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[Michał Sawicz <michal.sawicz@xxxxxxxxxxxxx>]

> 2. ResultsModel::data() seems to be called constantly while scrolling a
> scope up and down. I assume this should only be called once per result
> item (This doesn't explain the horizontal scrolling lag, but it may
> explain some vertical lag).

That's not exactly right. It will be called for every role*every result
that a delegate is created for. Delegates are recycled and created as
they go out and into view, respectively.

*If* it's called on the same result and same role multiple times, while
the result is constantly in view, that'd be a problem indeed.

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https://bugs.launchpad.net/bugs/1395994

Title:
  UI lags when scrolling through scopes

Status in “unity8” package in Ubuntu:
  New

Bug description:
  Some causes ruled out:

  1. OA in scopes can't be to blame as I have tested with and without it
  enabled, and with and without it present in the code
  (https://bugs.launchpad.net/ubuntu/+source/unity-scopes-
  api/+bug/1392615/comments/4).

  2. Location can't be blamed either (for the same reasons as above, as
  well as removing location service usage from the shell plugin itself).

  3. When profiling the QML and C++ code, I saw no indication that the
  shell plugin was doing anything process intensive enough to cause any
  hold up. In fact, it seems to be doing a pretty good job handling
  calls from the UI thread asynchronously.

  
  Observations made when profiling unity8:

  1. The QML ListView component updates its current item index when the
  next item reaches half-way across the screen (therefore, in mid
  animation between scopes). There doesn't look to be any serious
  blocking actions taking place during the onIsCurrentChange, setActive,
  etc. methods that I could find, but we should keep this in mind.

  2. ResultsModel::data() seems to be called constantly while scrolling
  a scope up and down. I assume this should only be called once per
  result item (This doesn't explain the horizontal scrolling lag, but it
  may explain some vertical lag).

  3. If you pull down to refresh any scope, then continuously move the
  dash left and right, the dash movement is smooth until results start
  showing up in the scope, at which point you see a lag as they appear.
  The lag observed here is almost definitely the same lag we see when
  scrolling through scopes (jittering as results start to pop up in
  scopes as they populate).

  (NOTE: All of the above points were observed with and without the
  latest unity8 performance fixes in rtm silo 8)

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