Question

我试图在create_task的实现之外定义Uri ^。在java中，如果你有一个异步任务，添加final修饰符将允许你在异步代码中使用该变量（带有final修饰符）。

如何在异步代码中使用以下代码中的Uri ^ source？

void addDownload(Uri^ source, StorageFolder^ destinationFolder, String^ fileName) {
    boolean requestUnconstrainedDownload = false;
    IAsyncOperation<StorageFile^>^ asyncOperationStorageFile = destinationFolder->CreateFileAsync(fileName, CreationCollisionOption::GenerateUniqueName);
    auto createStorageFileTask = create_task(asyncOperationStorageFile);
    createStorageFileTask.then([] (StorageFile^ destinationFile) {
        BackgroundDownloader^ downloader = ref new BackgroundDownloader();
        DownloadOperation^ downloadOperation = downloader->CreateDownload(source, destinationFile);
        downloadOperation->Priority = BackgroundTransferPriority::Default;
        HandleDownloadAsync(downloadOperation, true);
    });
}

Answer 1

只需捕获lambda中的变量 source ，就可以在任务的lambda主体中访问它：

void addDownload(Uri^ source, StorageFolder^ destinationFolder, String^ fileName) {

boolean requestUnconstrainedDownload = false;
IAsyncOperation<StorageFile^>^ asyncOperationStorageFile = destinationFolder->CreateFileAsync(fileName, CreationCollisionOption::GenerateUniqueName);
auto createStorageFileTask = create_task(asyncOperationStorageFile);
createStorageFileTask.then([source] (StorageFile^ destinationFile) {
    BackgroundDownloader^ downloader = ref new BackgroundDownloader();
    DownloadOperation^ downloadOperation = downloader->CreateDownload(source, destinationFile);
    downloadOperation->Priority = BackgroundTransferPriority::Default;
    HandleDownloadAsync(downloadOperation, true);
});

}

C ++ / CX Metro Windows - create_task实现可防止使用未在该范围内定义的变量

1 个答案: