Hi Johannes,just stop the execution in QgsTask::run(), for example using a break that stops a loop, and return False instead of True.
From https://api.qgis.org/api/3.30/classQgsTask.html
Performs the task's operation.This method will be called when the task commences (ie via calling start() ), and subclasses should implement the operation they wish to perform in the background within this method.A task must return a boolean value to indicate whether the task was completed successfully or terminated before completion.virtual bool QgsTask::run ( ) protectedpure virtual
Greetings, BenjaminAm 02.06.2023 um 14:19 schrieb Johannes Kröger (WhereGroup) via QGIS-Developer:
Hi, https://qgis.org/pyqgis/3.30/core/QgsTask.html says> Long running tasks should periodically check the isCanceled() flag to detect if the task has been canceled via some external event. If this flag is True then the task should clean up and terminate at the earliest possible convenience.How are we supposed to "terminate" a task properly?If we check for task.isCanceled() (or call task.cancel() ourselves for testing) and simply return:- QGIS will still show a notification that the "Task completed" (instead of "Task failed") (https://github.com/qgis/QGIS/blob/d3d0431685883b837caa571ad7282828cfbde65e/src/app/qgisapp.cpp#L15707)- The taskTerminated signal is not emitted. But the taskCompleted signal is emitted.- The task's status is QgsTask.CompleteThere is no obvious way to "terminate" the task from the Python side. We cannot set the task's status attribute to QgsTask.Terminated (https://qgis.org/pyqgis/3.30/core/QgsTask.html#qgis.core.QgsTask.Terminated).If an exception happens in the task then it seems to terminate "properly", i.e. its status will be QgsTask.Terminated, the taskTerminated signal is emitted and a notification "Task failed" is shown. This is the only way I found today to get into this state. I have not found this documented and only by accident...Example for a QgsTask.fromFuncction(..., flags=QgsTask.CanCancel), uncomment one of the final lines:def foo(task):time.sleep(6) # must be >= minTaskLengthForSystemNotification (5 by default)#task.cancel(); return # -> Task complete #raise Exception("fail plz") # -> Task failedIt feels like a bug to me (either task.cancel should directly set its status to QgsTask.Terminated or we should have a task.setStatus() method) but everything in and around QgsTask is fairly opaque and hard to grasp so maybe I am just missing something obvious.Cheers, HannesPS: I did not test the example function in a clean environment, in my test cases I have code like this as methods of a class.
-- Dr. Benjamin Jakimow Earth Observation Lab | Geography Department | Humboldt-Universität zu Berlin e-mail:[email protected] web:https://hu-berlin.de/eo-lab phone: +49 (0) 30 2093 6846 mobile: +49 (0) 157 5656 8477 fax: +49 (0) 30 2093 6848 mail: Unter den Linden 6 | 10099 Berlin | Germany room: 2'211 linkedin:https://www.linkedin.com/in/benjamin-jakimow matrix: @jakimowb:hu-berlin.de
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