Os trechos de código desta página precisam das seguintes importações se você estiver fora do console do pyqgis:

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from qgis.core import (
  QgsProcessingContext,
  QgsTaskManager,
  QgsTask,
  QgsProcessingAlgRunnerTask,
  Qgis,
  QgsProcessingFeedback,
  QgsApplication,
  QgsMessageLog,
)

15. Tarefas - trabalho pesado em segundo plano

15.1. Introdução

O processamento em segundo plano usando threads é uma maneira de manter uma interface de usuário responsdendo quando o processamento pesado está em andamento. As tarefas podem ser usadas para obter threading no QGIS.

Uma tarefa (QgsTask) é um contêiner para o código a ser executado em segundo plano, e o gerenciador de tarefas (QgsTaskManager) é usado para controlar a execução das tarefas. Essas classes simplificam o processamento em segundo plano no QGIS, fornecendo mecanismos para sinalização, relatórios de progresso e acesso ao status dos processos em segundo plano. As tarefas podem ser agrupadas usando subtarefas.

O gerenciador de tarefas global (encontrado com QgsApplication.taskManager()) é normalmente usado. Isso significa que suas tarefas podem não ser as únicas controladas pelo gerenciador de tarefas.

Existem várias maneiras de criar uma tarefa QGIS:

  • Crie sua própria tarefa estendendo QgsTask

    class SpecialisedTask(QgsTask):
    pass

  • Criar uma tarefa a partir de uma função

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    def heavyFunction():
    # Some CPU intensive processing ...
    pass

def workdone():
    # ... do something useful with the results
    pass

task = QgsTask.fromFunction('heavy function', heavyFunction,
                     onfinished=workdone)

  • Criar uma tarefa a partir de um algoritmo de processamento

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    params = dict()
context = QgsProcessingContext()
feedback = QgsProcessingFeedback()

buffer_alg = QgsApplication.instance().processingRegistry().algorithmById('native:buffer')
task = QgsProcessingAlgRunnerTask(buffer_alg, params, context,
                           feedback)

Aviso

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 addSubTask() function of 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 setDependentLayers() function of 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 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 addTask().

O status das tarefas pode ser monitorado usando QgsTask e sinais e funções de QgsTaskManager.

15.2. Exemplos

15.2.1. Estendendo QgsTask

Neste exemplo, RandomIntegerSumTask estende QgsTask e gerará 100 números inteiros aleatórios entre 0 e 500 durante um período especificado. Se o número aleatório for 42, a tarefa será abortada e uma exceção será gerada. Várias instâncias de RandomIntegerSumTask (com subtarefas) são geradas e adicionadas ao gerenciador de tarefas, demonstrando dois tipos de dependências.

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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(
                'RandomTask "{name}" completed\n' \
                'RandomTotal: {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(
                    'RandomTask "{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(
                    'RandomTask "{name}" Exception: {exception}'.format(
                        name=self.description(),
                        exception=self.exception),
                    MESSAGE_CATEGORY, Qgis.Critical)
                raise self.exception

    def cancel(self):
        QgsMessageLog.logMessage(
            'RandomTask "{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)

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RandomIntegerSumTask(0): Started task "waste cpu subtask shortest"
RandomIntegerSumTask(0): Started task "waste cpu short"
RandomIntegerSumTask(0): Started task "waste cpu mini"
RandomIntegerSumTask(0): Started task "waste cpu subtask long"
RandomIntegerSumTask(3): Task "waste cpu subtask shortest" completed
RandomTotal: 25452 (with 100 iterations)
RandomIntegerSumTask(3): Task "waste cpu mini" completed
RandomTotal: 23810 (with 100 iterations)
RandomIntegerSumTask(3): Task "waste cpu subtask long" completed
RandomTotal: 26308 (with 100 iterations)
RandomIntegerSumTask(0): Started task "waste cpu long"
RandomIntegerSumTask(3): Task "waste cpu long" completed
RandomTotal: 22534 (with 100 iterations)

15.2.2. Tarefa da função

Crie uma tarefa a partir de uma função (doSomething neste exemplo). O primeiro parâmetro da função conterá QgsTask para a função. Um parâmetro importante (nomeado) é on_finished, que especifica uma função que será chamada quando a tarefa for concluída. A função doSomething neste exemplo possui um parâmetro adicional com nome wait_time.

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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

# Create 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)

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RandomIntegerSumTask(0): Started task "waste cpu subtask short"
RandomTaskFromFunction(0): Started task Waste cpu 1
RandomTaskFromFunction(0): Started task Waste cpu 2
RandomTaskFromFunction(0): Task Waste cpu 2 completed
RandomTotal: 23263 ( with 100 iterations)
RandomTaskFromFunction(0): Task Waste cpu 1 completed
RandomTotal: 25044 ( with 100 iterations)

15.2.3. Tarefa de um algoritmo de processamento

Crie uma tarefa que use o algoritmo qgis:randompointsinextent para gerar 50000 pontos aleatórios dentro de uma extensão especificada. O resultado é adicionado ao projeto de forma segura.

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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)

Veja também: https://opengis.ch/2018/06/22/threads-in-pyqgis3/.