Allow a background task to use the message bar

[Rikuoja]

Add support for bar_msg in setup_task_logger. Will make a PR on this.

[Rikuoja]

Probably not doable? Looks like message bar cannot be accessed from outside the main thread :(

[Rikuoja]

Will cause an Exception:unknown at https://github.com/GispoCoding/qgis_plugin_tools/blob/master/tools/custom_logging.py#L183

[Joonalai]

Background tasks can use feedback and the UI thread could use that feedback message to display messages on message bar 🤔

[ivanlonel]

This is how I got it working.

In the UI thread:

@staticmethod
def pop_widget(mbi: QgsMessageBarItem) -> None:
    if mbi in iface.messageBar().items():
        iface.messageBar().popWidget(mbi)

@classmethod
def prepare_task(cls, task: QgsTask, message: str, maximum: Optional[int] = None) -> None:
    msg_bar = iface.messageBar()
    mbi_progress = msg_bar.createMessage(message)

    progress_bar = QProgressBar(mbi_progress)
    progress_bar.setAlignment(Qt.AlignLeft | Qt.AlignVCenter)
    if maximum is not None:
        progress_bar.setMaximum(maximum)
        task.progressChanged.connect(progress_bar.setValue)
    else:
        progress_bar.setRange(0, 0)
    mbi_progress.layout().addWidget(progress_bar)
    msg_bar.pushWidget(mbi_progress, Qgis.Info)

    btn_close: QToolButton = msg_bar.findChildren(QToolButton)[0]

    for slot in (
        functools.partial(cls.pop_widget, mbi_progress),
        functools.partial(btn_close.clicked.disconnect, task.cancel),
    ):
        task.taskCompleted.connect(slot)
        task.taskTerminated.connect(slot)

    btn_close.clicked.connect(task.cancel)
    QgsApplication.taskManager().addTask(task)

Then, in task's run method, keep updating progress with self.setProgress() and checking self.isCanceled() to see if the message bar's close button was clicked.