How to update ui (XAML element) after ppl tasks completed

Question

Hi there, title sais all.

Below sample code generates "Unhandled exception at 0x60C09AD1 (msvcr110d.dll) in ******: An invalid parameter was passed to a function that considers invalid parameters fatal.", when reaches the Append. And in general what is the best practice to  update ui thread after ppl tasks got completed?

concurrent_vector<int> vec;

concurrent_vector<task<void>> tasks;

for (int i = 0; i<200; ++i){

tasks.push_back(

task<void>([&vec,i]{vec.push_back(i);}));

}

when_all(begin(tasks),end(tasks)).then([

this,&vec](){

this->MyListView->Items->Append(vec.size());

});

Answer 1

found the answer myself, using Chanel9 (video file: AsyncMadeSimplewithCPPL).

After last .then in order to schedule bk to main UI thread need to specify continuation context, so last line of the code above becomes:

},

task_continuation_context::use_current());

Hope that will help other nabs like me, messing around due to lack of ANY C++/CX book or online training out there.

Answer 2

Thanks for sharing the solution.

Best regards,
Jesse

---

Jesse Jiang [MSFT]
MSDN Community Support | Feedback to us

Answer 3

I think current() means current thread when the task is scheduled. It will be UI thread if code is called by user interaction. But may be other thread.

I didn't find a good way to explicit ask for UI thread. Maybe I can save the value of current() in UI thread and use it later? (I assume the value is just a number indicating the thread, and I also assume that number won't change).

Answer 4

I think current() means current thread when the task is scheduled. It will be UI thread if code is called by user interaction. But may be other thread.

I didn't find a good way to explicit ask for UI thread. Maybe I can save the value of current() in UI thread and use it later? (I assume the value is just a number indicating the thread, and I also assume that number won't change).

You are correct - ::use_current() might mean "run on current thread, when you schedule (or when started ?) task;". If someone knows how to arbitrary and to explicitly specify the UI thread (smtg like ,task_continuation_context::use_UI_thread();), or other way around, that would help tons.

Anyone advanced enough to help with this?

Answer 5

 If someone knows how to arbitrary and to explicitly specify the UI thread (smtg like ,task_continuation_context::use_UI_thread();), or other way around, that would help tons.

Anyone advanced enough to help with this?

It`s impossible, as there can be several UI threads (one per view), and you must specify which one you need.

auto ui_ctx = task_continuation_context::use_current() captures context of the thread where it is called, and you should pass this ui_ctx to be able to run continuation on this specific UI thread.