Make connection adapter for array slices

[wincent]

The existing connectionFromArray requires an array representing all the records in a connection. Actually materializing such an array may be prohibitively expensive when dealing with large connections (containing thousands of items, or more). This largely defeats the purpose of pagination, which is supposed to free us from having to deal with total data sets when subsets suffice.

At the moment, it is possible to do some ghastly workarounds to trick connectionFromArray into thinking it has a fully materialized array. For example:

const paddedItems = new Array(offset).concat(...items);
if (count > offset + args.first) {
  paddedResults.length++;
}
const connection = connectionFromArray(paddedItems, args);

In other words, we create an array with, say, 1000 empty slots, then insert the 10 items actually retrieved, then increment the length property on the array to trick connectionFromArray into thinking that hasNextPage should be true.

This commit seeks to address this common use case by implementing connectionFromArraySlice. This effectively allows you to fetch the data for a subset of a connection, and pass that in with some metadata indicating where the subsequence starts and how long the total array would be if materialized, and get a connection object back.

We also add connectionFromPromisedArraySlice, for parity with connectionFromPromisedArray, and we reimplement the existing connectionFromArray to call through to the new function.

All specs pass, except one, which I actually think is a bug in the old implementation, so I changed it. The test case uses first: 0 with no before or after. Previously, the code returned hasNextPage: false, but according to the spec, it seems it should actually return true.

HasNextPage is specified as follows:

1. If first was not set, return false.
2. Let edges be the result of calling ApplyCursorsToEdges(allEdges, before, after).
3. If edges contains more than first elements, return true.
4. Return false.

Given that there are no before or after calls, and ApplyCursorsToEdges specifies that allEdges should be returned in this case, it seems we should be returning true as per step 3 above.

One final thing to note: I've exported cursorToOffset as it can be useful to schema authors when they prepare array slices suitable for passing in to connectionFromArraySlice.

[wincent]

Adding some Relay peeps: @steveluscher, @yungsters, @josephsavona.

I'll add a separate test for connectionFromPromisedArraySlice which I think should satisfy the coverage complaint.

[josephsavona]

👍