wasm: add local file access private API

Access to the local file system is restricted by the
Web sandbox, and a separate API an implementation is
needed to facilitate this for Qt applications.

This adds a private asynchronous callback-based C++
API for opening a file dialog and reading file content.

The implementation uses a file input html element to
show a file dialog, and then the uses the native File
and FileReader APIs to read the selected file(s).

Change-Id: I4e28baa032d7c3cd63241465f0ae55efd219a05b
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/gui/gui.pro

@@ -48,6 +48,7 @@ include(opengl/opengl.pri)
 qtConfig(animation): include(animation/animation.pri)
 include(itemmodels/itemmodels.pri)
 include(vulkan/vulkan.pri)
+include(platform/platform.pri)
 
 QMAKE_LIBS += $$QMAKE_LIBS_GUI
 

src/gui/platform/platform.pri

@@ -0,0 +1 @@
+wasm:include(wasm/wasm.pri)

src/gui/platform/wasm/qwasmlocalfileaccess.cpp

@@ -0,0 +1,169 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtGui module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qwasmlocalfileaccess_p.h"
+#include <private/qstdweb_p.h>
+#include <emscripten.h>
+#include <emscripten/bind.h>
+#include <emscripten/html5.h>
+#include <emscripten/val.h>
+
+QT_BEGIN_NAMESPACE
+
+namespace QWasmLocalFileAccess {
+
+void streamFile(const qstdweb::File &file, uint32_t offset, uint32_t length, char *buffer, const std::function<void ()> &completed)
+{
+    // Read file in chunks in order to avoid holding two copies in memory at the same time
+    const uint32_t chunkSize = 256 * 1024;
+    const uint32_t end = offset + length;
+    // assert end < file.size
+    auto fileReader = std::make_shared<qstdweb::FileReader>();
+
+    auto chunkCompleted = std::make_shared<std::function<void (uint32_t, char *buffer)>>();
+    *chunkCompleted = [=](uint32_t chunkBegin, char *chunkBuffer) mutable {
+
+        // Copy current chunk from JS memory to Wasm memory
+        qstdweb::ArrayBuffer result = fileReader->result();
+        qstdweb::Uint8Array(result).copyTo(chunkBuffer);
+
+        // Read next chunk if not at buffer end
+        uint32_t nextChunkBegin = std::min(chunkBegin + result.byteLength(), end);
+        uint32_t nextChunkEnd = std::min(nextChunkBegin + chunkSize, end);
+        if (nextChunkBegin == end) {
+            completed();
+            chunkCompleted.reset();
+            return;
+        }
+        char *nextChunkBuffer = chunkBuffer + result.byteLength();
+        fileReader->onLoad([=]() { (*chunkCompleted)(nextChunkBegin, nextChunkBuffer); });
+        qstdweb::Blob blob = file.slice(nextChunkBegin, nextChunkEnd);
+        fileReader->readAsArrayBuffer(blob);
+    };
+
+    // Read first chunk. First iteration is a dummy iteration with no available data.
+    (*chunkCompleted)(offset, buffer);
+}
+
+void streamFile(const qstdweb::File &file, char *buffer, const std::function<void ()> &completed)
+{
+    streamFile(file, 0, file.size(), buffer, completed);
+}
+
+void readFiles(const qstdweb::FileList &fileList,
+    const std::function<char *(uint64_t size, const std::string name)> &acceptFile,
+    const std::function<void ()> &fileDataReady)
+{
+    auto readFile = std::make_shared<std::function<void(int)>>();
+
+    *readFile = [=](int fileIndex) mutable {
+        // Stop when all files have been processed
+        if (fileIndex >= fileList.length()) {
+            readFile.reset();
+            return;
+        }
+
+        const qstdweb::File file = fileList[fileIndex];
+
+        // Ask caller if the file should be accepted
+        char *buffer = acceptFile(file.size(), file.name());
+        if (buffer == nullptr) {
+            (*readFile)(fileIndex + 1);
+            return;
+        }
+
+        // Read file data into caller-provided buffer
+        streamFile(file, buffer, [=]() {
+            fileDataReady();
+            (*readFile)(fileIndex + 1);
+        });
+    };
+
+    (*readFile)(0);
+}
+
+typedef std::function<void (const qstdweb::FileList &fileList)> OpenFileDialogCallback;
+void openFileDialog(const std::string &accept, FileSelectMode fileSelectMode,
+                    const OpenFileDialogCallback &filesSelected)
+{
+    // Create file input html element which will display a native file dialog
+    // and call back to our onchange handler once the user has selected
+    // one or more files.
+    emscripten::val document = emscripten::val::global("document");
+    emscripten::val input = document.call<emscripten::val>("createElement", std::string("input"));
+    input.set("type", "file");
+    input.set("style", "display:none");
+    input.set("accept", emscripten::val(accept));
+    input.set("multiple", emscripten::val(fileSelectMode == MultipleFiles));
+
+    // Note: there is no event in case the user cancels the file dialog.
+    static std::unique_ptr<qstdweb::EventCallback> changeEvent;
+    auto callback = [=]() { filesSelected(qstdweb::FileList(input["files"])); };
+    changeEvent.reset(new qstdweb::EventCallback(input, "change", callback));
+
+    // Activate file input
+    emscripten::val body = document["body"];
+    body.call<void>("appendChild", input);
+    input.call<void>("click");
+    body.call<void>("removeChild", input);
+}
+
+void openFiles(const std::string &accept, FileSelectMode fileSelectMode,
+    const std::function<void (int fileCount)> &fileDialogClosed,
+    const std::function<char *(uint64_t size, const std::string name)> &acceptFile,
+    const std::function<void()> &fileDataReady)
+{
+    openFileDialog(accept, fileSelectMode, [=](const qstdweb::FileList &files) {
+        fileDialogClosed(files.length());
+        readFiles(files, acceptFile, fileDataReady);
+    });
+}
+
+void openFile(const std::string &accept,
+    const std::function<void (bool fileSelected)> &fileDialogClosed,
+    const std::function<char *(uint64_t size, const std::string name)> &acceptFile,
+    const std::function<void()> &fileDataReady)
+{
+    auto fileDialogClosedWithInt = [=](int fileCount) { fileDialogClosed(fileCount != 0); };
+    openFiles(accept, FileSelectMode::SingleFile, fileDialogClosedWithInt, acceptFile, fileDataReady);
+}
+
+} // namespace QWasmLocalFileAccess
+
+QT_END_NAMESPACE

src/gui/platform/wasm/qwasmlocalfileaccess_p.h

@@ -0,0 +1,78 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtGui module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QWASMLOCALFILEACCESS_P_H
+#define QWASMLOCALFILEACCESS_P_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include <qglobal.h>
+#include <cstdint>
+#include <functional>
+
+QT_BEGIN_NAMESPACE
+
+namespace QWasmLocalFileAccess {
+
+enum FileSelectMode { SingleFile, MultipleFiles };
+
+void openFiles(const std::string &accept, FileSelectMode fileSelectMode,
+    const std::function<void (int fileCount)> &fileDialogClosed,
+    const std::function<char *(uint64_t size, const std::string name)> &acceptFile,
+    const std::function<void()> &fileDataReady);
+
+void openFile(const std::string &accept,
+    const std::function<void (bool fileSelected)> &fileDialogClosed,
+    const std::function<char *(uint64_t size, const std::string name)> &acceptFile,
+    const std::function<void()> &fileDataReady);
+
+} // namespace QWasmLocalFileAccess
+
+QT_END_NAMESPACE
+
+#endif // QWASMLOCALFILEACCESS_P_H

src/gui/platform/wasm/wasm.pri

@@ -0,0 +1,3 @@
+INCLUDEDIR += $$PWD
+HEADERS += $$PWD/qwasmlocalfileaccess_p.h
+SOURCES += $$PWD/qwasmlocalfileaccess.cpp