Review Request 111322: Prevent some unnecessary relayoutings when the view size is changed

[Commit Hook]

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This review has been submitted with commit 1594edb157df8bb6bbac8ca773682d0d98f753d7 by Frank Reininghaus to branch master.

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On June 30, 2013, 1:07 p.m., Frank Reininghaus wrote:
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> (Updated June 30, 2013, 1:07 p.m.)
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> Review request for Dolphin.
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> Description
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> Currently, we always trigger a relayouting of the view if the view geometry changes. However, in Icons/Details View the view height doesn't matter at all for the layout, and neither does the view width in Compact View.
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> I propose to not force a realayouting in these cases, but only a recalculation of the visible indexes.
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> Enabling the debug output in KItemListViewLayouter showed me that we actually re-layout the view far too often: for example, every time the desktop is switched. This seems to be due to a glitch in KUrlNavigator which changes the view height by 1 pixel every time the window becomes visible or invisible after a desktop switch. This might be worth investigating and fixing in kdelibs, but I think that this should be fixed on the Dolphin side as well.
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> I have a few more ideas how we can speed up the layouting, which can become a major bottleneck in Icons View in folders with many items (at least the first time, when no cached item sizes are available in KItemListSizeHintResolver yet): in many cases, file names are so short that we can tell that only one line is needed for showing the name without letting QTextLayout do a very expensive full calculation in
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> QSizeF KStandardItemListWidgetInformant::itemSizeHint(int index, const KItemListView* view).
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> Moreover, everything in that function seems to be reentrant, so we can actually speed up the itemSizeHint calculation by making use of multiple CPU cores. But these things require changes that might be too intrusive to push to master at this point of the release cycle. I'll investigate this for KDE 4.12.
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> Diffs
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>   dolphin/src/kitemviews/private/kitemlistviewlayouter.cpp c15b44e 
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> Diff: http://git.reviewboard.kde.org/r/111322/diff/
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> Testing
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> No unnecessary layoutings are done any more. Everything else still seems to work nicely.
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> Thanks,
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> Frank Reininghaus
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