Wt

This is an excellent question, and I'm sorry I can't properly answer it. The feature you're looking for is special in web technologies and called "Server Push". For your edification:

https://en.wikipedia.org/wiki/Push_technology#HTTP_server_push

I found this project by Roel that claims to demonstrate Wt's implementation of server push:

https://github.com/emweb/wt/tree/master/examples/feature/serverpush

It look like you'll be looking a Wt::WApplication::enableUpdates and Wt::WApplication::triggerUpdate. Good luck!

Please also note that you can't just touch the widget tree from a thread without locking the widget tree. Wt does this locking automatically for browser-generated events, but for your own events you'll have to grab the WApplication::UpdateLock before touching the widget tree. An alternative is to post an event to your session through WServer::post().

Best regards,

Wim.

I have a similar case: a map that needs to be updated say, every 1 second.

I tried to modify the serverpush example to not use a separate thread , but what happens is that the interface is only updated at the end (at the end of the cycle on the example; in my example I just add a new text widget every 1 second 3 times)

separate question: how to trigger the event automatically at application start ? (without the need for the push button event)

here's the code

class BigWorkWidget : public WContainerWidget
{
public:
  BigWorkWidget()
    : WContainerWidget()
  {
    Wt::WApplication *app = Wt::WApplication::instance();
    startButton_ = this->addWidget(cpp14::make_unique<WPushButton>("Start"));
    startButton_->clicked().connect(this, &BigWorkWidget::startBigWork);
  }

private:
  WPushButton *startButton_;

  void startBigWork() 
  {
    WApplication *app = WApplication::instance();
    app->enableUpdates(true);
    doBigWork(app);
  }

  void doBigWork(WApplication *app)
  {
    for (unsigned i = 0; i < 3; ++i) 
    {
      std::this_thread::sleep_for(std::chrono::milliseconds(1000));

      WApplication::UpdateLock uiLock(app);
      if (uiLock) 
      {
        addWidget(Wt::cpp14::make_unique<Wt::WText>("Plot  "));
        app->triggerUpdate();
      }
      else
        return;
    }
  }
};

//////////////////////////////////////////////////////////////////////////////////////
//Application_update
///////////////////////////////////////////////////////////////////////////////////////

class Application_update : public WApplication
{
public:
  Application_update(const WEnvironment& env) : WApplication(env)
  {
    std::unique_ptr<BigWorkWidget> update = cpp14::make_unique<BigWorkWidget>();
    root()->addWidget(std::move(update));
  }
};

I have yet to do anything which requires server push, but I would expect that you need to return from your big work function instead of looping there. If you want to add a widget every second (or do anything regularly like that), use Wt::WTimer: https://www.webtoolkit.eu/wt/doc/reference/html/classWt_1_1WTimer.html

If I understand correctly, this will not require server push since the client can poll the server at set intervals. Also, it will start whenever you start it (at application load time if you like) rather than requiring a button event initiated by the client.

Hey,

While doing time consuming work in the UI thread, you can render changes that happened until now (e.g. progress bar or intermediate updates) and react on pending user events (like the click on a cancel button that you may show during the long operation) by invoking WApplication::processEvents().

I'd recommend to use a server push mechanism rather than a WTimer for implementing server push.

Wim.

Thanks, Wim

that worked, by adding that call in the above code

if (uiLock)
{
  addWidget(Wt::cpp14::make_unique<Wt::WText>("Plot  "));
  app->triggerUpdate();
  app->processEvents();
}

I solved the problem for the push to start at application start by running a timer 1 time and make the push start there, then stop the timer.

I searched the examples for a case of a signal/slot that would allow to do this (start the push without a user event, like the start button)

but could not find any... if this is possible please let me know

///////////////////////////////////////////////////////////////////////////////////////
//PushWidget
///////////////////////////////////////////////////////////////////////////////////////

class PushWidget : public WContainerWidget
{
public:
  PushWidget()
    : WContainerWidget()
  {
    Wt::WApplication *app = Wt::WApplication::instance();
    startButton_ = this->addWidget(cpp14::make_unique<WPushButton>("Start"));
    startButton_->clicked().connect(this, &PushWidget::startBigWork);

    timer_ = cpp14::make_unique<WTimer>();
    timer_->setInterval(std::chrono::milliseconds{ 1000 });
    timer_->timeout().connect(this, &PushWidget::timerTick);
    timer_->start();
  }

private:
  WPushButton *startButton_;
  std::unique_ptr<WTimer> timer_;
  void timerTick()
  {
    timer_->stop();
    startBigWork();
  }

  void startBigWork()
  {
    WApplication *app = WApplication::instance();
    app->enableUpdates(true);
    doBigWork(app);
  }

  void doBigWork(WApplication *app)
  {
    for (unsigned i = 0; i < 5; ++i)
    {
      std::this_thread::sleep_for(std::chrono::milliseconds(2000));

      WApplication::UpdateLock uiLock(app);
      if (uiLock)
      {
        addWidget(Wt::cpp14::make_unique<Wt::WText>("Plot  "));
        app->triggerUpdate();
        app->processEvents();
      }
      else
        return;
    }
  }
};