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Qt async call: how to run something after an async call has finished its job

I have code like below which does an async call:

QMetaObject::invokeMethod(this, "endSelectionHandling", Qt::QueuedConnection);


I want to modify the code like this:

QMetaObject::invokeMethod(this, "endSelectionHandling", Qt::QueuedConnection);

// I want to add statements here which depend on the result of the above async call.
// How can I wait for the above async call to finish its jobs?


How can I wait for Qt asycn call to finish its job? Is there a better approach?

1 Answer
1

In your question, it looks like you don't need an asynchronous call at all because you are waiting for the result right after making it an async call.

But, in case you have some code in between, you may use C++11's std::async to call the function asynchronously and then wait on its std::future whenever and wherever you need its result after doing other stuff.

std::async

std::future

Here's an example:

#include <iostream>
#include <future>
#include <thread>
#include <chrono>

#define LOG() std::cout << __func__ << " : "

void test()
{
LOG() << "INn";

using namespace std::chrono_literals;
std::this_thread::sleep_for( 1s );

LOG() << "OUTn";
}

int main()
{
LOG() << "Calling test()...n";

auto f = std::async( std::launch::async, test );

LOG() << "Running test()...n";

// ... ...
// ... You can do other stuff here ...
// ... ...

f.wait(); // Blocking call to wait for the result to be available

LOG() << "Exiting...n";

return 0;
}


Here's the output:

main : Calling test()...
main : Running test()...
test : IN
test : OUT
main : Exiting...


Here's the live example: https://ideone.com/OviYU6

UPDATE:

However, in Qt realm, you may want to use QtConcurrent::run and QFuture to do things in Qt way.

QtConcurrent::run

QFuture

Here's the example:

#include <QDebug>
#include <QtConcurrent>
#include <QFuture>
#include <QThread>

#define LOG() qDebug() << __func__ << ": "

void test()
{
LOG() << "IN";

QThread::sleep( 1 );

LOG() << "OUT";
}

int main()
{
LOG() << "Calling test()...";

auto f = QtConcurrent::run( test );

LOG() << "Running test()...";

// ... ...
// ... You can do other stuff here ...
// ... ...

f.waitForFinished(); // Blocking call to wait for function to finish

LOG() << "Exiting...";

return 0;
}


Here's the output:

main : Calling test()...
main : Running test()...
test : IN
test : OUT
main : Exiting...


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