winrt c++/cx 并发访问冲突异常

Question

我要做的是检查本地文件夹中是否存在文件，如果找不到，则将其复制到那里（该文件以前作为资产添加到项目中）。

代码如下：

Windows::Storage::StorageFile^ MainPage::GetCustomFileAsync(Platform::String^ fileName)
{
using Windows::Storage::StorageFile;
using Windows::Storage::StorageFolder;

auto localFolder = Windows::Storage::ApplicationData::Current->LocalFolder;
auto localTask = concurrency::create_task(localFolder->GetFileAsync(fileName));
StorageFile^ retVal = nullptr;
localTask.then([&](StorageFile^ t){
    retVal = t;
}).then([](concurrency::task<void> t)
{
    try
    {
        t.get();
        OutputDebugString(L"Found\n");
    }
    catch (Platform::COMException^ e)
    {
        OutputDebugString(e->Message->Data());
    }
}).wait();
return retVal;
}


StorageFile^ fileVar;
if ((fileVar = this->GetCustomFileAsync("somefile.txt")) == nullptr)
{
    String^ path = Windows::ApplicationModel::Package::Current->InstalledLocation->Path + "\\Assets";
    concurrency::create_task(Windows::Storage::StorageFolder::GetFolderFromPathAsync(path)).then([](StorageFolder^ folder){
        return (folder->GetFileAsync("somefile.txt"));
    }).then([](StorageFile^ file){
        return (file->CopyAsync(Windows::Storage::ApplicationData::Current->LocalFolder));
    }).then([&](StorageFile^ file){
        fileVar = file;
        OutputDebugString(file->DisplayName->Data());
    });

}

发生的情况是，在将“file”分配给“fileVar”时出现访问冲突异常（可能是因为跨线程访问？）。如何解决这个问题？

编辑：我无法在那里进行所有处理，因为该文件将被多次访问。简而言之，我需要知道它何时被成功复制并处理它。这是有效的代码

Windows::Storage::StorageFile^ GetFile(Platform::String^ fileName)
{
using Windows::Storage::StorageFile;
using Windows::Storage::StorageFolder;
using Windows::Foundation::AsyncOperationCompletedHandler;
using Windows::Foundation::AsyncStatus;
using Windows::Foundation::IAsyncOperation;
using Platform::String;

auto localFolder = Windows::Storage::ApplicationData::Current->LocalFolder;
bool completed = false;
StorageFile^ retVal = nullptr;
localFolder->GetFileAsync(fileName)->Completed = ref new AsyncOperationCompletedHandler<StorageFile^>([&completed, &retVal, &fileName](IAsyncOperation<StorageFile^>^ fileOperation, AsyncStatus status)
{
    if (status == AsyncStatus::Error)
    {
        String^ path = Windows::ApplicationModel::Package::Current->InstalledLocation->Path + "\\Assets";
        Windows::Storage::StorageFolder::GetFolderFromPathAsync(path)->Completed = ref new AsyncOperationCompletedHandler<Windows::Storage::StorageFolder^>(
            [&completed, &retVal, &fileName](IAsyncOperation<Windows::Storage::StorageFolder^>^ folderOperation, AsyncStatus status)->void{
            auto assetFolder = folderOperation->GetResults();
            assetFolder->GetFileAsync(fileName)->Completed = ref new AsyncOperationCompletedHandler<Windows::Storage::StorageFile^>([&completed, &retVal, &fileName](IAsyncOperation<Windows::Storage::StorageFile^>^ fileOperation, AsyncStatus status)->void{
                auto file = fileOperation->GetResults();
                file->CopyAsync(Windows::Storage::ApplicationData::Current->LocalFolder)->Completed = ref new AsyncOperationCompletedHandler<Windows::Storage::StorageFile^>
                    ([&completed, &retVal, &fileName](IAsyncOperation<Windows::Storage::StorageFile^>^ fileOperation, AsyncStatus status)->void {
                    retVal = fileOperation->GetResults();
                    completed = true;
                });
            });
        });
    }
    else
    {
        retVal = fileOperation->GetResults();
        completed = true;
    }
});
while (completed == false);
return retVal;
}

Answer 1

与其将委托作为参数传递并返回void，不如让您的方法返回task<StorageFile^>，然后调用者可以执行.then() 以在操作成功后继续工作。

或者，如果它作为公共 WinRT 方法（不是内部/私有 C++ 方法）公开，则使用 IAsyncOperation<StorageFile^>^ 作为返回类型，并将整个内容包装在 create_async() 中：

IAsyncOperation<StorageFile^>^ DoStuff(params)
{
  return concurrency::create_async([params]
  { 
    // function body goes here
  });
}

Answer 2

这是我整理的一个解决方案。有两件事需要了解：

1. 当使用 concurrency::create_task 执行异步操作时，当父函数返回时，异步操作仍然可以执行。因此，捕获的变量必须在父函数的上下文中存活。如果它们通过引用传递，这显然不会发生。花了一段时间才意识到这一点。

2. WinRT 对并发运行时施加了某些限制。调用 concurrency::task::get() 或 concurrency::task::wait() 将在 STA 线程中抛出异常，除非调用是在任务继续中。

这篇文章的更多信息： http://social.msdn.microsoft.com/Forums/windowsapps/en-US/ae54980b-41ce-4337-a059-2213b549be4b/concurrencyinvalidoperation-when-calling-tasktget?forum=winappswithnativecode

在这种情况下，如何知道函数何时完成了它的工作？我选择传入回调（AKA 委托）。

delegate void FileOperation(Windows::Storage::StorageFile^ file);
void GetFileConcurrency(Platform::String^ fileName, FileOperation^ fileOp)
{
using Windows::Storage::StorageFile;
using Windows::Storage::StorageFolder;
using Platform::String;

auto localFolder = Windows::Storage::ApplicationData::Current->LocalFolder;
String^ assetFolderPath = Windows::ApplicationModel::Package::Current->InstalledLocation->Path + "\\Assets";


auto localFolderTask = concurrency::create_task(localFolder->GetFileAsync(fileName));
localFolderTask.then([localFolder, assetFolderPath, fileName, fileOp](concurrency::task<StorageFile^> theTask){
    try
    {
        StorageFile^ theFile = theTask.get();
        fileOp(theFile);
    }
    catch (Platform::Exception^ e)
    {
        OutputDebugString(e->Message->Data());
        auto assetFolderTask = concurrency::create_task(StorageFolder::GetFolderFromPathAsync(assetFolderPath));
        assetFolderTask.then([localFolder, assetFolderPath, fileName, fileOp](StorageFolder^ assetFolder){
            auto assetFileTask = concurrency::create_task(assetFolder->GetFileAsync(fileName));
            assetFileTask.then([localFolder, assetFolderPath, fileName, fileOp](StorageFile^ file){
                auto copyFileTask = concurrency::create_task(file->CopyAsync(localFolder));
                copyFileTask.then([localFolder, assetFolderPath, fileName, fileOp](StorageFile^ file){
                    OutputDebugString(file->Path->Data());
                    fileOp(file);
                });
            });
        });
    }
});
}