.. only:: html .. index:: Tasks .. _tasks: ****************************************** Tasks - doing heavy work in the background ****************************************** 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 `) is a container for the code to be performed in the background, and the task manager (:class:`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() `) 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 ` .. code-block:: python class SpecialisedTask(QgsTask): * Create a task from a function .. code-block:: python QgsTask.fromFunction('heavy function', heavyFunction, onfinished=workdone) * Create a task from a processing algorithm .. code-block:: python QgsProcessingAlgRunnerTask('native:buffer', params, context, feedback) .. warning:: Any background task (regardless of how it is created) must NEVER perform any GUI based operations, such as creating new widgets or interacting with existing widgets. Qt widgets must only be accessed or modified from the main thread. Attempting to use them from background threads will result in crashes. Dependencies between tasks can be described using the :meth:`addSubTask ` function of :class:`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 ` function of :class:`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 ` 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 `. The status of tasks can be monitored using :class:`QgsTask ` and :class:`QgsTaskManager ` signals and functions. Examples -------- Extending QgsTask ................. In this example ``RandomIntegerSumTask`` extends :class:`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. .. code-block:: python import random from time import sleep from qgis.core import ( QgsApplication, QgsTask, QgsMessageLog, ) MESSAGE_CATEGORY = 'RandomIntegerSumTask' class RandomIntegerSumTask(QgsTask): """This shows how to subclass QgsTask""" def __init__(self, description, duration): super().__init__(description, QgsTask.CanCancel) self.duration = duration self.total = 0 self.iterations = 0 self.exception = None def run(self): """Here you implement your heavy lifting. Should periodically test for isCanceled() to gracefully abort. This method MUST return True or False. Raising exceptions will crash QGIS, so we handle them internally and raise them in self.finished """ QgsMessageLog.logMessage('Started task "{}"'.format( self.description()), MESSAGE_CATEGORY, Qgis.Info) wait_time = self.duration / 100 for i in range(100): sleep(wait_time) # use setProgress to report progress self.setProgress(i) arandominteger = random.randint(0, 500) self.total += arandominteger self.iterations += 1 # check isCanceled() to handle cancellation if self.isCanceled(): return False # simulate exceptions to show how to abort task if arandominteger == 42: # DO NOT raise Exception('bad value!') # this would crash QGIS self.exception = Exception('bad value!') return False return True def finished(self, result): """ This function is automatically called when the task has completed (successfully or not). You implement finished() to do whatever follow-up stuff should happen after the task is complete. finished is always called from the main thread, so it's safe to do GUI operations and raise Python exceptions here. result is the return value from self.run. """ if result: QgsMessageLog.logMessage( 'Task "{name}" completed\n' \ 'Total: {total} (with {iterations} '\ 'iterations)'.format( name=self.description(), total=self.total, iterations=self.iterations), MESSAGE_CATEGORY, Qgis.Success) else: if self.exception is None: QgsMessageLog.logMessage( 'Task "{name}" not successful but without '\ 'exception (probably the task was manually '\ 'canceled by the user)'.format( name=self.description()), MESSAGE_CATEGORY, Qgis.Warning) else: QgsMessageLog.logMessage( 'Task "{name}" Exception: {exception}'.format( name=self.description(), exception=self.exception), MESSAGE_CATEGORY, Qgis.Critical) raise self.exception def cancel(self): QgsMessageLog.logMessage( 'Task "{name}" was canceled'.format( name=self.description()), MESSAGE_CATEGORY, Qgis.Info) super().cancel() longtask = RandomIntegerSumTask('waste cpu long', 20) shorttask = RandomIntegerSumTask('waste cpu short', 10) minitask = RandomIntegerSumTask('waste cpu mini', 5) shortsubtask = RandomIntegerSumTask('waste cpu subtask short', 5) longsubtask = RandomIntegerSumTask('waste cpu subtask long', 10) shortestsubtask = RandomIntegerSumTask('waste cpu subtask shortest', 4) # Add a subtask (shortsubtask) to shorttask that must run after # minitask and longtask has finished shorttask.addSubTask(shortsubtask, [minitask, longtask]) # Add a subtask (longsubtask) to longtask that must be run # before the parent task longtask.addSubTask(longsubtask, [], QgsTask.ParentDependsOnSubTask) # Add a subtask (shortestsubtask) to longtask longtask.addSubTask(shortestsubtask) QgsApplication.taskManager().addTask(longtask) QgsApplication.taskManager().addTask(shorttask) QgsApplication.taskManager().addTask(minitask) Task from function .................. Create a task from a function (``doSomething`` in this example). The first parameter of the function will hold the :class:`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``. .. code-block:: python import random from time import sleep MESSAGE_CATEGORY = 'TaskFromFunction' def doSomething(task, wait_time): """ Raises an exception to abort the task. Returns a result if success. The result will be passed, together with the exception (None in the case of success), to the on_finished method. If there is an exception, there will be no result. """ QgsMessageLog.logMessage('Started task {}'.format(task.description()), MESSAGE_CATEGORY, Qgis.Info) wait_time = wait_time / 100 total = 0 iterations = 0 for i in range(100): sleep(wait_time) # use task.setProgress to report progress task.setProgress(i) arandominteger = random.randint(0, 500) total += arandominteger iterations += 1 # check task.isCanceled() to handle cancellation if task.isCanceled(): stopped(task) return None # raise an exception to abort the task if arandominteger == 42: raise Exception('bad value!') return {'total': total, 'iterations': iterations, 'task': task.description()} def stopped(task): QgsMessageLog.logMessage( 'Task "{name}" was canceled'.format( name=task.description()), MESSAGE_CATEGORY, Qgis.Info) def completed(exception, result=None): """This is called when doSomething is finished. Exception is not None if doSomething raises an exception. result is the return value of doSomething.""" if exception is None: if result is None: QgsMessageLog.logMessage( 'Completed with no exception and no result '\ '(probably manually canceled by the user)', MESSAGE_CATEGORY, Qgis.Warning) else: QgsMessageLog.logMessage( 'Task {name} completed\n' 'Total: {total} ( with {iterations} ' 'iterations)'.format( name=result['task'], total=result['total'], iterations=result['iterations']), MESSAGE_CATEGORY, Qgis.Info) else: QgsMessageLog.logMessage("Exception: {}".format(exception), MESSAGE_CATEGORY, Qgis.Critical) raise exception # Creae a few tasks task1 = QgsTask.fromFunction('Waste cpu 1', doSomething, on_finished=completed, wait_time=4) task2 = QgsTask.fromFunction('Waste cpu 2', doSomething, on_finished=completed, wait_time=3) QgsApplication.taskManager().addTask(task1) QgsApplication.taskManager().addTask(task2) Task from a processing algorithm ................................ Create a task that uses the algorithm :ref:`qgis:randompointsinextent ` to generate 50000 random points inside a specified extent. The result is added to the project in a safe way. .. code-block:: python from functools import partial from qgis.core import (QgsTaskManager, QgsMessageLog, QgsProcessingAlgRunnerTask, QgsApplication, QgsProcessingContext, QgsProcessingFeedback, QgsProject) MESSAGE_CATEGORY = 'AlgRunnerTask' def task_finished(context, successful, results): if not successful: QgsMessageLog.logMessage('Task finished unsucessfully', MESSAGE_CATEGORY, Qgis.Warning) output_layer = context.getMapLayer(results['OUTPUT']) # because getMapLayer doesn't transfer ownership, the layer will # be deleted when context goes out of scope and you'll get a # crash. # takeMapLayer transfers ownership so it's then safe to add it # to the project and give the project ownership. if output_layer and output_layer.isValid(): QgsProject.instance().addMapLayer( context.takeResultLayer(output_layer.id())) alg = QgsApplication.processingRegistry().algorithmById( 'qgis:randompointsinextent') context = QgsProcessingContext() feedback = QgsProcessingFeedback() params = { 'EXTENT': '0.0,10.0,40,50 [EPSG:4326]', 'MIN_DISTANCE': 0.0, 'POINTS_NUMBER': 50000, 'TARGET_CRS': 'EPSG:4326', 'OUTPUT': 'memory:My random points' } task = QgsProcessingAlgRunnerTask(alg, params, context, feedback) task.executed.connect(partial(task_finished, context)) QgsApplication.taskManager().addTask(task) See also: https://www.opengis.ch/2018/06/22/threads-in-pyqgis3/.