QgsTask Class Reference

Abstract base class for long running background tasks. More...

#include <qgstaskmanager.h>

Inheritance diagram for QgsTask:

Public Types
enum	Flag {   CanCancel = 1 << 1 , CancelWithoutPrompt = 1 << 2 , Hidden = 1 << 3 , Silent = 1 << 4 ,   AllFlags = CanCancel }
	Task flags. More...
enum	SubTaskDependency { SubTaskIndependent = 0 , ParentDependsOnSubTask }
	Controls how subtasks relate to their parent task. More...
enum	TaskStatus {   Queued , OnHold , Running , Complete ,   Terminated }
	Status of tasks. More...

Signals
void	begun ()
	Will be emitted by task to indicate its commencement. More...
void	progressChanged (double progress)
	Will be emitted by task when its progress changes. More...
void	statusChanged (int status)
	Will be emitted by task when its status changes. More...
void	taskCompleted ()
	Will be emitted by task to indicate its successful completion. More...
void	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). More...

Public Member Functions
	QgsTask (const QString &description=QString(), QgsTask::Flags flags=AllFlags)
	Constructor for QgsTask. More...
	~QgsTask () override
void	addSubTask (QgsTask *subTask, const QgsTaskList &dependencies=QgsTaskList(), SubTaskDependency subTaskDependency=SubTaskIndependent)
	Adds a subtask to this task. More...
bool	canCancel () const
	Returns true if the task can be canceled. More...
virtual void	cancel ()
	Notifies the task that it should terminate. More...
QList< QgsMapLayer * >	dependentLayers () const
	Returns the list of layers on which the task depends. More...
QString	description () const
	Returns the task's description. More...
qint64	elapsedTime () const
	Returns the elapsed time since the task commenced, in milliseconds. More...
Flags	flags () const
	Returns the flags associated with the task. More...
void	hold ()
	Places the task on hold. More...
bool	isActive () const
	Returns true if the task is active, ie it is not complete and has not been canceled. More...
double	progress () const
	Returns the task's progress (between 0.0 and 100.0) More...
void	setDependentLayers (const QList< QgsMapLayer * > &dependentLayers)
	Sets a list of layers on which the task depends. More...
void	setDescription (const QString &description)
	Sets the task's description. More...
TaskStatus	status () const
	Returns the current task status. More...
void	unhold ()
	Releases the task from being held. More...
bool	waitForFinished (int timeout=30000)
	Blocks the current thread until the task finishes or a maximum of timeout milliseconds. More...

Protected Slots
void	setProgress (double progress)
	Sets the task's current progress. More...

Protected Member Functions
virtual void	finished (bool result)
	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). More...
bool	isCanceled () const
	Will return true if task should terminate ASAP. More...
virtual bool	run ()=0
	Performs the task's operation. More...

Friends
class	QgsTaskManager
class	QgsTaskRunnableWrapper
class	TestQgsTaskManager

Detailed Description

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.

Since

QGIS 3.0

Definition at line 54 of file qgstaskmanager.h.

Member Enumeration Documentation

Flag

enum QgsTask::Flag

Task flags.

Enumerator
CanCancel	Task can be canceled.
CancelWithoutPrompt	Task can be canceled without any users prompts, e.g. when closing a project or QGIS.
Hidden	Hide task from GUI (since QGIS 3.26)
Silent	Don't show task updates (such as completion/failure messages) as operating-system level notifications (since QGIS 3.26)
AllFlags	Task supports all flags.

Definition at line 72 of file qgstaskmanager.h.

SubTaskDependency

enum QgsTask::SubTaskDependency

Controls how subtasks relate to their parent task.

Enumerator
SubTaskIndependent	Subtask is independent of the parent, and can run before, after or at the same time as the parent.
ParentDependsOnSubTask	Subtask must complete before parent can begin.

Definition at line 167 of file qgstaskmanager.h.

TaskStatus

enum QgsTask::TaskStatus

Status of tasks.

Enumerator
Queued	Task is queued and has not begun.
OnHold	Task is queued but on hold and will not be started.
Running	Task is currently running.
Complete	Task successfully completed.
Terminated	Task was terminated or errored.

Definition at line 61 of file qgstaskmanager.h.

Constructor & Destructor Documentation

QgsTask()

QgsTask::QgsTask	(	const QString &	description = QString(),
		QgsTask::Flags	flags = AllFlags
	)

Constructor for QgsTask.

Parameters

description	text description of task
flags	task flags

Definition at line 29 of file qgstaskmanager.cpp.

~QgsTask()

QgsTask::~QgsTask ( )

override

Definition at line 37 of file qgstaskmanager.cpp.

Member Function Documentation

addSubTask()

void QgsTask::addSubTask	(	QgsTask *	subTask,
		const QgsTaskList &	dependencies = QgsTaskList(),
		SubTaskDependency	subTaskDependency = 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.

Definition at line 155 of file qgstaskmanager.cpp.

begun

void QgsTask::begun ( )

signal

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

canCancel()

bool QgsTask::canCancel ( ) const

inline

Returns true if the task can be canceled.

Definition at line 106 of file qgstaskmanager.h.

cancel()

void QgsTask::cancel ( )

virtual

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()

Reimplemented in QgsVectorWarperTask, QgsMapRendererTask, QgsNetworkContentFetcherTask, QgsPointCloudLayerExporterTask, QgsProcessingAlgRunnerTask, QgsProviderSublayerTask, QgsProxyProgressTask, QgsVectorFileWriterTask, QgsVirtualLayerTask, QgsRasterFileWriterTask, QgsVectorLayerExporterTask, and QgsVectorLayerFeatureCounter.

Definition at line 91 of file qgstaskmanager.cpp.

dependentLayers()

QList< QgsMapLayer * > QgsTask::dependentLayers

(

)

const

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.

See also

setDependentLayers()

Definition at line 163 of file qgstaskmanager.cpp.

description()

QString QgsTask::description ( ) const

inline

Returns the task's description.

Definition at line 122 of file qgstaskmanager.h.

elapsedTime()

qint64 QgsTask::elapsedTime

(

)

const

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

The value is undefined for tasks which have not begun.

Since

QGIS 3.4

Definition at line 56 of file qgstaskmanager.cpp.

finished()

virtual void QgsTask::finished ( bool  result )

inlineprotectedvirtual

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.

Reimplemented in QgsMapRendererTask, QgsPointCloudLayerExporterTask, QgsProcessingAlgRunnerTask, QgsVectorFileWriterTask, QgsRasterFileWriterTask, and QgsVectorLayerExporterTask.

Definition at line 284 of file qgstaskmanager.h.

flags()

Flags QgsTask::flags ( ) const

inline

Returns the flags associated with the task.

Definition at line 94 of file qgstaskmanager.h.

hold()

void QgsTask::hold

(

)

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()

Definition at line 124 of file qgstaskmanager.cpp.

isActive()

bool QgsTask::isActive ( ) const

inline

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

Definition at line 112 of file qgstaskmanager.h.

isCanceled()

bool QgsTask::isCanceled ( ) const

protected

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.

Definition at line 118 of file qgstaskmanager.cpp.

progress()

double QgsTask::progress ( ) const

inline

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

Definition at line 127 of file qgstaskmanager.h.

progressChanged

void QgsTask::progressChanged ( double  progress )

signal

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()

run()

virtual bool QgsTask::run ( )

protectedpure virtual

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.

Implemented in QgsVectorWarperTask, QgsMapRendererTask, QgsNetworkContentFetcherTask, QgsPointCloudLayerExporterTask, QgsProcessingAlgRunnerTask, QgsProviderSublayerTask, QgsCopyFileTask, QgsProxyProgressTask, QgsVectorFileWriterTask, QgsVirtualLayerTask, QgsRasterFileWriterTask, QgsVectorLayerExporterTask, and QgsVectorLayerFeatureCounter.

setDependentLayers()

void QgsTask::setDependentLayers

(

const QList< QgsMapLayer * > &

dependentLayers

)

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.

See also

dependentLayers()

Definition at line 196 of file qgstaskmanager.cpp.

setDescription()

void QgsTask::setDescription

(

const QString &

description

)

Sets the task's description.

This must be called before adding the task to a QgsTaskManager, changing the description after queuing the task has no effect.

Since

QGIS 3.10

Definition at line 51 of file qgstaskmanager.cpp.

setProgress

void QgsTask::setProgress ( double  progress )

protectedslot

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

Definition at line 232 of file qgstaskmanager.cpp.

status()

TaskStatus QgsTask::status ( ) const

inline

Returns the current task status.

Definition at line 117 of file qgstaskmanager.h.

statusChanged

void QgsTask::statusChanged ( int  status )

signal

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

taskCompleted

void QgsTask::taskCompleted ( )

signal

Will be emitted by task to indicate its successful completion.

Note

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

taskTerminated

void QgsTask::taskTerminated ( )

signal

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

unhold()

void QgsTask::unhold

(

)

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()

Definition at line 139 of file qgstaskmanager.cpp.

waitForFinished()

bool QgsTask::waitForFinished

(

int

timeout = 30000

)

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 \c true.

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

Definition at line 168 of file qgstaskmanager.cpp.

Friends And Related Function Documentation

QgsTaskManager

friend class QgsTaskManager

friend

Definition at line 370 of file qgstaskmanager.h.

QgsTaskRunnableWrapper

friend class QgsTaskRunnableWrapper

friend

Definition at line 371 of file qgstaskmanager.h.

TestQgsTaskManager

friend class TestQgsTaskManager

friend

Definition at line 372 of file qgstaskmanager.h.

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The documentation for this class was generated from the following files:

• /build/qgis-3.28.0+99sid/src/core/qgstaskmanager.h
• /build/qgis-3.28.0+99sid/src/core/qgstaskmanager.cpp