Been doing quite a bit of research but can't find the answer to this. I have an application which is built up by creating instances derived from QObject that I move to different threads. In these "threads" I have a QNetworkAccessManager where I do rest request to azure. My issue now is that I have some retry logic that is happening way to quickly. For example, if a container doesn't exists, it needs to be created for the request to be retried (put blob). The issue is if the put blob request happens way too quickly after the container was created, it will fail to upload anything. Also, I would like to increase the time between the reties so that they dont happen to quickly after eachother. What is the best way to force a thread to sleep from within itself?
I'd probably end up using some combination of QTimer and signals/slots. For example:
// Slot
void retry() { ... }
...
// Execute retry code in 1 second:
QTimer::singleShot( 1000, this, SLOT(retry()) );
Use QThread::wait()
Related
I have an ordinary GUI Thread (Main Window) and want to attach a Worker thread to it. The Worker thread will be instantiated, moved to its own thread and then fired away to run on its own independently, running a messaging routine (non-blocking).
This is where the worker is created:
void MainWindow::on_connectButton_clicked()
{
Worker* workwork;
workwork= new Worker();
connect(workwork,SIGNAL(invokeTestResultsUpdate(int,quint8)),
this,SLOT(updateTestResults(int,quint8)),Qt::QueuedConnection);
connect(this,SIGNAL(emitInit()),workwork,SLOT(init()));
workwork->startBC();
}
This is where the Worker starts:
void Worker::startBC()
{
t1553 = new QThread();
this->moveToThread(t1553);
connect(t1553,SIGNAL(started()),this,SLOT(run1553Process()));
t1553->start();
}
I have two problems here, regarding the event queue of the new thread:
The first and minor problem is that, while I can receive the signals from the Worker thread (namely: invokeTestResultsUpdate), I cannot invoke the init method by emitting the emitInit signal from MainWindow. It just doesn't fire unless I call it directly or connect it via Qt::DirectConnection . Why is this happening? Because I have to start the Worker thread's own messaging loop explicitly? Or some other thing I'm not aware of? (I really fail to wrap my head around the concept of Thread/Event Loop/Signal Slot mechanism and the relation between each other even though I try. I welcome any fresh perspective here too.)
The second and more obscure problem is: run1553process method does some heavy work. By heavy work, I mean a very high rate of data. There is a loop running, and I try to receive the data flowing from a device (real-time) as soon as it lands in the buffer, using mostly extern API functions. Then throw the mentioned invokeTestResultsUpdate signal towards the GUI each time it receives a message, updating the message number box. It's nothing more than that.
The thing I'm experiencing is weird; normally the messaging routine is mostly unhindered but when I resize the main window, move it, or hide/show the window, the Worker thread skips many messages. And the resizing action is really slow (not responds very fast). It's really giving me a cancer.
(Note: I have tried subclassing QThread before, it did not mitigate the problem.)
I've been reading all the "Thread Affinity" topics and tried to apply them but it still behaves like it is somehow interrupted by the GUI thread's events at some point. I can understand MainWindow's troubles since there are many messages at the queue to be executed (both the invoked slots and the GUI events). But I cannot see as to why a background thread is affected by the GUI events. I really need to have an extremely robust and unhindered message routine running seperately behind, firing and forgetting the signals and not giving a damn about anything.
I'm really desperate for any help right now, so any bit of information is useful for me. Please do not hesitate to throw ideas.
TL;DR: call QCoreApplication::processEvents(); periodiacally inside run1553process.
Full explanation:
Signals from the main thread are put in a queue and executed once the event loop in the second thread takes control. In your implementation you call run1553Process as soon as the thread starts. the control will not go back to the event loop until the end of that function or QCoreApplication::processEvents is manually invoked so signals will just sit there waiting for the event loop to pick them up.
P.S.
you are leaking both the worker and the thread in the code above
P.P.S.
Data streams from devices normally provide an asynchronous API instead of you having to poll them indefinetly
I finally found the problem.
The crucial mistake was connecting the QThread's built in start() signal to run1553Process() slot. I had thought of this as replacing run() with this method, and expected everything to be fine. But this caused the actual run() method to get blocked, therefore preventing the event loop to start.
As stated in qthread.cpp:
void QThread::run()
{
(void) exec();
}
To fix this, I didn't touch the original start() signal, instead connected another signal to my run1553Process() independently. First started the thread ordinarily, allowed the event loop to start, then fired my other signals. That did it, now my Worker can receive all the messages.
I think now I understand the relation between threads and events better.
By the way, this solution did not take care of the message skipping problem entirely, but I feel that's caused by another factor (like my message reading implementation).
Thanks everyone for the ideas. I hope the solution helps some other poor guy like me.
in my C++ based BlackBerry 10 app I have a custom class that uses QNetworkAccessManager to handle network connections. The requestFinished(..) method of QNetworkAccessManager object emits a signal when it receives some data.
The thing is, many outside classes are interested in this signal. So, I have many slots connected to this signal. My problem is that, in those slots, I may be doing some UI related stuff -- so I may not want that once the signal is emitted, ALL slots get called simultaneously all the time.
Rather I may want that, at one point in time, the signal calls only one slot, at another point in time, another slot, and etc. What are the ways to do this???
I thought I could do is using different QNetworkAccessManager objects (below) -- but I have heard it is not recommended??? e.g., what are other ways??? Thank you.
MyNetworkClass *network1 = new MyNetworkClass();
bool res = QObject::connect(network1, SIGNAL(signalSuccess(QVariant)), this, SLOT(CustomSLot1(QVariant)));
MyNetworkClass *network2 = new MyNetworkClass();
bool res = QObject::connect(network2, SIGNAL(signalSuccess(QVariant)), this, SLOT(CustomSLot2(QVariant)));
MyNetworkClass *network3 = new MyNetworkClass();
bool res = QObject::connect(network3, SIGNAL(signalSuccess(QVariant)), this, SLOT(CustomSLot3(QVariant)));
By default, Qt application runs only 1 thread. This means nothing can be asyncronous. When one signal is fired, the slot function is run before anything else happens.
If you want to keep UI responsive and handle 3 functions at the same time, you need to thread them. At the beginning of the slot-function, start a thread where you execute the actual functionality.
http://doc.qt.digia.com/stable/thread-basics.html
Qt also has asynchronous functions:
http://doc.qt.digia.com/stable/qtconcurrentrun.html
Basically what Gjordis is saying you have to run the custom class where the QNetworkAccessManager resides asynchronously. You have 2 simple choices :
Either invoke asynchronous the methods of your class using QtConcurrent::run
Either handle this class events in another thread, see this excellent minimal example. The only thing is that the doWork() mentionned is really a doInit() or startWork() depending on the situation.
With both of these cases it is primordial to interact with the class only using signals and slots , otherwise you may encounter races conditions. Which means the "consumer" UI thread should not use getters\setters. Locking is not a good option, as the UI thread will require to perform an eventually blocking operation to get his data.
You should emit different signals in sequence from the same Object: a signal for every different object that is interested.
In this way you can decide the order and decide if send message to all or only to some.
Hi I have a QNetworkAccessManager which I use to send request to get image data from server. This call is asynchronous. I do multiple calls with it. Each call is done by opening a new instance of QNetworkAccessManager So when a specific condition occurs I want to stop the QNetworkAccessManager from receiving the replies from its network requests. Is there any way to do it? Thanks.
Don't use a new QNetworkAccessManager for each request but share the manager. It's usually fine to have just one. Of course one can have multiple if the application design suggests it - but e.g. managing multiple of them in a single controlling object is usually unnecessary. Just have one manager with the same lifetime as the object controlling the network requests.
To cancel running operations, keep the QNetworkReply* pointers QNetworkAccessManager::get/put/post return and call abort() when your condition occurs.
Connect to the finished() signal to remove them from the bookkeeping (as otherwise you would end up with dangling pointers).
If that becomes too complicated, think about using the command pattern. In this answer I describe why I find it particularly useful in this context.
I have encountered the need to use multithreading in my windows form GUI application using C++. From my research on the topic it seems background worker threads are the way to go for my purposes. According to example code I have
System::Void backgroundWorker1_DoWork(System::Object^ sender, System::ComponentModel::DoWorkEventArgs^ e)
{
BackgroundWorker^ worker = dynamic_cast<BackgroundWorker^>(sender);
e->Result = SomeCPUHungryFunction( safe_cast<Int32>(e->Argument), worker, e );
}
However there are a few things I need to get straight and figure out
Will a background worker thread make my multithreading life easier?
Why do I need e->Result?
What are the arguments passed into the backgroundWorker1_DoWork function for?
What is the purpose of the parameter safe_cast(e->Argument)?
What things should I do in my CPUHungryFunction()?
What if my CPUHungryFunction() has a while loop that loops indefinitely?
Do I have control over the processor time my worker thread gets?
Can more specifically control the number of times the loop loops within a set period? I don’t want to be using up cpu looping 1000s of times a second when I only need to loop 30 times a second.
*Is it necessary to control the rate at which the GUI is updated?
Will a background worker thread make my multithreading life easier?
Yes, very much so. It helps you deal with the fact that you cannot update the UI from a worker thread. Particularly the ProgressChanged event lets you show progress and the RunWorkerCompleted event lets you use the results of the worker thread to update the UI without you having to deal with the cross-threading problem.
Why do I need e->Result?
To pass back the result of the work you did to the UI thread. You get the value back in your RunWorkerCompleted event handler, e->Result property. From which you then update the UI with the result.
What are the arguments passed into the function for?
To tell the worker thread what to do, it is optional. Otherwise identical to passing arguments to any method, just more awkward since you don't get to chose the arguments. You typically pass some kind of value from your UI for example, use a little helper class if you need to pass more than one. Always favor this over trying to obtain UI values in the worker, that's very troublesome.
What things should I do in my CPUHungryFunction()?
Burn CPU cycles of course. Or in general do something that takes a long time, like a dbase query. Which doesn't burn CPU cycles but takes too long to allow the UI thread to go dead while waiting for the result. Roughly, whenever you need to do something that takes more than a second then you should execute it on a worker thread instead of the UI thread.
What if my CPUHungryFunction() has a while loop that loops indefinitely?
Then your worker never completes and never produces a result. This may be useful but it isn't common. You would not typically use a BGW for this, just a regular Thread that has its IsBackground property set to true.
Do I have control over the processor time my worker thread gets?
You have some by artificially slowing it down by calling Thread.Sleep(). This is not a common thing to do, the point of starting a worker thread is to do work. A thread that sleeps is using an expensive resource in a non-productive way.
Can more specifically control the number of times the loop loops within a set period? I don’t want to be using up cpu looping 1000s of times a second when I only need to loop 30 times a second.
Same as above, you'd have to sleep. Do so by executing the loop 30 times and then sleep for a second.
Is it necessary to control the rate at which the GUI is updated?
Yes, that's very important. ReportProgress() can be a fire-hose, generating many thousands of UI updates per second. You can easily get into a problem with this when the UI thread just can't keep up with that rate. You'll notice, the UI thread stops taking care of its regular duties, like painting the UI and responding to input. Because it keeps having to deal with another invoke request to run the ProgressChanged event handler. The side-effect is that the UI looks frozen, you've got the exact problem back you were trying to solve with a worker. It isn't actually frozen, it just looks that way, it is still running the event handler. But your user won't see the difference.
The one thing to keep in mind is that ReportProgress() only needs to keep human eyes happy. Which cannot see updates that happen more frequently than 20 times per second. Beyond that, it just turns into an unreadable blur. So don't waste time on UI updates that just are not useful anyway. You'll automatically also avoid the fire-hose problem. Tuning the update rate is something you have to program, it isn't built into BGW.
I will try to answer you question by question
Yes
DoWork is a void method (and need to be so). Also DoWork executes
in a different thread from the calling one, so you need to have a
way to return something to the calling thread. The e->Result
parameter will be passed to the RunWorkerCompleted event inside
the RunWorkerCompletedEventArgs
The sender argument is the backgroundworker itself that you can use
to raise events for the UI thread, the DoWorkEventArgs eventually
contains parameters passed from the calling thread (the one who has
called RunWorkerAsync(Object))
Whatever you have need to do. Paying attention to the userinterface
elements that are not accessible from the DoWork thread. Usually, one
calculate the percentage of work done and update the UI (a progress
bar or something alike) and call ReportProgress to communicate with
the UI thread. (Need to have WorkerReportProgress property set to
True)
Nothing runs indefinitely. You can always unplug the cord.
Seriously, it is just another thread, the OS takes care of it and
destroys everything when your app ends.
Not sure what do you mean with this, but it is probably related
to the next question
You can use the Thread.Sleep or Thread.Join methods to release the
CPU time after one loop. The exact timing to sleep should be fine
tuned depending on what you are doing, the workload of the current
system and the raw speed of your processor
Please refer to MSDN docs on BackgroundWorker and Thread classes
I'm attempting to create a library whose API will be used in the following way:
WebService *service = new WebService( username, password );
User *user = service->getAuthenticatedUser();
UserAssets *assets = user->assets();
// And so on
Neither the authenticated user, nor their assets, will be downloaded when the WebServer instance is created, rather they will only be retrieved if explicitly requested.
Whenever I've had to retrieve data from the network before using Qt, I've followed the standard pattern of connection the finished() signal of the QNetworkReply to the appropriate slot and using that slot to handle the returned data.
My problem here is that pattern does not seem to accommodate my desired use-case. I would like the users of my library (other developers) to be able to use a single line to request and receive the data they desire, but the 'Qt way' seems, at least from my point of view, to require them to initiate the request on one line, and then connect some signal to some other slot to get the data back, which is not the experience I would like them to have.
I'm new to network programming, both in general and with Qt, but I've used libraries written in Python, communicating with the exact same service, that are able to achieve this, so it does seem to be possible.
Is it possible to perform the full lifecycle of a HTTP request with Qt with a single function call?
Your best bet is probably to use a QEventLoop. This would allow you to 1) initiate the HTTP connection and, from your caller's perspective, 2) effectively block until you get a response.
For example:
QNetworkReply wait for finished
As already other have mentioned you could use QEventLoop to wait for finished() or error() signals, and the quitting event loop. This solution while working, have some serious disadvantages.
If it takes longer to download given address, then you might be stuck in your event loop for quite a while. The event loop is processing events nicely, so your app doesn't frezze, but there are some quirks connected to it anyway. Imagine that user is waiting for load, and then presses another button, to load something else. Then you will have multiple loop-in-loop, and first file will have to wait for the second to finish downloading.
Doing things in single call suggest to many programmers, that this will happen at one instant. But your function is processing events internally, so this might not hold. Imagine a code like
// some pointer accessible to many functions/methods (eg. member, global)
MyData* myData=0;
Then a code calling your function:
if (myData){
QNetworkReply* reply = getMyWobsite(whatever);
myData->modify(reply);
}
Seems fine, but what if some other slot happens to call
myData=0;
If this slot will be executed while waiting for request, application will crash. If you decide to use QEventLoop in your function, be sure to mention it in function documentation, so programmers using it will be able to avoid such problems.
If you are not using qt for anything else, you might even consider some alternative libraries (eg. libcurl) that might have what you need already implemented.