[Tkinter-discuss] get quickly, very fast and directly items in a canvas from a moving button mouse

[Mister Vanhalen]

Hi,

Thank you very much for your answer. I chose your second advice, because
the virtual rectangle seemed to be more adapted for my application. But I
keep in mind also the tag_bind.
Thank you again for your help,
Regards,
Eduard

On Sat, Feb 4, 2012 at 9:34 PM, Michael Lange <klappnase at web.de> wrote:

> Hi,
>
> Thus spoketh Mister Vanhalen <mistervanhalen at gmail.com>
> unto us on Sat, 4 Feb 2012 14:33:43 +0100:
>
> > Hello everyone,
> >
> > I have got a canvas, where I created some items (rectangles and
> > letters). I want to catch quickly and directly every item when I move
> > my mouse holding a button.
> > I managed to do something but when the movement is too fast, all items
> > are not selected....
> >
> (...)
>
> there seems to be a certain point where even in the simplest example
> some events fail to be handled, not sure if it is Tk- or window manager
> related, but here it is the same, if the mouse is *very* fast, the event
> might be missed.
>
> >From your example it is hard to tell if there is an additional problem in
> your event callback that slows things down further, because the snippet
> does not show any visible effect when an item is being selected. Maybe in
> your "real world" code some bottleneck is in what happens to "item" later
> within your select() method. Maybe some optimization might help.
>
> Two things come to mind which you might try to improve things at least a
> little:
>
> * first, using the canvases tag_bind() method instead of bind().
> * second, try to reduce the amount of Tk calls within the select()
> callback; in cases where optimization becomes an issue, tk callbacks
> prove to be processed relatively slow and often considerable improvements
> can be achieved by storing some Tk related information in Python objects
> instead of querying them every time through Tk, or sometimes a trick can
> be used to pass them directly to the callback, as in this example:
>
> ######
> from Tkinter import *
>
> root = Tk()
> canv = Canvas(root)
> canv.pack(fill='both', expand=1)
> t1 = canv.create_text(10, 10, text='foobar', anchor='nw')
> t2 = canv.create_text(10, 60, text='foobar', anchor='nw')
> t3 = canv.create_text(10, 110, text='foobar', anchor='nw')
> t4 = canv.create_text(10, 160, text='foobar', anchor='nw')
>
> def select(event, item):
>    print 'selected', item
>
> for t in (t1, t2, t3, t4):
>    canv.tag_bind(t, '<Any-Motion>',
>                   lambda event, item=t : select(event,item))
>
> root.mainloop()
> #######
>
> Now, whem you talk about "selecting" a totally different approach comes
> to mind; it sounds to me like you want to draw some kind of "virtual
> rectangle" across the canvas to select all the items within this
> rectangle, much as you would do in a file browser to select items? If
> yes, then maybe you would better actually draw such a rectangle, and then
> later "select" all the items within, maybe the ButtonPress and
> ButtonRelease event's coords are easier to track than all the coords
> within the motion. Just a guess of course.
>
> Regards
>
> Michael
>
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> unknown
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