Retain previous result when partialRefetching

[glentakahashi]

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

Version

@apollo/client 3.0.0-beta.38

Right now if a query is marked as partial, during the refresh loading will properly be set to "true" however data is returned as undefined.

This causes undesirable behavior as when a partial refetch is occurring you cannot render the component with the previous data unless you use an external state mechanism like a useState to store the value in-between runs.

In some cases it might be desirable depending on the use case to show a null or loading state during a partial refetch, but this could be handled by using the loading state instead of relying on data being undefined.

You can also get around this today by setting returnPartialData: true, however that is not desirable in many situations as well.

Relates to apollographql/react-apollo#2114
A partial port of apollographql/react-apollo#3606
(apollo-client 3.x fixed the bug causing partialRefetches to not happen, this only ports over setting previousData) cc @minznerjosh

[codecov]

Codecov Report

Merging #6012 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6012   +/-   ##
=======================================
  Coverage   95.20%   95.20%           
=======================================
  Files          88       88           
  Lines        3674     3674           
  Branches      873      903   +30     
=======================================
  Hits         3498     3498           
  Misses        154      154           
  Partials       22       22           

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[glentakahashi]

@benjamn or @hwillson Sorry to ping, but this ticket has been open a while and was curious if you had thoughts or opinions on mergeability

[glentakahashi]

Actually it appears as if this bug was fixed in #6221.

Now it appears that when refetching, the component with partialRefetch: true won't re-render until the refetch has completed unless it has returnPartialData set as true