately not totally helpful.
Questions
A. When you authenticate through the newest Facebook iOS SDK, what is the default lifetime of the access token you get? This document says an extended token request will give you one that lasts 60 days. This other document talks about the first access token request and mentions varying validities but it's unclear and does it talk about specific validity times:
(emphasis is mine)
When you obtain an access token from Facebook, it will be valid immediately and usable in requests to the API for some time period defined by Facebook. After that period has elapsed, the access token is considered to have expired and the user will need to be authenticated again in order for your app to obtain a fresh access token. The duration for which a given access token is valid depends on how it was generated.
There are also events which may cause an access token to become invalid before its expected expiry time. Such events include the user changing their password, an application refreshing it's App Secret. Dealing with varying access token expiry times, and handling the case when an access token becomes invalid before its expected expiry time is essential for building robust social experiences.
B. For the client, now that the access token isn't necessarily long lived, is the right approach for us to:
Let use login through FB, then detect whenever the access token is expired. If it is, then call into FB iOS SDK to re-authentication/re-authorize? (this should just trigger user to bounce out to FB iOS app, and in most cases come immediately back to our app with a new access token).
C. According to this blog post I found, you can only extend an access token once:
Can I exchange my 60 day access token for a new 60 day access token?
No, sorry you cannot. You can only exchange a valid (meaning current) user access token for an extended one. You cannot extend an already extended access token.
On the client, I can just handle this by prompting a re-authentication/re-authorization as I mentioned in Question B. However, this doesn't work on our server. We could certainly have the server renew it once to 60 days, but what happens on the 61st day? The server just stops being able to sync the friend's list?
D. It seems to make sense to check the validity of the FB access token every time the app starts or re-hydrates from sleep. What is the best way for our iOS app to check this? Is there a recommended endpoint to call to validate a token? Should we just call into https://graph.facebook.com/me passing the access token and checking the response?
Note: we can certainly record the expires time when we get the initially extended token, but this isn't reliable since the user could revoke our app's permission anytime which makes the expires time an unreliable data point on validity
回答1:
Overview
I believe that the root of what facebook is trying to achieve is to prevent an app from having perpetual ever-lasting access to a user's account. So, with the new migration an app can only access an account for 60 days unless the user signs in again.
I don't work for facebook, but here are my findings from playing around with the facebook graph api.
General Solution
- Whenever a user signs in, take their access token and immediately extend/refresh it, and save it
- Record the expiration date of the access token
- When an access token expires (either from the recorded date, or a graph API exception telling you so), then notify the user that you don't have access, and ask them to sign in again.
Answers
A. When you authenticate through the newest Facebook iOS SDK, what is the default lifetime of the access token you get? This document says an extended token request will give you one that lasts 60 days. This other document talks about the first access token request and mentions varying validities but it's unclear and does it talk about specific validity times:
Here's how it works:
- The first sign-in grants you approximately two hours
- By refreshing the access token, you can get up to 60 days
- If the user doesn't sign in to those 60 days, there is no way to get access for longer without having them sign in.
- If the user de-authorizes your app, that 60 day windows ends immediately, and you will no longer have access.
B. For the client, now that the access token isn't necessarily long lived, is the right approach for us to: Let use login through FB, then detect whenever the access token is expired. If it is, then call into FB iOS SDK to re-authentication/re-authorize? (this should just trigger user to bounce out to FB iOS app, and in most cases come immediately back to our app with a new access token).
If the users access token is expired, your only option is to have them go through a login loop like you are talking about.
C. According to this blog post I found, you can only extend an access token once. On the client, I can just handle this by prompting a re-authentication/re-authorization as I mentioned in Question B. However, this doesn't work on our server. We could certainly have the server renew it once to 60 days, but what happens on the 61st day? The server just stops being able to sync the friend's list?
You can only extend an access token once. On the 61st day, you are out of luck. Best notify the user and let them know that unless they sign in, you won't be able to do anything.
D. It seems to make sense to check the validity of the FB access token every time the app starts or re-hydrates from sleep. What is the best way for our iOS app to check this? Is there a recommended endpoint to call to validate a token? Should we just call into https://graph.facebook.com/me passing the access token and checking the response?
I haven't be able to find an API equivalent of the Debug Console. This FB blog article talks about invalidated access tokens, but doesn't mention any API methods in particular meant to test the API.
I your suggestion of hitting https://graph.facebook.com/me would work just fine is exactly what they recommend in their example. In fact, I might use this approach in my app as a pro-active way of checking an access token.
Tid Bits
- When you "refresh" an access token, a new access token will be returned. The response looks like:
access_token=TOKEN&expires=5183912 - You can only "refresh" an access token once. If you try to "refresh" the long-lived token returned from a previous call, it will return the same token, but doesn't throw an exception unless the token has expired. (in other words, you can safely try to refresh your token)
- The default access token length seems to be around 2 hours
- If you "refresh" an access token, that new access tokens seems to be the one that you'll get from the facebook API afterwards (instead of returning the original, short-lived access token)
Also, if you want to play around, these tools make it easy to test out your use case in a browser before burying it in your code:
- Graph API Explorer - For creating and getting access tokens
- Debug Console - For checking the expiry date of tokens before/after refresh
- Refresh Endpoint - For manually testing extending your tokens
回答2:
Great answer, one important addition : the default token lasts between 1 and 2 hours. You get the remaining of the hour during which the user signs up, plus 1 full hour. For example if a user signs up at 3:45pm, the access token will expire at 5pm. To be safe developers should assume it only lasts 1hour.
来源：https://stackoverflow.com/questions/10132610/how-to-handle-facebooks-deprecation-of-offline-access-when-you-use-token-both-i