[dolphin] [Bug 423537] New: hold N previews in memory (i.e. multi-level preview caching)

[Harald Sitter]

https://bugs.kde.org/show_bug.cgi?id=423537

            Bug ID: 423537
           Summary: hold N previews in memory (i.e. multi-level preview
                    caching)
           Product: dolphin
           Version: unspecified
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: dolphin-bugs-null at kde.org
          Reporter: sitter at kde.org
                CC: kfm-devel at kde.org
  Target Milestone: ---

Not super sure who should do this, might be KIO.

When one browses through directories there's a good chance one may go up and
down and up and down. The trouble is every time the dir changes we PreviewJob
the new entries even when we know that they haven't changed. At the best of
times that will simply result in the job poking the thumbnailer and the
thumbnailer reading the cache file from disk, over and over again. At the worst
of times the previewable content has caching disabled entirely (Text thumbs,
directory thumbs) and we spend energy on needlessly updating stuff all the
time. The current way of doing things also means there can be a noticeable
amount of time between going up to the previous directory and the (unchanged)
previews appearing again.

To improve things I'd propose having a global static FIFO in-memory cache of
previews with some N entries that attaches a QFileSystemWatcher or something to
kick the cache entry out in case the file changes.
When loading previews for $URI first try to pop them out of the in-memory cache
and only if that misses start a PreviewJob.

For example:
I'm in dolphin's src dir and descend into the views dir only to find that the
file I was looking for is in fact not here so I go back up and into the
kitemviews dir instead.

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