allow users to pass link compositions and custom caches

[jpstrikesback]

We need to be able to pass in our own links along with the lovely links from AWS.

Likewise for an Apollo cache (especially since we want to use the same cache we use for the client in say apollo-link-state, cause that's what I was told to do)

This should help with: #52 #36 #24 #21 #3

Let me know if this looks good, interesting, undesirable, or needs work.

Cheers,
JP

[alexeygolev]

This is really needed as unions and interfaces are pretty much useless with apollo without a fragmentMatcher which can't be provided at the moment

[ceich]

This assignment would be cleaner if you swap the ternary operators:

const cache = ownCache ? ownCache
    : disableOffline ? new InMemoryCache()
    : new OfflineCache(store);

There also seems to be a typo on line 56: owncache should be ownCache.

[M1chaelTran]

Agreed with this feedback as it is a lot more readable.

[dguisinger]

I think there should be a way to add links without having to redefine the AWS links. I don't want to have to manually keep track of AWS internal SDK changes in order to get all of the out-of-the-box functionality when supplying a custom link. Maybe a function that can return an array of the AWS supplied links that can be used in the following way:
[ myLink, ...AwsAppSyncDefaultLinks() ]

in my case, I'm looking to use a link to stripout __typename properties on objects submitted to mutations, not override any AWS provided functionality.

[manueliglesias]

Hey

This should be possible soon, take a look at #96 and these comments with usage examples:

• How can I add a custom link to the link chain? #3 (comment)
• Local state story #24 (comment)