Firstly, I've never used threads, but have found lots of examples on the internet about their use but nothing that obviously answers my question.
I have a class that loads and manipulates a file(s). It is fairly CPU intensive so I intend to put it in its own thread so that the GUI remains responsive. However, I would also like to use a progress bar to indicate the current status of the file operations.
The question is, what is the best way of approaching this, i.e. How do I get my file class to tell the app where it's up to? Do I have to add thread specific code to my class? Or is there an interface I can implement? Or, am I approaching this all wrong. Additionally (sorry, another stupid question) I assume I need an indicator in my file class to tell the thread when it's finished?
I'm using VS2010 and intend to build the app with WPF (if that's relevant)
Thanks for any advice,
Bob
Since you are intending to use WPF, use a Dispatcher:
Build More Responsive Apps With The Dispatcher
(Otherwise, for Winforms use a BackgroundWorker)
Related
I've an application witch update execute commands on a database. I've a function witch execute that commands and in the in this function I display some messages to the user.
My problem is that the texts written into the textEdit are displayed at the end of the function work.
I've tried to use QThread but it does'nt resolve the problem
this->moveToThread(&threadText);
connect(&threadText, SIGNAL(started()), this, SLOT(writeTexte()));
threadText.start();
Does anyone have an idea how to proceed to write into textEdit and not block the UI ?
In Qt you never need threads in standard UI applications! That is because Qt uses something called event loops that process events smoothly without ever blocking. If you experience blocking, that is a sign that you have a bug somewhere in your code. For any beginner there are some pitfalls. The official documentation on the subject is quite good.
It is hard to tell what your exact bug is because there is littel code to look at. I suggest you post more of your code, especially the naive approach that did not work before you added threads to the mix.
On a side note, using threads in Qt is of course possible and has it's usages, however it can be un-intuitive. I have found this article to be a really good starting point to understand how threads is used best in Qt.
I want to make a tool which I can attach to another process and see where it writes to the memory, but I don't even know where to begin.
Any idea on how would I do this?
To do this, you will essentially have to write a debugger. Windows has The Debugging Application Programming Interface which you will need to use to accomplish this. This API is not simple and probably isn't designed to be called easily from VB.NET, so you probably have a challenging task ahead. But that's how you would begin.
I am working with the QuickTime API and need to perform a few lengthy (as in hours) operations in the background. Unfortunately, it is not multi-thread friendly, so I am falling back to perform the tasks in a separate process, so all QuickTime related calls can happen in its main thread.
After launching it, I need a way of getting feedback on its progress, since the operations can take very long.
I am unsure of how to do this, specifically:
Should the separate process be compiled as another cocoa app or a command line tool?
How to launch it from the main cocoa app?
How to periodically get an object from it to get status information?
How to determine when it finished?
How to avoid showing a window/console when called?
How to have it part of the .app bundle so that it does not appear as a separate executable to the user?
These are really 6+ questions, but they are very related and very specific, and I think anyone needing to launch external processes (instead of spawning worker threads) can benefit from their answers. Generic code examples would be very helpful.
If it is possible, then implement the functionality in a command line tool, or another form of GUI-less application. For Cocoa applications it is possible to prevent them appearing on the Dock or in the Force Quit dialog, however a command line tool is a single binary file which does that anyway, so that would probably be a better way.
In terms of launching the tool, NSTask & NSPipe are your friends in this endeavour. The tool can definitely be kept inside your main Application's bundle, inside the Resources directory or some such, and then launched when needed. You can use the pipe to communicate back and forth.
I don't have any example code to hand, and its been a long while since I've had occasion to use either of these classes so the information I can give is limited, but it should be enough to point you in the right direction.
I have a VB.NET program. In proerties/Application I checked Make a Single Instance Application. But it still allows someone from opening the program while already opened. How can I prevent this?
The short answer is you can't. VB does some tricks behind the scenes to help you accomplish this since it's a common business requirement. However anything you create that is actually exclusive opens you to a denial of service attack by someone else maliciously creating/holding that resource and denying it to you.
Common ways are creating a well known mutex/semaphore/named pipe/something similar and if it already exists, exiting the instance just launched. There just isn't a secure way of doing what you want to do.
When you select Make Single Instance Application, starting a new copy of the program will automatically switch to the existing copy.
You can customize this behavior in the Application class.
The commonest way to do this is to implement a mutex, however, I will point you to an article or two, here, and here on CodeProject.
Hope this helps,
Best regards,
Tom.
I have a college assignment due quite soon, and I need to be able to call a C++ dll that takes a long time (possibly infinte, it relies on user input)to execute. Im calling this through VB. My VB GUI freezes up when this happens, and I would like to keep the GUI responsive, so that the user can stop this possibly infinte loop.
Can anyone suggest the best/fastest way of doing this?
A bit of background, the C++ is trying to keep score on a snooker table using a webcam, and while the VB scoreboard updates easily, I would like to script it so that the analysis is almost continuous, while still allowing the user to interact. Currently the project requires the user to press a button to start the shot analysis, but it would be preferable if the program scripted itself. I have only realised this problem now and the deadline is very soon.
Update: Our lecturer suggested an option to solve the problem, but it would appear that most options here and the one he suggested will not work for us as the processing time required for the webcam image capture is too great to handle due to hardware constraints. Thanks for taking the time to help, it was much appreciated!
The best way to handle threading in VB.NET is via the System.Threading namespace.
You might also look into Application.DoEvents()
Try the system.Threading as Mark said, also try looking at Background Worker Process which is a bit simpler in VB.NET. Readup here
I would definitely use the Background Worker process. You can drag it onto your form and use the DoWork sub routine to actually do the work that is freezing your GUI thread. You can also use the ReportProgress event to actually provide progress back to your form.
As for your question regarding two separate threads, If both steps take a long time to complete I would run them in the same thread one after the other.
The one thing that could bite you with using the thread is cross-threading. In the context of your problem this means not having your second thread update form controls.
A really good resource for how to implement this code along with dealing with cross-threading is this PDF.
I also should point out that if you are using .net 1.0/1.1 you can still do the multi-threading, but don't have the luxary of having a background worker control. You'd just have to create a new thread from the System.Threading Namespace.
Just as an alternative, you could have your C++ actually processing in the background all the time. When called from VB, it would just be retrieving data from it or sending it a command (start, quit, ???) all of which would return instantly.
This could also give you more reliability since C++ would never miss video frames while VB was collecting the garbage or doing the dishes or whatever VB does in the background--C++ is going to let you be closer to a real time system.
RE: the comment about how.
What I'd probably do is have my VB programs send "Messages" to the C++ (As I said). A message is just a way to think of a function/method call--but generally they return quickly.
The "Start" message would tell the C++ code to start it's thread running and return. This is a Linux C++ thread howto, I'm not sure if you need to do something different in windows (I'd hope not, but I haven't used C++ as my main dev. language in decades).
If that doesn't work, just google "C++ Threads"
Sending a "Stop" message would stop the thread (and probably free resources).
A "Get Data" call would go to the location that the C++ thread used to store data, grab it and return.
Sorry to be so general, I'm pretty heavily Java these days.
I'm surprised nobody has suggested using a BackgroundWorker yet.