Why hangs the app by the parallel loading of images?

Question

Hi all,

there are four sub-folders, each with 2 images. First, all images must be opened and loaded to a memory objects (BitmapImage class). Then the main-UI can be started, because they used these images in the binding. But this does not happen. The loading process hangs while waiting for the results from the BitmapImage::ImageOpened or BitmapImage::ImageFailed. Why?

Here is a code snippet:

void ImagingLoad::MainPage::Page_Loaded_1(Platform::Object^ sender, Windows::UI::Xaml::RoutedEventArgs^ e)
{
	task<void>([this]()
	{
		auto spRootSoundpool = KnownFolders::PicturesLibrary;
		String^ csFolderPool = L"Test"; // Root folder

		std::vector<String^> xSubFolders;
		xSubFolders.push_back(L"Pic1"); // Folder with 2 pictures
		xSubFolders.push_back(L"Pic2"); // Folder with 2 pictures
		xSubFolders.push_back(L"Pic3"); // Folder with 2 pictures
		xSubFolders.push_back(L"Pic4"); // Folder with 2 pictures

		task<StorageFolder^> xOpenPool(spRootSoundpool->GetFolderAsync(csFolderPool));
		auto spPoolFolder = xOpenPool.get();

		if(spPoolFolder != nullptr)
		{
			HANDLE hWait[2] = {::CreateEventExW(NULL, NULL, 0, EVENT_ALL_ACCESS), ::CreateEventExW(NULL, NULL, 0, EVENT_ALL_ACCESS)};
			HANDLE *pWait = hWait;

			for(std::vector<String^>::iterator it = xSubFolders.begin(); it != xSubFolders.end(); ++it)
			{
				auto csName = *it;
				Dispatcher->RunAsync(CoreDispatcherPriority::Normal, ref new DispatchedHandler([=]()
				{
					task<StorageFolder^>(spPoolFolder->GetFolderAsync(csName)).then([=](StorageFolder^ spFolder)
					{
						// Query only picture files.
						Platform::Collections::Vector<String^>^ spList = ref new Platform::Collections::Vector<String^>;
						spList->Append(L".jpg");
						spList->Append(L".png");
						auto spOptions = ref new QueryOptions(CommonFileQuery::DefaultQuery, spList);
						auto spResult = spFolder->CreateFileQueryWithOptions(spOptions);

						task<uint32>(spResult->GetItemCountAsync()).then([=](uint32 nCount)
						{
							if(nCount >= 2) // We must have at least 2 pictures.
							{
								task<IVectorView<StorageFile^>^>(spResult->GetFilesAsync()).then([=](IVectorView<StorageFile^>^ spListFiles)
								{
									auto spPic1 = spListFiles->GetAt(0);
									auto spPic2 = spListFiles->GetAt(1);

									// Open and load a first picture.
									task<Streams::IRandomAccessStream^>(spPic1->OpenAsync(FileAccessMode::Read)).then([=](Streams::IRandomAccessStream^ spStream)
									{
										auto spBmp = ref new Windows::UI::Xaml::Media::Imaging::BitmapImage;
										spBmp->ImageOpened += ref new RoutedEventHandler([pWait](Object^ sender, RoutedEventArgs^ e)
										{
											::SetEvent(pWait[0]);
										});
										spBmp->ImageFailed += ref new ExceptionRoutedEventHandler([pWait](Object^ sender, ExceptionRoutedEventArgs^ e)
										{
											::SetEvent(pWait[0]);
										});
										spBmp->SetSource(spStream);
										m_mapPictures1->Insert(csName, spBmp);
									});

									// Open and load a second picture.
									task<Streams::IRandomAccessStream^>(spPic2->OpenAsync(FileAccessMode::Read)).then([=](Streams::IRandomAccessStream^ spStream)
									{
										auto spBmp = ref new Windows::UI::Xaml::Media::Imaging::BitmapImage;
										spBmp->ImageOpened += ref new RoutedEventHandler([pWait](Object^ sender, RoutedEventArgs^ e)
										{
											::SetEvent(pWait[1]);
										});
										spBmp->ImageFailed += ref new ExceptionRoutedEventHandler([pWait](Object^ sender, ExceptionRoutedEventArgs^ e)
										{
											::SetEvent(pWait[1]);
										});
										spBmp->SetSource(spStream);
										m_mapPictures2->Insert(csName, spBmp);
									});
								});
							}
							else
							{
								::SetEvent(pWait[0]);
								::SetEvent(pWait[1]);
							}
						});
					});
				}));

				::WaitForMultipleObjectsEx(2, hWait, TRUE, INFINITE, FALSE);
			}

			::CloseHandle(hWait[0]);
			::CloseHandle(hWait[1]);
		}
	}).then([this]()
	{
		Dispatcher->RunAsync(CoreDispatcherPriority::Normal, ref new DispatchedHandler([=]()
		{
			m_ring->IsActive = false; // Stop the waiting.
		}));
	});
}

Best regards, Anry

Answer 1

 

Hello,

 

Did you do this in UI thread?

 

We suggest that we cannot wait any operator in UI thread, we should wait them in work thread.

Please follow this document

http://msdn.microsoft.com/en-us/library/hh750082(v=vs.110).aspx

 

Best regards,

Jesse

---

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

Answer 2

How are these defined?

m_mapPictures1

m_mapPictures2

---

David Lamb

Answer 3

Hello,

1. All this is executed in a separate thread (see in the code):

task<void>([this](){ <... THE ACTUAL WORK ...> });

2. m_mapPicture1 and m_mapPicture2 are defined as Platform::Collections::Map<String^, BitmapImage^>.

Best regards, Anry

Answer 4

My understanding is the BitmapImage will only read / decode the stream when added to the visual tree or ResourceDictionary. See if adding them to a

Windows::UI::Xaml::ResourceDictionary^ m_MyResources;

will trigger the ImageOpened event.

---

David Lamb

Answer 5

Hi David,

this is very strange, because your recommendation does not work sometimes, although this has improved :-)

My scenario:

1. Some XAML-elements show an image. Each of this images has a binding to the data context:

<GridView.ItemTemplate>
    <DataTemplate>
        <Grid Height="150" Width="310">
            <Grid.Background>
                <ImageBrush ImageSource="{Binding Icon}" Stretch="Uniform"/>
            </Grid.Background>
        </Grid>
    </DataTemplate>
</GridView.ItemTemplate>

2. The data context has a property of type Windows::UI::Xaml::Media::ImageSource^. The get() function provides an object of BitmapImage. The BitmapImage-instance is created only by first call of the get(). I want to be notified, if the ImageOpened/Failed event is triggered, to raise a PropertyChanged event for "Icon" and to force a show of this image again (in this case, the BitmapImage-instance is already there and ready to be showed).

Best regards, Anry

Answer 6

Don know if it help to you, but i'm loading images this way :

void TCover::OnLoadComplete(Platform::Object^ sender, Windows::UI::Xaml::RoutedEventArgs^ e)
{
  if( m_parent != nullptr)
  {
    m_parent->RaisePropertyChanged( "AlbumArt"); // this is calling notify
  }
}

void TCover::OnLoadFailed(Platform::Object^ sender, Windows::UI::Xaml::ExceptionRoutedEventArgs^ e)
{
  #ifdef _DEBUG
    OutputDebugString( ("Cannot open file - error: " + e->ErrorMessage)->Data());
  #endif
}

void TCover::loadCoverFromURI()
{
  String^ uri = m_name;
  m_image = ref new BitmapImage(ref new Uri( uri));
  m_image->ImageOpened += ref new Windows::UI::Xaml::RoutedEventHandler(this, &TCover::OnLoadComplete);
  m_image->ImageFailed += ref new Windows::UI::Xaml::ExceptionRoutedEventHandler(this, &TCover::OnLoadFailed);
}