wasm: Local file access using standard QFileDialog API

Intro

The expected way to access local files in Qt Code is to use the QFileDialog API (widgets or QML) to show a file dialog to the user, and then access the file using the returned file path or url.

QString filePath = QFileDialog::getOpenFileName("file.txt");

...

QFile file(filePath); // open and read file

However, this is not supported on by Qt WebAssembly, to access local files on the host file system, for two main reasons:

• The Web file dialog API returns a File object instead of a path. The File object is the only way to access file content; opening a file from a path is not supported.
• The Web File API is asynchronous when reading or writing to a file.

Background

The Web platform provides access to local files through the File System API. (https://developer.mozilla.org/en-US/docs/Web/API/File_System_API) This API provides functionality for opening a native file dialog, where the user can give the web application access to files.

File access is provided through the Web File API (https://developer.mozilla.org/en-US/docs/Web/API/File). This is a non-POSIX file API, which does not map directly to the current QFile abstraction:

• Web File is capability based: opening a file via a path is not possible; the File object obtained from the file dialog must be used
• Web File is async: read/write calls complete via JS Promise

Current Support for QFileDialog on WebAssembly

QFileDialog provides two functions for reading and writing file content: These are supported on all platforms with a special implementation for WebAssembly and a generic fallback for other platforms.{}

QByteArray QFileDialog::getOpenFileContent();
QFileDialog::saveFileContent(QByteArray);

Using this API makes local file access possible today, but has a couple of limitations:

• Can be difficult to use from existing code bases which works with QFile and file paths, especially from QML.
• Has no support for partial file reads or writes (useful when working with large files)

Related Emscripten / Web Platform work

• WasmFS (https://github.com/orgs/emscripten-core/projects/1), a new C++ based file system implementation for Emscripten.
• The Origin private file system (OPFS)(https://developer.mozilla.org/en-US/docs/Web/API/File_System_API/Origin_private_file_system): provides synchronous access to a sandboxed file system

Qt Development Tasks

• Add a native backend for QFileDialog
  • Support the async QFileDialog API (the fileSelected signal)
  • Support QML usage (also async)
  • Support the synchronous QFileDialog API (the static functions)
• Find a way to handle File objects
  • TODO