How to implement async waiting loop with packaged_task in C++? - c++

Question

Let us say, that we use packaged_task for implementing async work in a program.
#include <map>
#include <iostream>
#include <vector>
#include <algorithm>
#include <sstream>
#include <iterator>
#include <future>
#include <thread>
#include <chrono>
int main()
{
std::packaged_task<int()> packagedTaskOne([]()
{
using namespace std::chrono_literals;
std::this_thread::sleep_for(4s);
return 4;
});
std::future<int> futureResultOne = packagedTaskOne.get_future();
std::thread tOne(std::move(packagedTaskOne));
futureResultOne.wait();
tOne.join();
std::cout << "done\n";
return 0;
}
I can barely imagine how to implement waiting cursor in C#, because I can use something like while(!Task.Run(async () => await _service.GetValue()).IsCompleted) and output some dots that show loading. I want the same with packaged_task but I do not understand how to fuse it with while loop because I do not get what a condition I should for checking is this thread completed. How to make the same loop in C++ with packaged_task?

Answer 1

If you don't want to use futureResultOne.get(); to avoid blocking, the following is a way to check if your task has finished or not in the main (you can use an independent one of course) thread.
#include <iostream>
#include <future>
#include <thread>
#include <chrono>
#include <condition_variable>
using namespace std::chrono_literals;
std::condition_variable cv;
bool theThreadHasFinished;
std::mutex mut;
int main()
{
std::packaged_task<int()> packagedTaskOne([]{
std::this_thread::sleep_for(4s);
return 4;
});
std::future<int> futureResultOne = packagedTaskOne.get_future();
std::thread tOne([&]{packagedTaskOne();
std::lock_guard lg{mut};
theThreadHasFinished = true;
cv.notify_all();});
tOne.detach();
std::unique_lock<std::mutex> lock{ mut };
cv.wait(lock, [ ](){ return theThreadHasFinished; });
std::cout << "done\n";
return 0;
}
You can use a while loop ad wait_for but this consumes your resources