Subgroup: Task

Class: QgsTask

class qgis.core.QgsTask(description: str = '', flags: Union[QgsTask.Flags, QgsTask.Flag] = QgsTask.AllFlags)

Bases: PyQt5.QtCore.QObject

Constructor for QgsTask.

Parameters:
  • description – text description of task
  • flags – task flags

Abstract base class for long running background tasks. Tasks can be controlled directly, or added to a QgsTaskManager for automatic management.

Derived classes should implement the process they want to execute in the background within the run() method. This method will be called when the task commences (ie via calling run() ).

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.

New in version 3.0: Methods

addSubTask Adds a subtask to this task.
canCancel Returns true if the task can be canceled.
cancel Notifies the task that it should terminate.
childEvent
connectNotify
customEvent
dependentLayers Returns the list of layers on which the task depends.
description Returns the task’s description.
disconnectNotify
elapsedTime Returns the elapsed time since the task commenced, in milliseconds.
finished If the task is managed by a QgsTaskManager, this will be called after the task has finished (whether through successful completion or via early termination).
flags Returns the flags associated with the task.
fromFunction Creates a new QgsTask task from a python function.
hold Places the task on hold.
isActive Returns true if the task is active, ie it is not complete and has not been canceled.
isCanceled Will return true if task should terminate ASAP.
isSignalConnected
progress Returns the task’s progress (between 0.0 and 100.0)
receivers
run Performs the task’s operation.
sender
senderSignalIndex
setDependentLayers Sets a list of layers on which the task depends.
setProgress Sets the task’s current progress.
status Returns the current task status.
timerEvent
unhold Releases the task from being held.
waitForFinished Blocks the current thread until the task finishes or a maximum of timeout milliseconds.

Signals

begun Will be emitted by task to indicate its commencement.
progressChanged Will be emitted by task when its progress changes.
statusChanged Will be emitted by task when its status changes.
taskCompleted Will be emitted by task to indicate its successful completion.
taskTerminated Will be emitted by task if it has terminated for any reason other then completion (e.g., when a task has been canceled or encountered an internal error).

Attributes

AllFlags
CanCancel
Complete
OnHold
ParentDependsOnSubTask
Queued
Running
SubTaskIndependent
Terminated
AllFlags = 2
CanCancel = 2
Complete = 3
class Flag

Bases: int

class Flags

Bases: sip.wrapper

QgsTask.Flags(Union[QgsTask.Flags, QgsTask.Flag]) QgsTask.Flags(QgsTask.Flags)

OnHold = 1
ParentDependsOnSubTask = 1
Queued = 0
Running = 2
class SubTaskDependency

Bases: int

SubTaskIndependent = 0
class TaskStatus

Bases: int

Terminated = 4
addSubTask(self, subTask: QgsTask, dependencies: object = QgsTaskList(), subTaskDependency: QgsTask.SubTaskDependency = QgsTask.SubTaskIndependent)

Adds a subtask to this task.

Subtasks allow a single task to be created which consists of multiple smaller tasks. Subtasks are not visible or indepedently controllable by users. Ownership of the subtask is transferred. Subtasks can have an optional list of dependent tasks, which must be completed before the subtask can begin. By default subtasks are considered independent of the parent task, ie they can be run either before, after, or at the same time as the parent task. This behavior can be overridden through the subTaskDependency argument. Note that subtasks should NEVER be dependent on their parent task, and violating this constraint will prevent the task from completing successfully.

The parent task must be added to a QgsTaskManager for subtasks to be utilized. Subtasks should not be added manually to a QgsTaskManager, rather, only the parent task should be added to the manager.

Subtasks can be nested, ie a subtask can legally be a parent task itself with its own set of subtasks.

begun

Will be emitted by task to indicate its commencement.

Note

derived classes should not emit this signal directly, it will automatically be emitted when the task begins [signal]

canCancel(self) → bool

Returns true if the task can be canceled.

cancel(self)

Notifies the task that it should terminate. Calling this is not guaranteed to immediately end the task, rather it sets the isCanceled() flag which task subclasses can check and terminate their operations at an appropriate time. Any subtasks owned by this task will also be canceled. Derived classes must ensure that the base class implementation is called from any overridden version.

See also

isCanceled()

childEvent()
connectNotify()
customEvent()
dependentLayers(self) → List[QgsMapLayer]

Returns the list of layers on which the task depends. The task will automatically be canceled if any of these layers are about to be removed.

description(self) → str

Returns the task’s description.

disconnectNotify()
elapsedTime(self) → int

Returns the elapsed time since the task commenced, in milliseconds.

The value is undefined for tasks which have not begun.

New in version 3.4.

finished(self, result: bool)

If the task is managed by a QgsTaskManager, this will be called after the task has finished (whether through successful completion or via early termination). The result argument reflects whether the task was successfully completed or not. This method is always called from the main thread, so it is safe to create widgets and perform other operations which require the main thread. However, the GUI will be blocked for the duration of this method so tasks should avoid performing any lengthy operations here.

flags(self) → QgsTask.Flags

Returns the flags associated with the task.

static fromFunction(description, function, *args, on_finished=None, flags=2, **kwargs)

Creates a new QgsTask task from a python function.

Example:

def calculate(task):
# pretend this is some complex maths and stuff we want # to run in the background return 5*6
def calculation_finished(exception, value=None):
if not exception:
iface.messageBar().pushMessage(
‘the magic number is {}’.format(value))
else:
iface.messageBar().pushMessage(
str(exception))
task = QgsTask.fromFunction(‘my task’, calculate,
on_finished=calculation_finished)

QgsApplication.taskManager().addTask(task)

hold(self)

Places the task on hold. If the task in not queued (ie it is already running or has finished) then calling this has no effect. Calling this method only has an effect for tasks which are managed by a QgsTaskManager.

See also

unhold()

isActive(self) → bool

Returns true if the task is active, ie it is not complete and has not been canceled.

isCanceled(self) → bool

Will return true if task should terminate ASAP. If the task reports the CanCancel flag, then derived classes’ run() methods should periodically check this and terminate in a safe manner.

isSignalConnected()
progress(self) → float

Returns the task’s progress (between 0.0 and 100.0)

progressChanged

Will be emitted by task when its progress changes.

Parameters:progress – percent of progress, from 0.0 - 100.0

Note

derived classes should not emit this signal directly, instead they should call setProgress() [signal]

receivers()
run(self) → bool

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.

sender()
senderSignalIndex()
setDependentLayers(self, dependentLayers: Iterable[QgsMapLayer])

Sets a list of layers on which the task depends. The task will automatically be canceled if any of these layers are about to be removed.

setProgress(self, progress: float)

Sets the task’s current progress. The derived class should call this method whenever the task wants to update its progress. Calling will automatically emit the progressChanged signal.

Parameters:progress – percent of progress, from 0.0 - 100.0
status(self) → QgsTask.TaskStatus

Returns the current task status.

statusChanged

Will be emitted by task when its status changes.

Parameters:status – new task status

Note

derived classes should not emit this signal directly, it will automatically be emitted [signal]

taskCompleted

Will be emitted by task to indicate its successful completion.

Note

derived classes should not emit this signal directly, it will automatically be emitted [signal]

taskTerminated

Will be emitted by task if it has terminated for any reason other then completion (e.g., when a task has been canceled or encountered an internal error).

Note

derived classes should not emit this signal directly, it will automatically be emitted [signal]

timerEvent()
unhold(self)

Releases the task from being held. For tasks managed by a QgsTaskManager calling this will re-add them to the queue. If the task in not currently being held then calling this has no effect.

See also

hold()

waitForFinished(self, timeout: int = 30000) → bool

Blocks the current thread until the task finishes or a maximum of timeout milliseconds. If timeout is 0 the thread will be blocked forever. In case of a timeout, the task will still be running. In case the task already is finished, the method will return immediately while returning true.

The result will be false if the wait timed out and true in any other case.