Background worker populate QListView thumbnails

I have a simple QListview that displays a list of items Names. I would like to display the thumbnail of each item once it has been downloaded. How can i do the following as I'm new to using something like a background worker and I'm not sure how to achieve this.

This explains what i think would be the best approach...

Use a custom QStyledItemDelegate that overrides the initStyleOption() function.
Detects the lack of an icon and issues an asynchronous request to load it.
In the meantime, display default empty icon so user sees placeholder
When the asynchronous request to download the icon is done, it signals my widget which updates the items icon.

When i create all of my QStandardModelItems, I give them a custom piece of data (a custom role) that holds the path of the thumbnail for each item

import os
import sys
from PySide2 import QtCore, QtGui, QtWidgets

try:
    # python 2
    from urllib import urlretrieve
    from urllib2 import urlopen
except Exception as e:
    # python 3
    from urllib.request import urlretrieve, urlopen

import time
from urllib.parse import urlparse

def getThumbnail(url, output):
    if os.path.exists(output):
        return output
    # # download 1
    # # urlretrieve(url, output)
    # # return os.path.abspath(output)

    # download 2
    response = urlopen(url, timeout=5000)
    f = open(output, "wb")
    try:
        f.write(response.read())
    finally:
        f.close()
    return output

class ExampleDialog(QtWidgets.QDialog):

    def __init__(self):
        super(ExampleDialog, self).__init__()

        self.itemModel = QtGui.QStandardItemModel()

        self.uiListView = QtWidgets.QListView()
        # self.uiListView.setViewMode(QtWidgets.QListView.IconMode)
        self.uiListView.setIconSize(QtCore.QSize(80, 60))  #set icon size
        self.uiListView.setGridSize(QtCore.QSize(90, 70)) #set icon grid display
        self.uiListView.setModel(self.itemModel)

        self.mainLayout = QtWidgets.QVBoxLayout(self)
        self.mainLayout.addWidget(self.uiListView)

        self.populateImages()


    def populateImages(self):
        root = os.path.join(os.getenv('APPDATA'), 'MyApp\\cache')
        if not os.path.exists(root):
            os.makedirs(root)

        print('IMAGES:', root)

        for x in range(20):
            url = 'https://picsum.photos/id/{}/80/60.jpg'.format(x)

            p = urlparse(url).path
            ext = os.path.splitext(p)[-1]
            output = os.path.join(root, '{}{}'.format(x, ext))

            # get thumbnail
            getThumbnail(url, output)

            # Item
            item = QtGui.QStandardItem('{}'.format(x))
            item.setData(QtGui.QPixmap(output), QtCore.Qt.DecorationRole)
            item.setData(output, QtCore.Qt.UserRole)
            self.itemModel.appendRow(item)

if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    window =  ExampleDialog()
    window.show()
    window.raise_()
    sys.exit(app.exec_())

Solution

Instead of using a background worker you can use QNetworkAccessManager for asynchronous downloading.

from dataclasses import dataclass
from functools import cached_property
import sys

from PySide2 import QtCore, QtGui, QtWidgets, QtNetwork


@dataclass
class IconDownloader(QtCore.QObject):
    url: QtCore.QUrl
    index: QtCore.QPersistentModelIndex
    _parent: QtCore.QObject = None

    def __post_init__(self):
        super().__init__()
        self.setParent(self._parent)

    @cached_property
    def network_manager(self):
        manager = QtNetwork.QNetworkAccessManager()
        manager.finished.connect(self._handle_finished)
        return manager

    def start(self):
        if self.index.isValid():
            request = QtNetwork.QNetworkRequest(self.url)
            request.setAttribute(
                QtNetwork.QNetworkRequest.FollowRedirectsAttribute, True
            )
            self.network_manager.get(request)

    def _handle_finished(self, reply):
        if reply.error() == QtNetwork.QNetworkReply.NoError:
            pixmap = QtGui.QPixmap()
            ok = pixmap.loadFromData(reply.readAll())
            if ok and self.index.isValid():
                model = self.index.model()
                model.setData(
                    QtCore.QModelIndex(self.index), pixmap, QtCore.Qt.DecorationRole
                )
        else:
            print(reply.error(), reply.errorString())
        reply.deleteLater()
        self.deleteLater()


class ExampleDialog(QtWidgets.QDialog):
    def __init__(self):
        super(ExampleDialog, self).__init__()

        self.itemModel = QtGui.QStandardItemModel()

        self.uiListView = QtWidgets.QListView()
        # self.uiListView.setViewMode(QtWidgets.QListView.IconMode)
        self.uiListView.setIconSize(QtCore.QSize(80, 60))  # set icon size
        self.uiListView.setGridSize(QtCore.QSize(90, 70))  # set icon grid display
        self.uiListView.setModel(self.itemModel)

        self.mainLayout = QtWidgets.QVBoxLayout(self)
        self.mainLayout.addWidget(self.uiListView)

        self.populateImages()

    def populateImages(self):
        for x in range(20):
            url = f"https://picsum.photos/id/{x}/80/60.jpg"
            item = QtGui.QStandardItem(f"x")
            self.itemModel.appendRow(item)
            downloader = IconDownloader(
                QtCore.QUrl(url),
                QtCore.QPersistentModelIndex(self.itemModel.indexFromItem(item)),
                self,
            )
            downloader.start()


if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    window = ExampleDialog()
    window.show()
    window.raise_()
    sys.exit(app.exec_())