JavaFX拖拽至外部程序：拖放完成后解密再执行系统操作的实现

Great question—this is a common pain point with drag-and-drop for processed/large files, and there are two solid approaches to solve this without blocking users or wasting resources.

Approach 1: Defer Decryption Until Drop Completion (Preferred) #

Most UI frameworks (WinForms/WPF, Qt, Electron, etc.) support deferred data provision for drag-and-drop, which is exactly what you need. Here's how it works:

Instead of decrypting files in onDragDetected, you only pass metadata (like the encrypted file's ID or path) during the initial drag start. The actual decryption only kicks in when the user completes the drop (i.e., releases the mouse over the target), or when the target application requests the file data.

Step-by-Step Implementation #

1. In onDragDetected:
   Don’t decrypt anything. Instead, create a drag data object that uses a callback to provide the file content only when requested. For example, in WPF:

   private void OnDragDetected(object sender, DragEventArgs e)
   {
       var dataObject = new DataObject();
       // Register a deferred callback for file data
       dataObject.SetData(DataFormats.FileDrop, false, (data, format) => {
           // This code runs ONLY when the target (e.g., File Explorer) requests the file
           string tempFilePath = DecryptToTempFile(selectedEncryptedFile);
           // Pass the decrypted temp file path to the target
           data.SetData(DataFormats.FileDrop, new string[] { tempFilePath });
           // Clean up the temp file after the operation finishes
           RegisterTempFileCleanup(tempFilePath);
       });
       DragDrop.DoDragDrop((DependencyObject)sender, dataObject, DragDropEffects.Copy);
   }
   

2. Handle Cleanup:
   Use system-level temp file flags (like Windows FILE_FLAG_DELETE_ON_CLOSE) or listen for the drag operation’s completion event to delete the temp file once the copy/move is done.

Why This Works #

• If the user cancels the drag (e.g., presses Esc), the callback never runs—no wasted CPU/IO on decryption.
• Decryption happens asynchronously after the drop, so the user doesn’t wait during the drag initiation.
• It follows standard drag-and-drop protocols, so it works with all compliant target apps (File Explorer, Office, etc.).

Approach 2: Get Drop Target Folder and Manually Execute Move/Copy #

If your framework doesn’t support deferred data (unlikely, but possible), you can try to retrieve the target folder path and handle the file transfer manually. Note this is a fallback with limited compatibility:

How to Get the Target Path #

On Windows, you can use Win32 APIs to extract the target folder from the drop destination window:

• For File Explorer, use SHGetPathFromIDListEx to get the folder path associated with the target window handle.
• For the desktop, the target path is the user’s desktop directory (retrievable via SHGetFolderPath).

Example Workflow #

// Pseudocode for Windows Win32
HWND targetWindow = GetDropTargetWindowHandle();
TCHAR targetFolder[MAX_PATH];

if (SHGetPathFromIDListEx(GetExplorerFolderFromHWND(targetWindow), targetFolder, MAX_PATH, GPFIDL_DEFAULT))
{
    // Decrypt to temp file first
    string tempFilePath = DecryptFile(selectedEncryptedFile);
    // Build the final destination path
    string destPath = PathCombine(targetFolder, selectedEncryptedFile.GetOriginalFileName());
    // Execute copy/move manually
    if (CopyFile(tempFilePath.c_str(), destPath.c_str(), FALSE))
    {
        // If moving, delete the original encrypted file here
        DeleteFile(selectedEncryptedFile.GetPath());
    }
    // Clean up temp file
    DeleteFile(tempFilePath.c_str());
}

Caveats #

• Poor Compatibility: This only works with File Explorer and a handful of file managers. It won’t work with apps like Word, where the drop target isn’t a file system folder.
• Error Handling: You’ll need to handle edge cases like permission errors, target path not existing, or interrupted transfers manually.

Final Tips #

• Always decrypt large files asynchronously—never block the UI thread. Add a progress bar or status message so users know the operation is in progress.
• Prefer the deferred data approach first—it’s the standard, robust solution for this exact scenario.

内容的提问来源于stack exchange，提问作者Chaoz