Issue calling something on the UI Thread from inside a callback in c++/cx

Question

This is my Callback function:

void VideoCapturerSampleCallback(struct LmiVideoCapturer_* capturer, const LmiVideoFrame* videoFrame, LmiVoidPtr userData)
{
  //TODO will have to be on ui thread;
  Windows::ApplicationModel::Core::CoreApplication::MainView->CoreWindow->Dispatcher->RunAsync(0, ref new Windows::UI::Core::DispatchedHandler([this]()
{
    Windows::UI::Xaml::Media::Imaging::BitmapImage^ bitmapImage =
        ref new Windows::UI::Xaml::Media::Imaging::BitmapImage();
    auto uri = ref new Windows::Foundation::Uri("ms-appx:///Assets/1.jpg");
    if (first){
        first = false;
    }
    else{
        uri = ref new Windows::Foundation::Uri("ms-appx:///Assets/1.jpg");
        first = true;
    }
    bitmapImage->UriSource = uri;
    media->Source = bitmapImage;
}));
}

Which will basically change the picture every time the callback will be called, for now, I just change 2 pictures from Assets, in order to see if it works. This is the error I get in the logs:

Error   17  error C1903: unable to recover from previous error(s); stopping compilation C:\Users\Alin Rosu\Downloads\App2\App2\MainPage.xaml.cpp    109 1   App2
Error   16  error C3482: 'this' can only be used as a lambda capture within a non-static member function    C:\Users\Alin Rosu\Downloads\App2\App2\MainPage.xaml.cpp    93  1   App2
19  IntelliSense: 'this' may only be used inside a nonstatic member function    c:\Users\Alin Rosu\Downloads\App2\App2\MainPage.xaml.cpp    93  144 App2
18  IntelliSense: function "Windows::UI::Core::CoreDispatcher::RunAsync" cannot be called with the given argument list
        argument types are: (int, Windows::UI::Core::DispatchedHandler ^)
        object type is: Windows::UI::Core::CoreDispatcher ^ c:\Users\Alin Rosu\Downloads\App2\App2\MainPage.xaml.cpp    93  84  App2

Now is it possible to use that [this] inside the callback, without getting an error? I am a Android developer, so for me the logic response to this, was creating a global variable App2::MainPage, and then in the constructor, save "this" in it, so that I would have the context over there, and try to call RunAsync with that, but it does not work. Is it possible to instantiate the DispatchedHandler another way?

score 0 · Accepted Answer · answered Jan 09 '15 at 13:24

Declared the context like this:

App2::MainPage^ context;

Then set it in the MainPage function:

MainPage::MainPage()
{
InitializeComponent();
media = this->player;
context = this;
}

And then I did it this way inside the VideoCapturerSampleCallback:

void VideoCapturerSampleCallback(struct LmiVideoCapturer_* capturer, const LmiVideoFrame* videoFrame, LmiVoidPtr userData)
{
App2::MainPage^ ctx = context;
Windows::ApplicationModel::Core::CoreApplication::MainView->CoreWindow->Dispatcher->RunAsync(Windows::UI::Core::CoreDispatcherPriority::Normal, ref new Windows::UI::Core::DispatchedHandler([ctx]()
{
    //TODO will need to set in here
    Windows::UI::Xaml::Media::Imaging::BitmapImage^ bitmapImage =
        ref new Windows::UI::Xaml::Media::Imaging::BitmapImage();
    auto uri = ref new Windows::Foundation::Uri("ms-appx:///Assets/1.jpg");
    if (first){
        XTRACE(L"===================FIIIIIIIIIIIIRST =======================\n");
        first = false;
    }
    else{
        XTRACE(L"===================FIIIIIIIIIIIIRST FAAAALSEEEEE=======================\n");
        uri = ref new Windows::Foundation::Uri("ms-appx:///Assets/2.jpg");
        first = true;
    }
    bitmapImage->UriSource = uri;
    media->Source = bitmapImage;
}));
}

This resolved my issues and I am able to modify the UI, being on the UI Thread.

Issue calling something on the UI Thread from inside a callback in c++/cx

1 Answers1