[Waiting for #374] Add fetchMore method to the observable query

[rricard]

• This PR is based onto Strip apollo directives #374.
• It must be rebased regularly onto Strip apollo directives #374 until it's merged.
• Once Strip apollo directives #374 is merged a new PR must be created against master.

TODO:

• Update CHANGELOG.md with your change
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

Linked to the work of #350

Usage example

let handleCount = 0;
const handle = client.watchQuery({
  query: gql`{
    allPeople @apolloFetchMore(name: "people") {
      id
      name
   }
  `
}).subscribe({
  next({data}) {
    handleCount ++;
    if(handleCount === 1) {
      // data === {allPeople: [{id: 'luke', name: 'Luke Skywalker'}]}
      handle.fetchMore(['people']);
    } else {
      // data === {allPeople: [{id: 'luke', name: 'Master Luke Skywalker'}, {id: 'jarjar', name: 'Jar Jar Binks'}]}
      // Note: if the id matches, consider the item new and discard the old one
    }
  }
});

You can't control the order in which the data is merged, that's why, for now, you should define the order in your rendering function. A next PR may change that but we have the issue of serialising functions into the store ...

Usage example after #453

let handleCount = 0;
const handle = client.watchQuery({
  query: gql`query people($cur: ID){
    allPeople(cursor: $cur) @apolloFetchMore(name: "people") {
      id
      name
   }
  `,
  paginationArguments: ['cursor'],
}).subscribe({
  next({data}) {
    handleCount ++;
    if(handleCount === 1) {
      // data === {allPeople: [{id: 'luke', name: 'Luke Skywalker'}]}
      handle.fetchMore(['people'], {cur: "afterluke"});
    } else {
      // data === {allPeople: [{id: 'luke', name: 'Luke Skywalker'}, {id: 'jarjar', name: 'Jar Jar Binks'}]}
    }
  }
});

[Slava]

Thanks for extracting your work from #350!

I just published a different proposal for a feature that might solve a similar provlem in #455. I would be curious to see if the two can co-exist or we should choose one.

[rricard]

I'll try to take a look on Monday. Thanks for the pointers!