Add downloader example with QNetworkAccessManager

This widget allows to download a file and show a progress bar.
Unlike other example on the internet, bytes are read from the
readyRead() method and not from finished(). This makes possible to
download large file without consuming memory.

Task-number: PYSIDE-841
Change-Id: Ic314ef1fbc299be6c3636fcb502b3c532d713cfd
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
(cherry picked from commit 4769e8fd93b2b434c6b38d755d1090eca5b62893)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 218 insertions, 0 deletions

diff --git a/examples/network/downloader/downloader.py b/examples/network/downloader/downloader.py
new file mode 100644
index 000000000..538bb8866
--- /dev/null
+++ b/examples/network/downloader/downloader.py
@@ -0,0 +1,215 @@
+#############################################################################
+##
+## Copyright (C) 2021 The Qt Company Ltd.
+## Contact: https://www.qt.io/licensing/
+##
+## This file is part of the Qt for Python examples of the Qt Toolkit.
+##
+## $QT_BEGIN_LICENSE:BSD$
+## You may use this file under the terms of the BSD license as follows:
+##
+## "Redistribution and use in source and binary forms, with or without
+## modification, are permitted provided that the following conditions are
+## met:
+##   * Redistributions of source code must retain the above copyright
+##     notice, this list of conditions and the following disclaimer.
+##   * Redistributions in binary form must reproduce the above copyright
+##     notice, this list of conditions and the following disclaimer in
+##     the documentation and/or other materials provided with the
+##     distribution.
+##   * Neither the name of The Qt Company Ltd nor the names of its
+##     contributors may be used to endorse or promote products derived
+##     from this software without specific prior written permission.
+##
+##
+## THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+## "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+## LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+## A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+## OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+## SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+## LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+## DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+## THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+## (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+## OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+##
+## $QT_END_LICENSE$
+##
+#############################################################################
+
+from PySide6.QtWidgets import (
+    QWidget,
+    QApplication,
+    QMessageBox,
+    QLineEdit,
+    QProgressBar,
+    QPushButton,
+    QHBoxLayout,
+    QVBoxLayout,
+    QStyle,
+    QFileDialog,
+)
+from PySide6.QtCore import QStandardPaths, QUrl, QFile, QSaveFile, QDir, QIODevice, Slot
+from PySide6.QtNetwork import QNetworkReply, QNetworkRequest, QNetworkAccessManager
+import sys
+
+
+class DownloaderWidget(QWidget):
+    """A widget to download a http file to a destination file"""
+
+    def __init__(self, parent=None):
+        super().__init__(parent)
+
+        self.manager = QNetworkAccessManager(self)
+        self.link_box = QLineEdit()
+        self.dest_box = QLineEdit()
+        self.progress_bar = QProgressBar()
+
+        self.start_button = QPushButton("Start")
+        self.abort_button = QPushButton("Abort")
+
+        self.link_box.setPlaceholderText("Download Link ...")
+
+        self._open_folder_action = self.dest_box.addAction(
+            qApp.style().standardIcon(QStyle.SP_DirOpenIcon), QLineEdit.TrailingPosition
+        )
+        self._open_folder_action.triggered.connect(self.on_open_folder)
+
+        #  Current QFile
+        self.file = None
+        # Current QNetworkReply
+        self.reply = None
+
+        #  Default http url
+        self.link_box.setText(
+            "http://master.qt.io/archive/qt/6.0/6.0.1/single/qt-everywhere-src-6.0.1.zip"
+        )
+
+        #  Default destination dir
+        self.dest_box.setText(
+            QDir.fromNativeSeparators(
+                QStandardPaths.writableLocation(QStandardPaths.DownloadLocation)
+            )
+        )
+
+        # buttons bar layout
+        hlayout = QHBoxLayout()
+        hlayout.addStretch()
+        hlayout.addWidget(self.start_button)
+        hlayout.addWidget(self.abort_button)
+
+        # main layout
+        vlayout = QVBoxLayout(self)
+        vlayout.addWidget(self.link_box)
+        vlayout.addWidget(self.dest_box)
+        vlayout.addWidget(self.progress_bar)
+        vlayout.addStretch()
+        vlayout.addLayout(hlayout)
+
+        self.resize(300, 100)
+
+        self.start_button.clicked.connect(self.on_start)
+        self.abort_button.clicked.connect(self.on_abort)
+
+    @Slot()
+    def on_start(self):
+        """When user press start button"""
+
+        #  http file
+        url_file = QUrl(self.link_box.text())
+
+        # destination file
+        dest_path = QDir.fromNativeSeparators(self.dest_box.text().strip())
+        dest_file = QDir(dest_path).filePath(url_file.fileName())
+
+        # Ask a question if file already exists
+        if QFile.exists(dest_file):
+            ret = QMessageBox.question(
+                self,
+                "File exists",
+                "Do you want to override the file ?",
+                QMessageBox.Yes | QMessageBox.No,
+            )
+            if ret == QMessageBox.No:
+                return
+
+            QFile.remove(dest_file)
+
+        self.start_button.setDisabled(True)
+        # Create the file in write mode to append bytes
+        self.file = QSaveFile(dest_file)
+
+        if self.file.open(QIODevice.WriteOnly):
+
+            # Start a GET HTTP request
+            self.reply = self.manager.get(QNetworkRequest(url_file))
+            self.reply.downloadProgress.connect(self.on_progress)
+            self.reply.finished.connect(self.on_finished)
+            self.reply.readyRead.connect(self.on_ready_read)
+            self.reply.errorOccurred.connect(self.on_error)
+        else:
+            error = self.file.errorString()
+            print(f"Cannot open device: {error}")
+
+    @Slot()
+    def on_abort(self):
+        """When user press abort button"""
+        if self.reply:
+            self.reply.abort()
+            self.progress_bar.setValue(0)
+
+        if self.file:
+            self.file.cancelWriting()
+
+        self.start_button.setDisabled(False)
+
+    @Slot()
+    def on_ready_read(self):
+        """ Get available bytes and store them into the file"""
+        if self.reply:
+            if self.reply.error() == QNetworkReply.NoError:
+                self.file.write(self.reply.readAll())
+
+    @Slot()
+    def on_finished(self):
+        """ Delete reply and close the file"""
+        if self.reply:
+            self.reply.deleteLater()
+
+        if self.file:
+            self.file.commit()
+
+        self.start_button.setDisabled(False)
+
+    @Slot(int, int)
+    def on_progress(self, bytesReceived: int, bytesTotal: int):
+        """ Update progress bar"""
+        self.progress_bar.setRange(0, bytesTotal)
+        self.progress_bar.setValue(bytesReceived)
+
+    @Slot(QNetworkReply.NetworkError)
+    def on_error(self, code: QNetworkReply.NetworkError):
+        """ Show a message if an error happen """
+        if self.reply:
+            QMessageBox.warning(self, "Error Occurred", self.reply.errorString())
+
+    @Slot()
+    def on_open_folder(self):
+
+        dir_path = QFileDialog.getExistingDirectory(
+            self, "Open Directory", QDir.homePath(), QFileDialog.ShowDirsOnly
+        )
+
+        if dir_path:
+            dest_dir = QDir(dir_path)
+            self.dest_box.setText(QDir.fromNativeSeparators(dest_dir.path()))
+
+
+if __name__ == "__main__":
+
+    app = QApplication(sys.argv)
+
+    w = DownloaderWidget()
+    w.show()
+    sys.exit(app.exec_())
diff --git a/examples/network/downloader/downloader.pyproject b/examples/network/downloader/downloader.pyproject
new file mode 100644
index 000000000..af1dbb3c2
--- /dev/null
+++ b/examples/network/downloader/downloader.pyproject
@@ -0,0 +1,3 @@
+{
+    "files": ["downloader.py"]
+}