QGIS API Documentation  3.4.3-Madeira (2f64a3c)
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QgsTask Class Referenceabstract

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

#include <qgstaskmanager.h>

Inheritance diagram for QgsTask:
Inheritance graph
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Public Types

enum  Flag { CanCancel = 1 << 1, 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...
 
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 53 of file qgstaskmanager.h.

Member Enumeration Documentation

◆ Flag

Task flags.

Enumerator
CanCancel 

Task can be canceled.

AllFlags 

Task supports all flags.

Definition at line 70 of file qgstaskmanager.h.

◆ 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 155 of file qgstaskmanager.h.

◆ 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 60 of file qgstaskmanager.h.

Constructor & Destructor Documentation

◆ QgsTask()

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

Constructor for QgsTask.

Parameters
descriptiontext description of task
flagstask flags

Definition at line 28 of file qgstaskmanager.cpp.

◆ ~QgsTask()

QgsTask::~QgsTask ( )
override

Definition at line 34 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 135 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 94 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 QgsVectorLayerExporterTask, QgsMapRendererTask, QgsVirtualLayerTask, QgsNetworkContentFetcherTask, QgsRasterFileWriterTask, QgsProcessingAlgRunnerTask, and QgsVectorFileWriterTask.

Definition at line 79 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 143 of file qgstaskmanager.cpp.

◆ description()

QString QgsTask::description ( ) const
inline

Returns the task's description.

Definition at line 110 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 45 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 QgsVectorLayerExporterTask, QgsMapRendererTask, QgsVectorFileWriterTask, QgsRasterFileWriterTask, and QgsProcessingAlgRunnerTask.

Definition at line 272 of file qgstaskmanager.h.

◆ flags()

Flags QgsTask::flags ( ) const
inline

Returns the flags associated with the task.

Definition at line 89 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 106 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 100 of file qgstaskmanager.h.

◆ isCanceled()

bool QgsTask::isCanceled ( ) const
inlineprotected

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 279 of file qgstaskmanager.h.

◆ progress()

double QgsTask::progress ( ) const
inline

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

Definition at line 115 of file qgstaskmanager.h.

◆ progressChanged

void QgsTask::progressChanged ( double  progress)
signal

Will be emitted by task when its progress changes.

Parameters
progresspercent 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 QgsVectorLayerExporterTask, QgsMapRendererTask, QgsVectorFileWriterTask, QgsRasterFileWriterTask, QgsNetworkContentFetcherTask, QgsVirtualLayerTask, QgsProcessingAlgRunnerTask, QgsProxyProgressTask, 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 172 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
progresspercent of progress, from 0.0 - 100.0

Definition at line 208 of file qgstaskmanager.cpp.

◆ status()

TaskStatus QgsTask::status ( ) const
inline

Returns the current task status.

Definition at line 105 of file qgstaskmanager.h.

◆ statusChanged

void QgsTask::statusChanged ( int  status)
signal

Will be emitted by task when its status changes.

Parameters
statusnew 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 120 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 true.

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

Definition at line 148 of file qgstaskmanager.cpp.

Friends And Related Function Documentation

◆ QgsTaskManager

friend class QgsTaskManager
friend

Definition at line 354 of file qgstaskmanager.h.

◆ QgsTaskRunnableWrapper

friend class QgsTaskRunnableWrapper
friend

Definition at line 355 of file qgstaskmanager.h.

◆ TestQgsTaskManager

friend class TestQgsTaskManager
friend

Definition at line 356 of file qgstaskmanager.h.


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