Question

I want to execute once a specific action delayed n seconds after the last asynchronous event occurred. So if successive events appear in less that n seconds, the specific action is delayed (deadline_timer is restarted).

I adapted the timer class from boost deadline_timer issue and for simplicity the events are generated synchronously. Running the code, I’m expecting something like:

1 second(s)
2 second(s)
3 second(s)
4 second(s)
5 second(s)
action       <--- it should appear after 10 seconds after the last event

but I get

1 second(s)
2 second(s)
action
3 second(s)
action
4 second(s)
action
5 second(s)
action
action

Why does this happen? How to solve this?

#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <iostream>

class DelayedAction
{
public:
    DelayedAction():        
        work( service),
        thread( boost::bind( &DelayedAction::run, this)),
        timer( service)
    {}

    ~DelayedAction()
    {
        thread.join();
    }

    void startAfter( const size_t delay)
    {
        timer.cancel();
        timer.expires_from_now( boost::posix_time::seconds( delay));
        timer.async_wait( boost::bind( &DelayedAction::action, this));
    }

private:
    void run()
    {
        service.run();
    }

    void action() 
    {
        std::cout << "action" << std::endl;
    }

    boost::asio::io_service         service;
    boost::asio::io_service::work   work;
    boost::thread                   thread;
    boost::asio::deadline_timer     timer;
};

int main()
{
    DelayedAction d;
    for( int i = 1; i < 6; ++i)
    {
        Sleep( 1000);
        std::cout << i << " second(s)\n";
        d.startAfter( 10);
    }
}

PS Writing this, I’m thinking the true issue is how boost::deadline_timer can be restarted once it was started.

Answer 1

When you call expires_from_now() it resets the timer, and the handler is immediately called with the error code boost::asio::error::operation_aborted.

If you handle the error code case in your handler, then you can make this work as expected.

void startAfter( const size_t delay)
{
    // no need to explicitly cancel
    // timer.cancel();
    timer.expires_from_now( boost::posix_time::seconds( delay));
    timer.async_wait( boost::bind( &DelayedAction::action, this, boost::asio::placeholders::error));
}

// ...

void action(const boost::system::error_code& e) 
{
    if(e != boost::asio::error::operation_aborted)
        std::cout << "action" << std::endl;
}

This is discussed in the documentation:
http://www.boost.org/doc/libs/1_47_0/doc/html/boost_asio/reference/deadline_timer.html

See specifically the section titled: Changing an active deadline_timer’s expiry time