Check FetchPolicy in Apollo Interceptor

[ersen-lw]

Use case

I want to write a interceptor to log when a cache miss occurs. I only want to log it when the FetchPolicy is CacheOnly. If a request is CacheOnly and the outcome is a miss implies something is wrong on my end so I want to log those cases whilst ignoring  other policies like CacheFirst where it is expected to be missed.

class MyApolloCacheMissLoggingInterceptor: ApolloInterceptor {
    override fun <D : Operation.Data> intercept(request: ApolloRequest<D>, chain: ApolloInterceptorChain): Flow<ApolloResponse<D>> {
        return chain.proceed(request).onEach {
            //TODO add check if fetch policy is CacheOnly
            if (it.exception is CacheMissException) {
                Timber.e(it.exception, "Apollo cache miss on ${request.operation.name()}")
            }
        }
    }
}

Describe the solution you'd like

I can access the executionContext in the request but FetchPolicyContext is marked as internal so I can't access it. Could this be made public ?

[BoD]

The FetchPolicyContext is internal, but you could add your own context to keep track of the fetch policy, Something like:

class MyFetchPolicyContext(val fetchPolicy: FetchPolicy) : ExecutionContext.Element {
  override val key: ExecutionContext.Key<*>
    get() = Key

  companion object Key : ExecutionContext.Key<MyFetchPolicyContext>
}

val <D : Operation.Data> ApolloRequest<D>.fetchPolicy: FetchPolicy?
  get() = executionContext[MyFetchPolicyContext]?.fetchPolicy

and an extension to use to set the policy instead of the Apollo one:

fun ApolloCall<*>.myFetchPolicy(fetchPolicy: FetchPolicy) =
  fetchPolicy(fetchPolicy)
  .addExecutionContext(MyFetchPolicyContext(fetchPolicy))

then in your interceptor you can check request.fetchPolicy.

[ersen-lw]

Thanks I'll look into it 👍