Re^4: Perl/Tk window contents disappear when obscured then revealed

[Another Ed]

Thanks BrowserUk for your very detailed explanations.

I'm amazed that we're facing this problem at all: if, for example, I obscure the Firefox window into which I'm currently typing, then reveal it again, it redisplays exactly as you would expect; I am sure that the Firefox code doesn't have to contain things like sub-second checking loops just to redisplay the contents of its own window. Surely the task of redrawing a window when it is revealed is down to the window manager, not the application whose window it is? So why is Perl/Tk any different? Is this a "feature" of Tk, or is it something that has gone wrong in the Perl interface to it? You say this in the nature of GUIs - but this is so crazy that I find that difficult to believe, with all respect to you.

I'm also surprised that this problem, which must surely be one that many many people will have faced, doesn't appear to have a standard workaround, let alone a fix.

Regarding the choices you list, I agree that (1) is not nice. I am very uncomfortable with (2), on the basis that making the machine do a lot of work just to wait for something to happen simply doesn't seem a sensible way to approach anything. I think your (3) is the only realistic option. I can't visualise in detail how this would be done, but I guess it could use Proc::Background as suggested by Anonymous Monk below. (However the 2 responses to that suggestion both involve the same kind of crazy loops as your choice (2)!)

I became diverted away from this problem, hence the very late response to you, and I now find I cannot devote any more time to it. I'll finish writing my application without trying to display a Perl/Tk window in parallel with another application's window, so this issue will no longer arise.

Many thanks to everyone who has contributed to this.

Ed

[BrowserUk]

I am sure that the Firefox code doesn't have to contain things like sub-second checking loops just to redisplay the contents of its own window.

You'd be wrong I'm afraid. And you can prove it to yourself. Find a website with a page that updates (say) once a minute. Cover the section that changes and wait 1 1/2 minutes then uncover it. Is what you uncover the same as it was 1/1/2 minutes ago? Or has it changed.

Surely the task of redrawing a window when it is revealed is down to the window manager, not the application whose window it is?

The best the window manager could do is cache the bits that you cover up and blit them back when you uncover it again; but that wouldn't then reflect any changes that have happened in the mean time. (Ask yourself: How could the window manager know what might have changed -- if anything -- without consulting the application?)

But that isn't what happens. When the window uncovers, An 'update rectangle' that denotes the uncovered region is sent as part of an update message to the applications window procedure and it is responsible for redrawing that rectangle.

That's a lightweight description of the process -- the real thing is far more complex (long ago I helped write and test the window manager used by OS/2's Presentation Manager) -- but close enough for discussion.

The best applications that have dynamic window content, draw into an off-screen window buffer; and simply blit the appropriate part of it on-screen in response to update messages. This effectively decouples the drawing process from user events. My biggest beef with Tk (and many other GUI toolkits) is that they do not give you direct access to the window buffers, which effectively prevents using this optimum strategy.

There have been attempts to move this functionality into some GUI toolkits, but with zoomable and scrollable windows, it means the application has to maintain the entire 'world content', even when large parts of it are off screen (and thus could be skipped for optimisation purposes), because the application is no longer in control of what is actually being displayed at any given time.

[choroba]

the task of redrawing a window when it is revealed is down to the window manager, not the application

When Firefox freezes (happens to me sometimes when running adobe flash plugin), its window becomes completely gray. I can switch to other applications, but Firefox does not update its windows. I fear you are not right here.

[Anonymous Monk]

If you want to see this in action, install http://maf.mozdev.org/ and save a file -- maf slows things down and blocks