RFC: Add mergeMode to simplePagination helper

[hoangvvo]

Summary

Relay Pagination allows setting mergeMode to define how the paginated items are merged. From the doc:

Example series of requests:

first: 1 => node 1, endCursor: a
first: 1, after: 1 => node 2, endCursor: b
...
last: 1 => node 99, startCursor: c
last: 1, before: c => node 89, startCursor: d

With inwards merging the nodes will be in this order: [1, 2, ..., 89, 99] And with outwards merging: [..., 89, 99, 1, 2, ...]

We should implement a similar feature in Simple Pagination.

A use case is for the infinite scroll that needs to load previous items when scrolling up, where the pages with higher skip or offset value should be inserted to the beginning of the array. Ex: Chat app. In this case, later paginated items should be added to the beginning instead of the end.

Proposed Solution

Add a new option mergeMode = "before" | "after" to define how pages will be merged. If mergeMode = "after" (default), (later) pages with higher offset will be added after the others. This matches the current behavior so it will not be a breaking change. Otherwise, if mergeMode = "before", (later) pages with higher offset will be added before the others.

Example:

Where mergeMode = "after" (default) makes sense:

offset = 0 => 1,2,3
offset = 3 => 4,5,6

mergeMode = "after" => [1,2,3,  4,5,6]
mergeMode = "before" => [4,5,6,  1,2,3]

Where mergeMode = "before" makes sense:

offset = 0 => 4,5,6
offset = 3 => 1,2,3

mergeMode = "after" => [4,5,6,  1,2,3]
mergeMode = "before" => [1,2,3,  4,5,6]

Here is a demo image (source):