wasm: Add saveFileContent()

Saves a file by file download, where the user can choose
the file name and location using a file dialog.

Change-Id: I4d2ecc76fc33bb65fdf3d7ca3fcd9566c62547dd
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/platform/wasm/qwasmlocalfileaccess.cpp

@@ -164,6 +164,43 @@ void openFile(const std::string &accept,
     openFiles(accept, FileSelectMode::SingleFile, fileDialogClosedWithInt, acceptFile, fileDataReady);
 }
 
+void saveFile(const char *content, size_t size, const std::string &fileNameHint)
+{
+    // Save a file by creating programatically clicking a download
+    // link to an object url to a Blob containing the file content.
+    // File content is copied once, so that the passed in content
+    // buffer can be released as soon as this function returns - we
+    // don't know for how long the browser will retain the TypedArray
+    // view used to create the Blob.
+
+    emscripten::val document = emscripten::val::global("document");
+    emscripten::val window = emscripten::val::global("window");
+
+    emscripten::val fileContentView = emscripten::val(emscripten::typed_memory_view(size, content));
+    emscripten::val fileContentCopy = emscripten::val::global("ArrayBuffer").new_(size);
+    emscripten::val fileContentCopyView = emscripten::val::global("Uint8Array").new_(fileContentCopy);
+    fileContentCopyView.call<void>("set", fileContentView);
+
+    emscripten::val contentArray = emscripten::val::array();
+    contentArray.call<void>("push", fileContentCopyView);
+    emscripten::val type = emscripten::val::object();
+    type.set("type","application/octet-stream");
+    emscripten::val contentBlob = emscripten::val::global("Blob").new_(contentArray, type);
+
+    emscripten::val contentUrl = window["URL"].call<emscripten::val>("createObjectURL", contentBlob);
+    emscripten::val contentLink = document.call<emscripten::val>("createElement", std::string("a"));
+    contentLink.set("href", contentUrl);
+    contentLink.set("download", fileNameHint);
+    contentLink.set("style", "display:none");
+
+    emscripten::val body = document["body"];
+    body.call<void>("appendChild", contentLink);
+    contentLink.call<void>("click");
+    body.call<void>("removeChild", contentLink);
+
+    window["URL"].call<emscripten::val>("revokeObjectURL", contentUrl);
+}
+
 } // namespace QWasmLocalFileAccess
 
 QT_END_NAMESPACE

src/gui/platform/wasm/qwasmlocalfileaccess_p.h

@@ -71,6 +71,8 @@ void openFile(const std::string &accept,
     const std::function<char *(uint64_t size, const std::string name)> &acceptFile,
     const std::function<void()> &fileDataReady);
 
+void saveFile(const char *content, size_t size, const std::string &fileNameHint);
+
 } // namespace QWasmLocalFileAccess
 
 QT_END_NAMESPACE

src/widgets/dialogs/qfiledialog.cpp

@@ -2447,6 +2447,51 @@ void QFileDialog::getOpenFileContent(const QString &nameFilter, const std::funct
 #endif
 }
 
+/*!
+    This is a convenience static function that saves \a fileContent to a file, using
+    a file name and location chosen by the user. \a fileNameHint can be provided to
+    suggest a file name to the user.
+
+    This function is used to save files to the local file system on Qt for WebAssembly, where
+    the web sandbox places restrictions on how such access may happen. Its implementation will
+    make the browser display a native file dialog, where the user makes the file selection.
+
+    It can also be used on other platforms, where it will fall back to using QFileDialog.
+
+    The function is asynchronous and returns immediately.
+
+    \snippet code/src_gui_dialogs_qfiledialog.cpp 16
+    \since 5.14
+*/
+void QFileDialog::saveFileContent(const QByteArray &fileContent, const QString &fileNameHint)
+{
+#ifdef Q_OS_WASM
+    QWasmLocalFileAccess::saveFile(fileContent.constData(), fileContent.size(), fileNameHint.toStdString());
+#else
+    QFileDialog *dialog = new QFileDialog();
+    dialog->setAcceptMode(QFileDialog::AcceptSave);
+    dialog->setFileMode(QFileDialog::AnyFile);
+    dialog->selectFile(fileNameHint);
+
+    auto fileSelected = [=](const QString &fileName) {
+        if (!fileName.isNull()) {
+            QFile selectedFile(fileName);
+            if (selectedFile.open(QIODevice::WriteOnly))
+                selectedFile.write(fileContent);
+        }
+    };
+
+    auto dialogClosed = [=](int code) {
+        Q_UNUSED(code);
+        delete dialog;
+    };
+
+    connect(dialog, &QFileDialog::fileSelected, fileSelected);
+    connect(dialog, &QFileDialog::finished, dialogClosed);
+    dialog->show();
+#endif
+}
+
 /*!
     This is a convenience static function that will return a file name selected
     by the user. The file does not have to exist.

src/widgets/dialogs/qfiledialog.h

@@ -284,6 +284,7 @@ public:
 
     static void getOpenFileContent(const QString &nameFilter,
                                    const std::function<void(const QString &, const QByteArray &)> &fileContentsReady);
+    static void saveFileContent(const QByteArray &fileContent, const QString &fileNameHint = QString());
 
 protected:
     QFileDialog(const QFileDialogArgs &args);

src/widgets/doc/snippets/code/src_gui_dialogs_qfiledialog.cpp

@@ -155,3 +155,8 @@ auto fileOpenCompleted = [](const QString &fileName, const QByteArray &fileConte
 }
 QFileDialog::getOpenFileContent("Images (*.png *.xpm *.jpg)",  fileContentReady);
 //! [15]
+
+//! [16]
+QByteArray imageData; // obtained from e.g. QImage::save()
+QFileDialog::saveFile("myimage.png", imageData);
+//! [16]