CODEWITHSUNDEEP
c++-winrt
user3743210
user3743210

Reputation: 211

How to read all bytes of a file in winrt with ReadBufferAsync?

I have an array of objects that each needs to load itself from binary file data. I create an array of these objects and then call an AsyncAction for each of them that starts it reading in its data. Trouble is, they are not loading entirely - they tend to get only part of the data from the files. How can I make sure that the whole thing is read? Here is an outline of the code: first I enumerate the folder contents to get a StorageFile for each file it contains. Then, in a for loop, each receiving object is created and passed the next StorageFile, and it creates its own Buffer and DataReader to handle the read. m_file_bytes is a std::vector.

m_buffer = co_await FileIO::ReadBufferAsync(nextFile);
  m_data_reader = winrt::Windows::Storage::Streams::DataReader::FromBuffer(m_buffer);
  m_file_bytes.resize(m_buffer.Length());
  m_data_reader.ReadBytes(m_file_bytes);

My thought was that since the buffer and reader are class members of the object they would not go out of scope and could finish their work uninterrupted as the next objects were asked to load themselves in separate AsyncActions. But the DataReader only gets maybe half of the file data or less. What can be done to make sure it completes? Thanks for any insights.

[Update] Perhaps what is going is that the file system can handle only one read task at a time, and by starting all these async reads each is interrupting the previous one -? But there must be a way to progressively read a folder full of files.

[Update] I think I have it working, by adopting the principle of concentric loops - the idea is not to proceed to the next load until the first one has completed. I think - someone can correct me if I'm wrong, that the file system cannot do simultaneous reads. If there is an accepted and secure example of how to do this I would still love to hear about it, so I'm not answering my own question.

Upvotes: 0

Views: 521

Answers (1)

Dylan-Huang
Dylan-Huang

Reputation: 48

    #include <wrl.h>
    #include <robuffer.h>   
    uint8_t* GetBufferData(winrt::Windows::Storage::Streams::IBuffer& buffer)
    {
        ::IUnknown* unknown = winrt::get_unknown(buffer);
        ::Microsoft::WRL::ComPtr<::Windows::Storage::Streams::IBufferByteAccess> bufferByteAccess;
        HRESULT hr = unknown->QueryInterface(_uuidof(::Windows::Storage::Streams::IBufferByteAccess), &bufferByteAccess);
        if (FAILED(hr))
            return nullptr;
        byte* bytes = nullptr;
        bufferByteAccess->Buffer(&bytes);
        return bytes;
   }

https://learn.microsoft.com/en-us/cpp/cppcx/obtaining-pointers-to-data-buffers-c-cx?view=vs-2017

https://learn.microsoft.com/en-us/windows/uwp/xbox-live/storage-platform/connected-storage/connected-storage-using-buffers

Upvotes: 1

Related Questions