Wt::WTextArea data update in detached thread

Added by Stefano Fattore about 2 years ago

Hello, I'm new to the forum, I hope I'm not wrong in posting here.

I'm developing a web application in Wt 4.9.1, I've come across a situation where I have to perform an important task (such as executing a code external to the application) that takes quite a long time.

I thought about running this task in a separate thread and capturing its stdout to pipe it into a WTextAreas.
I tried via Wt::WServer::post(...) passing the sessionID and a code to execute like in this example:

`void Dispatcher::run_src()
{
...

Wt::WServer::instance()->post(
    WApplication::instance()->sessionId(),
    [=] {
         // C99 functions
         begin_emulator(CompAndRun.bin.toUTF8().c_str());
         while (emulate());
         end_emulator();
         // end C99 functions

         reg_rend_->update(machine_status()); /* aka register_render (object that contains the WTextAreas), an instance variable of Dispatcher */
     }
);

...

machine_status() returns a char pointer containing a Json, if I try to print its correct content but when invoking reg_rend_->update(machine_status()) which calls an update the content of multiple WTextAreas based on the parsing of the json I get:

[error] "Wt: Exception in WApplication::notify(): boost::bad_get: failed value get using boost::get"

I also tried not using the lambda function or using std::thread but to no avail.

I think I'm getting the concept of how things work here wrong, I'm going crazy to find the solution, how can I do?

Finally, I apologize if my English is bad. Thank you.

Replies (2)

RE: Wt::WTextArea data update in detached thread - Added by Roel Standaert about 2 years ago

When you use WServer::post(), the lambda will be executed while the WApplication's UpdateLock is held. You don't want to hold the UpdateLock while you're doing your long-running task.

One approach, detailed in the serverpush example, is to run your work in a separate thread, and directly grab the UpdateLock whenever you want to update the UI.

Similarly, you can also use post(...) whenever you want to update the UI.

P.S.: I see you accidentally posted this twice, probably mistakenly because Redmine was being slow or something. I removed the other one.

RE: Wt::WTextArea data update in detached thread - Added by Stefano Fattore about 2 years ago

Yes I mistakenly clicked the send button twice, tried to remove a copy of the post without success.

Thank you for your quick reply, I will look into the example you gave me.

Thanks again.

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