# have a function that downloads text
def download_text(url: str) -> str:
    # ...

# create the filter store somewhere in your code, ideally you should
# only have one in your application
my_filter_store = WebKit2.UserContentFilterStore.new('some/cache/path')

# a name that will be used to save/retrieve blocklists from the store
BLOCKLIST_ID = 'blocklist'

def apply_adblock(
        webview: WebKit2.WebView,
        filter_store: WebKit2.UserContentFilterStore,
        blocklist_url: str
) -> None:
    # 0: Retrieve the WebKit2.UserContentManager from the WebView
    content_manager = webview.get_user_content_manager()

    def save_blocklist_cb(caller, res, *args):
        # 6: Once saving is done we can retrieve the newly created
        #    WebKit2.UserContentFilter with save_finish and finally add th
        #    filter to the WebKit2.UserContentManager
        try:
            filter = filter_store.save_finish(res)
            content_manager.add_filter(filter)
        except GLib.Error:
            print('Error saving blocklist')

    def download_blocklist_cb(blocklist: str):
        # 5: Now that we have the blocklist (as a string containing a json),
        #    we'll need to convert it to GLib.Bytes, then save it in the store.
        #    Yes, we need to save it even if we want to refresh it every time,
        #    this is just the way this works.
        #    This is also async and when it's done it will call continue on to
        #    save_blocklist_cb
        filter_store.save(
            BLOCKLIST_ID, GLib.Bytes.new(blocklist.encode()), None,
            save_blocklist_cb
        )

    def download_blocklist():
        # 4: Download the blocklist and continue on to download_blocklist_cb
        try:
            res = download_text(blocklist_url)
            GLib.idle_add(download_blocklist_cb, res)
        except:
            print('Error downloading the blocklist')

    def filter_load_cb(caller, res, *args):
        # 2: load_finish will either succeed, and return a
        #    WebKit2.UserContentFilter object or raise a GLib.Error in case
        #    it cannot find the blocklist in the store (ie: on the first run)
        try:
            filter = filter_store.load_finish(res)
            content_manager.add_filter(filter)
        except GLib.Error:
            # 3: If loading the blocklist from the store fails, download the
            #    blocklist (async to avoid blocking of course) and save it
            print('blocklist store not found, downloading...')
            Thread(target=download_blocklist, daemon=True).start()

    # 1: try to load the blocklist from the store, this is async and will call
    #    filter_load_cb when it's done
    filter_store.load(BLOCKLIST_ID, None, filter_load_cb, None)