[PyQt] Threading crashes

Mark Summerfield list at qtrac.plus.com
Tue Apr 20 15:39:24 BST 2010

On 2010-04-20, andre hotz wrote:
> Mark Summerfield schrieb:
> > On 2010-04-20, andre hotz wrote:
> >> Hey,
> >>
> >> I have a problem with my QThreads (running on windows). It is the first
> >> time I use multithreading, so I am probably doing something wrong ;)
> >>
> >> The situation is as follows: I have a QTreeWidget which works like a
> >> filebrowser, listing a few thousand files. Creating
> >> the treeitems from the files is pretty fast, but I also want the
> >> treeWidget to display more information about the files.
> >> This is taking some time depending on the quantity of the files. So 
> >> I did is to first fill the treeView with all the filenames.
> >> After that, I start an update-threat, which loops through all the items
> >> in the treeWidget, gathers its data and fills the additional
> >> columns in the treeView with the data. While the thread is updating the
> >> treeWidget, the user can already work with the files and does not need
> >> to wait for the update to finish.
> >>
> >> This all works pretty well.
> >
> > I think you've been pretty lucky to have it work at all!
> hehe, yeah, I was pretty happy when I got it working the first time ;)
> > Qt does not provide any means of locking items or views. As a
> > consequence, Qt does _not_ support updating models, scenes, or items in
> > models, or items in widgets such as QTreeWidgetItems or
> > QTableWidgetItems, etc. outside the main (GUI) thread. So any such
> > update is potentially the source of a crash.
> hm, yes, this might explain the crashes..
> > One technique that could be used is to hold the data in a thread-safe
> > data structure and read data from that into the GUI as needed (locking
> > the data structure or each item read from it as needed), and updating
> > the data from one or more secondary threads (locking as needed).
> That sounds like it could work. Would it be possible to hold the data as
> you said, but when the update thread is finished, let it emit a signal
> to the treeWidget which then updates itself with the given data
> structure? Or does emitting signals from another thread also not work?

One of the great things about signals and slots is that they work safely
across threads:-)

So you can certainly emit a signal in a secondary thread that's
connected to a main (GUI) thread slot. And since you can pass values via
signals, if you had simple things like ints, doubles, and QStrings, you
could even call the main thread every time you updated the data on a
single file in the secondary thread (at some cost in performance that
may or may not matter) to give the main thread the info you want.

Mark Summerfield, Qtrac Ltd, www.qtrac.eu
    C++, Python, Qt, PyQt - training and consultancy
        "Advanced Qt Programming" - ISBN 0321635906

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