WinUI3 : Can we get Main threads DispatcherQueue from worker thread

Question

WinUI3 : Can we get Main threads DispatcherQueue from worker thread

Harshithraj1871 1,706

Hi,

We were working on a multithreaded application where we wanted the UI Painting to be raised from different thread.

We came across DispatcherQueue, where we can get the Main threads dispatcher queue from Main thread, and then we can enqueue UI operations to main thread's DispatcherQueue object from different thread. For some reasons we don't want to pass on the main thread's DispatcherQueue object to worker/different threads.

Is there any way to access the main threads DispatcherQueue directly from worker thread, or post the UI operations to Main thread directly from the worker thread.

Thank you

Accepted answer

1 additional answer

Your answer

Answer 1

With this test, I update the UI from a worker thread :
(thread created with _beginthreadex, changing the Background color of myButton every 1/2 second)

At beginning

#include <winrt\Microsoft.UI.Dispatching.h>_  
#include <winrt\Windows.UI.Core.h>

Global variables

HANDLE g_hThread = NULL;  
HANDLE g_hEvent = NULL;  
BOOL g_bContinue = TRUE;  
int g_nCpt = 0;  
winrt::Microsoft::UI::Xaml::Window::Closed_revoker m_closedRevoker;

Test on myButton_Click

UINT idThread;  
g_hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);  
if (g_hEvent)  
{  
    g_bContinue = TRUE;  
    g_hThread = (HANDLE)_beginthreadex(NULL, 0, (unsigned int (WINAPI*)(void*))ChildThreadProc, (HANDLE)this, 0, &idThread);  
    SetEvent(g_hEvent);  
}

Thread (remove space at S leep (editor bug))

 static UINT WINAPI ChildThreadProc(LPDWORD lpdwData)  
    {  
        DWORD dwWait;  
        while (g_bContinue)  
        {  
            dwWait = WaitForSingleObject(g_hEvent, INFINITE);  
            if (dwWait == WAIT_OBJECT_0)  
            {  
                if (!g_bContinue)  
                    break;  
                auto pThis = (MainWindow*)(lpdwData);  
                bool isQueued = pThis->DispatcherQueue().TryEnqueue(  
                    Microsoft::UI::Dispatching::DispatcherQueuePriority::Normal,  
                    [pThis]()  
                    {  
                        Microsoft::UI::Xaml::Media::SolidColorBrush blueBrush{ Windows::UI::Colors::Blue() };  
                        Microsoft::UI::Xaml::Media::SolidColorBrush redBrush{ Windows::UI::Colors::Red() };  
                        if (g_nCpt % 2 == 0)  
                        {  
                            pThis->myButton().Background(blueBrush);  
                        }  
                        else  
                        {  
                            pThis->myButton().Background(redBrush);  
                        }  
                        g_nCpt++;  
                    });  
                    S leep(500);  
            }  
        }  
        CloseHandle(g_hEvent);  
        return 0;  
    }

In Constructor

   m_closedRevoker = this->Closed(winrt::auto_revoke, [&](auto&&, auto&&)  
   {     
   		g_bContinue = FALSE;  
    	SetEvent(g_hEvent);  
   });

Harshithraj1871 1,706 Reputation points

2022-12-12T07:39:01.19+00:00

Thank you, this was very helpful, here you have passed the window object to the child thread and extracted the dispatcher queue from that, is there any way we can do it without using any ref to the window/dispatcher queue? Is there any way where we can directly run a block of code in main thread with out having a reference to Window or its dispatcher queue.

I found a way, where we can get the MainView from core application

CoreApplication::MainView().CoreWindow().Dispatcher().RunAsync( CoreDispatcherPriority::Normal, [&]() { TextBox temp; });

But this is crashing, i was not able to figure it why, i have attached this as the comment in this answer.
Castorix31 90,686 Reputation points

2022-12-14T08:01:37.263+00:00

This code is for UWP : desktop-to-uwp-supported-api
or also : Change CoreDispatcher.RunAsync to DispatcherQueue.TryEnqueue
Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2022-12-15T01:45:43.703+00:00

As the document @Castorix31 attached said,

In your Windows App SDK app:

Instead of winrt::resume_foreground, you'll need to use wil::resume_foreground (from the Windows Implementation Libraries (WIL)).

And instead of CoreDispatcher, you'll need to use the Microsoft.UI.Dispatching.DispatcherQueue class, as described in Change Windows.UI.Core.CoreDispatcher to Microsoft.UI.Dispatching.DispatcherQueue.

And There is no reason to avoid passing information to other threads.
Harshithraj1871 1,706 Reputation points

2022-12-15T04:47:12.437+00:00

Thank you, this was helpful

Answer 2

Xiaopo Yang - MSFT 12,731 Microsoft External Staff

Hello,

Welcome to Microsoft Q&A!

You can spin off a worker thread carrying necessary information from the UI thread.
Advanced concurrency and asynchrony with C++/WinRT: use winrt::resume_foreground and winrt::resume_foreground.

Thank you.

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Castorix31 90,686 Reputation points

2022-12-09T07:02:59.293+00:00

You can use CoreDispatcher.RunAsync

In WinUI ?
Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2022-12-09T07:10:24.113+00:00

Should It be ok?
Harshithraj1871 1,706 Reputation points

2022-12-09T07:29:43.38+00:00

Thank you
Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2022-12-09T07:36:19.857+00:00

RunAsync': a function that returns 'auto' cannot be used before it is defined

CoreDispatcher.RunAsync which hasn't implemented yet cannot be used in WinUI3.
Harshithraj1871 1,706 Reputation points

2022-12-09T09:51:03.71+00:00

I tried using RunAsync from child thread like this

But it is giving debug error at line 24
call stack :
Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2022-12-14T02:55:06.413+00:00

@Harshithraj1871 I cannot use CoreApplication in Winui3 project. And as far as the code concerned, how do you update the main thread UI? a singleton?
Harshithraj1871 1,706 Reputation points

2022-12-15T04:49:13.927+00:00

Thank you, Issue is solved. I just searching for a way where we can paint on main thread with out using any parameter from the main thread/Window

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WinUI3 : Can we get Main threads DispatcherQueue from worker thread

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