/related #2293 (Query onCompleted not invoked)

See #2293 (comment), #2293 (comment), #2293 (comment), and #2293 (comment)

The issue

When passing both onCompleted and refetchQueries to a Mutation that un-mounts as a result of refetchQueries, onCompleted will never be called.

Consider the following example:

A logout button that invokes a mutation, which re-fetches a query to propagate this change to the rest of the app (e.g. removing references to the currently logged-in user).

On that mutation, we have an onCompleted to redirect to the login page after both a successful logout mutation AND a successful refetch .

With the current implementation, onCompleted will never be called because as soon as refetchQueries is finished, the logout mutation will unmount and the redirect will never happen.

{loggedIn && ( // ← mutation rendering is conditional 
  <Mutation
    awaitRefetchQueries
    mutation={logoutMutation}
    refetchQueries={[/* a query to update the current user info (e.g. loggedIn) */]}
    onCompleted={redirectToLogin /* this callback will never be called */}
  >
    {/* a logout button to run the mutation when clicked */}
  </Mutation>
)}

This might not be the best example, but hopefully it can demonstrate the real issue the PR is trying to address.

Replicating the Issue

I was able to replicate this issue easily when I tried to pass an onCompleted callback to the Mutation in this test and assert that it was called:

• https://github.com/apollographql/react-apollo/blob/master/test/client/Mutation.test.tsx#L1479-L1518

The Cause

It appears that this is intentional in order to avoid updating the Mutation internal state after it un-mounts.

Unless I'm missing something here or there is a reason not do so, this doesn't necessarily mean that an onCompleted callback should never be called when a mutation is resolved and has been unmounted.

While it can be hard to think that an unmounted component would have callbacks called, it is important to keep in mind that consumers of an unmounted Mutation have no indication to when the said mutation is resolved successfully or when its refetched queries are finished (same for errors).

Possible Fix

By changing the logic of onMutationCompleted a little bit, it looks like it's possible to invoke the user-provided onComplete even when the Mutation unmounts.

I also went ahead and applied the same changes to onError/onMutationError for consistency.