tree 3219ed003b571388ca727226a517f01b70b17ea9
parent b4f69961a9f7f4999943641a634a4b623d531a9d
author Liquan(Max) Gu <maxlg@chromium.org> 1565292358 +0000
committer Commit Bot <commit-bot@chromium.org> 1565292358 +0000

[LCP] Update LCP when both text and image are ready

Largest Contentful Paint compares the text and the image candidate
to decide which is the largest candidate. Currently, LCP generates
a result entry between updating the text candidate and the image
candidate, which produces an intermediate result. It causes
the issue in crbug.com/988115, #c4 explains the mechanism of the bug.

To remove the intermediate state, this CL changes the way
LCP-calculator takes the text candidate and image candidate. Instead
of notifying LCP-calculator of the text candidate's update right after
the update, we wait until both text candidate and image candidate have
been updated to notify LCP-calculator.

Also, LCP-calculator adds the logic of checking whether candidate
has changed, from ImagePaintTimingDetector and TextPaintTimingDetector.
Although it's redundant with |UpdateCandidate| in text and image,
these are necessary because going forwards, the "has_changed" logic in
both detectors would have to be removed along with the LIP and LTP
logic.

This CL also considers the chances where either detector is destroyed,
where it would no longer be able to find the largest candidate. In
this case, perf API would use the last reported candidate as the
candidate to compare with the candidate from another detector, in
order to decide the largest between text and image.

Bug: 982307, 988115

Change-Id: I562c7aeab6037678dd4bd7d6eddacdeb5be8bfbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733456
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685300}