task manager qgis

Note that it's vital to assign the task not just to a variable, but to a class attribute. Otherwise, the task will be deleted after download_tiles() finishes, but before the task itself finishes.

Finally, the task needs to be run. For this, we use the QgsTaskManager , which is a singleton class that takes care of delegating any background tasks. We add the task and the task manager makes sure that the task starts running.

And that's it! We have successfully created a small plugin that downloads Mapzen/Tilezen elevation tiles in the background by using QgsTask , so that the user can continue interacting with QGIS while the download runs.

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Issues with QgsTask and task manager

I am trying to use the task manager to run a long operation (uploading layer to a GeoServer instance, for the Geoserver Explorer plugin), but I am finding some issues.

Here is the code that I am using:

The issues I have are mainly two:

1) After adding the task to the task manager, it doesnt start. I have to add some call that refreshes the UI, such as logging a dummy message to the QGIS log or calling processEvents() to have it started. I have tried in Win and Mac, and in both cases the task is not triggered. Docs say that the task manager will take care of running it when it can, but that never happens, no matter what I do on the QGIS UI.

2) The finished () method is not always run when the task is finished. The layersUploaded method that is connected to the taskCompleted signal is always run when the task is correctly finished, but the finished method of the task is only called sometimes. That doesn't depend on the success of the task. If it fails, the finished method is only run sometimes, and I could not find any pattern in it.

Any ideas or suggestions?

taskmanager

4 Answers 4

QGIS version 3.4.2 If you define a task and add it to the task manager within a function scope, it will cause the task to either not execute properly or, at worst, crash QGIS. Consider the following code.

If you put this in the script editor in QGIS and execute it, it will run just fine. The user can cancel. The exceptions are raised, occasionally. All is well. Now if you replace the last block of the code to be within a function scope, like so:

Now when you run the script you will find that either task1 will appear to run, but the completed() function will never be reached (task2 will NOT run) or QGIS will crash. If you run it twice, QGIS will most surely crash. This will also happen when using the class method, as in OP's code. This is likely a bug and I may link to this answer and report it. A possible workaround would be to take the task creation and execution out of scope, as in my first example, and run it from the script editor.

Update: I have reported this issue to QGIS, however it may not be a bug and just a matter of scope. If you have the following code, in a module named task_example.py, you will find it to work correctly for both the subclassed and fromFunction methods:

You can now, in the python console (or another py file), make the following executions:

Please note that I commented out the parts of the code that cause an exception when the random integer is 42. I was tired of the answer to the ultimate question of life, the universe, and everything throwing exceptions at me.

Surprisingly, but on_finished=completed() instead of on_finished=completed worked for me. – BlackTesta Oct 26, 2022 at 12:40

As @Subgenius said, a workaround would be to create the task variable with global scope.

And to call it,

This way, you can call the task from inside a function and a class.

Full code -

At this time of writing this answer, I can make the task run properly by assigning the task to a python's object attribute.

For example:

In the snippet above, calling the run_task_without_self will run the task badly (no finished or completed signal is emitted). On the second function, run_task_with_self , it will run as expected.

Unfortunately, I don't really understand why, since I just found it somehow the first time, and when I remove the self , it doesn't work anymore. Another thing that I found is if I put the tasks in a dictionary, (e.g. self.task_dict = {'1': QgsTask(...), '2': QgsTask(...)} ), it also doesn't work properly.

Hmm, I've been using QgsTask.fromFunction I've copied in some code that works for me. (Though I cant get Exited by user to work.. that I posted here: My question ).

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Qgis-documentation / docs / pyqgis_developer_cookbook / tasks.rst.

Tasks - doing heavy work in the background

The code snippets on this page need the following imports if you're outside the pyqgis console:

Introduction

Background processing using threads is a way to maintain a responsive user interface when heavy processing is going on. Tasks can be used to achieve threading in QGIS.

A task ( :class:`QgsTask <qgis.core.QgsTask>` ) is a container for the code to be performed in the background, and the task manager ( :class:`QgsTaskManager <qgis.core.QgsTaskManager>` ) is used to control the running of the tasks. These classes simplify background processing in QGIS by providing mechanisms for signaling, progress reporting and access to the status for background processes. Tasks can be grouped using subtasks.

The global task manager (found with :meth:`QgsApplication.taskManager() <qgis.core.QgsApplication.taskManager>` ) is normally used. This means that your tasks may not be the only tasks that are controlled by the task manager.

There are several ways to create a QGIS task:

Create your own task by extending :class:`QgsTask <qgis.core.QgsTask>`

Create a task from a function

Create a task from a processing algorithm

Any background task (regardless of how it is created) must NEVER use any QObject that lives on the main thread, such as accessing QgsVectorLayer, QgsProject or perform any GUI based operations like creating new widgets or interacting with existing widgets. Qt widgets must only be accessed or modified from the main thread. Data that is used in a task must be copied before the task is started. Attempting to use them from background threads will result in crashes.

Dependencies between tasks can be described using the :meth:`addSubTask() <qgis.core.QgsTask.addSubTask>` function of :class:`QgsTask <qgis.core.QgsTask>` . When a dependency is stated, the task manager will automatically determine how these dependencies will be executed. Wherever possible dependencies will be executed in parallel in order to satisfy them as quickly as possible. If a task on which another task depends is canceled, the dependent task will also be canceled. Circular dependencies can make deadlocks possible, so be careful.

If a task depends on a layer being available, this can be stated using the :meth:`setDependentLayers() <qgis.core.QgsTask.setDependentLayers>` function of :class:`QgsTask <qgis.core.QgsTask>` . If a layer on which a task depends is not available, the task will be canceled.

Once the task has been created it can be scheduled for running using the :meth:`addTask() <qgis.core.QgsTaskManager.addTask>` function of the task manager. Adding a task to the manager automatically transfers ownership of that task to the manager, and the manager will cleanup and delete tasks after they have executed. The scheduling of the tasks is influenced by the task priority, which is set in :meth:`addTask() <qgis.core.QgsTaskManager.addTask>` .

The status of tasks can be monitored using :class:`QgsTask <qgis.core.QgsTask>` and :class:`QgsTaskManager <qgis.core.QgsTaskManager>` signals and functions.

Extending QgsTask

In this example RandomIntegerSumTask extends :class:`QgsTask <qgis.core.QgsTask>` and will generate 100 random integers between 0 and 500 during a specified period of time. If the random number is 42, the task is aborted and an exception is raised. Several instances of RandomIntegerSumTask (with subtasks) are generated and added to the task manager, demonstrating two types of dependencies.

Task from function

Create a task from a function ( doSomething in this example). The first parameter of the function will hold the :class:`QgsTask <qgis.core.QgsTask>` for the function. An important (named) parameter is on_finished , that specifies a function that will be called when the task has completed. The doSomething function in this example has an additional named parameter wait_time .

Task from a processing algorithm

Create a task that uses the algorithm :ref:`qgis:randompointsinextent <qgisrandompointsinextent>` to generate 50000 random points inside a specified extent. The result is added to the project in a safe way.

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This means that your tasks may not be the only tasks that are controlled by the task manager. There are several ways to create a QGIS task:.
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A task ( QgsTask ) is a container for the code to be performed in the background, and the task manager ( QgsTaskManager ) is used to control the running of the
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This is where the QGIS background tasks ( QgsTask ) come in very handy. ... We add the task and the task manager makes sure that the task
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QGIS version 3.4.2 If you define a task and add it to the task manager within a function scope, it will cause the task to either not execute
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The global task manager (found with :meth:`QgsApplication.taskManager() <qgis.core.QgsApplication.taskManager>`) is normally used. This means that your tasks