QgsProcessingAlgorithm class

enum QgsProcessingAlgorithm::Flag

Flags indicating how and when an algorithm operates and should be exposed to users.

QgsProcessingAlgorithm::QgsProcessingAlgorithm() defaulted

Constructor for QgsProcessingAlgorithm.

initAlgorithm() should be called after creating an algorithm to ensure it can correctly configure its parameterDefinitions() and outputDefinitions(). Alternatively, calling create() will return a pre-initialized copy of the algorithm.

QString QgsProcessingAlgorithm::asPythonCommand(const QVariantMap& parameters, QgsProcessingContext& context) const virtual

Returns a Python command string which can be executed to run the algorithm using the specified parameters.

Algorithms which cannot be run from a Python command should return an empty string.

bool QgsProcessingAlgorithm::canExecute(QString* errorMessage = nullptr) const virtual

Returns true if the algorithm can execute.

Algorithm subclasses can return false here to indicate that they are not able to execute, e.g. as a result of unmet external dependencies. If specified, the errorMessage argument will be filled with a localised error message describing why the algorithm cannot execute.

bool QgsProcessingAlgorithm::checkParameterValues(const QVariantMap& parameters, QgsProcessingContext& context, QString* message = nullptr) const virtual

Checks the supplied parameter values to verify that they satisfy the requirements of this algorithm in the supplied context.

The message parameter will be filled with explanatory text if validation fails. Overridden implementations should also check this base class implementation.

QgsProcessingAlgorithm* QgsProcessingAlgorithm::create(const QVariantMap& configuration = QVariantMap()) const

Creates a copy of the algorithm, ready for execution.

This method returns a new, preinitialized copy of the algorithm, ready for executing.

The configuration argument allows passing of a map of configuration settings to the algorithm, allowing it to dynamically adjust its initialized parameters and outputs according to this configuration. This is generally used only for algorithms in a model, allowing them to adjust their behavior at run time according to some user configuration.

QWidget* QgsProcessingAlgorithm::createCustomParametersWidget(QWidget* parent = nullptr) const virtual

If an algorithm subclass implements a custom parameters widget, a copy of this widget should be constructed and returned by this method.

The base class implementation returns nullptr, which indicates that an autogenerated parameters widget should be used.

QgsExpressionContext QgsProcessingAlgorithm::createExpressionContext(const QVariantMap& parameters, QgsProcessingContext& context, QgsProcessingFeatureSource* source = nullptr) const

Creates an expression context relating to the algorithm.

This can be called by algorithms to create a new expression context ready for evaluating expressions within the algorithm. Optionally, a source can be specified which will be used to populate the context if it implements the QgsExpressionContextGenerator interface.

QgsProcessingParameterDefinitions QgsProcessingAlgorithm::destinationParameterDefinitions() const

Returns a list of destination parameters definitions utilized by the algorithm.

QString QgsProcessingAlgorithm::displayName() const pure virtual

Returns the translated algorithm name, which should be used for any user-visible display of the algorithm name.

Algorithm display names should be short, e.g. ideally no more than 3 or 4 words. The name should use sentence case (e.g. "Raster layer statistics", not "Raster Layer Statistics").

Flags QgsProcessingAlgorithm::flags() const virtual

Returns the flags indicating how and when the algorithm operates and should be exposed to users.

Default flags are FlagSupportsBatch and FlagCanCancel.

QString QgsProcessingAlgorithm::group() const virtual

Returns the name of the group this algorithm belongs to.

This string should be localised.

QString QgsProcessingAlgorithm::groupId() const virtual

Returns the unique ID of the group this algorithm belongs to.

This string should be fixed for the algorithm, and must not be localised. The group id should be unique within each provider. Group id should contain lowercase alphanumeric characters only and no spaces or other formatting characters.

Q_DECL_DEPRECATED QString QgsProcessingAlgorithm::helpString() const virtual

Returns a localised help string for the algorithm.

Algorithm subclasses should implement either helpString() or helpUrl().

QString QgsProcessingAlgorithm::helpUrl() const virtual

Returns a url pointing to the algorithm's help page.

QIcon QgsProcessingAlgorithm::icon() const virtual

Returns an icon for the algorithm.

QString QgsProcessingAlgorithm::id() const

Returns the unique ID for the algorithm, which is a combination of the algorithm provider's ID and the algorithms unique name (e.g.

"qgis:mergelayers" ).

QString QgsProcessingAlgorithm::name() const pure virtual

Returns the algorithm name, used for identifying the algorithm.

This string should be fixed for the algorithm, and must not be localised. The name should be unique within each provider. Names should contain lowercase alphanumeric characters only and no spaces or other formatting characters.

const QgsProcessingOutputDefinition* QgsProcessingAlgorithm::outputDefinition(const QString& name) const

Returns a matching output by name.

Matching is done in a case-insensitive manner.

QgsProcessingOutputDefinitions QgsProcessingAlgorithm::outputDefinitions() const

Returns an ordered list of output definitions utilized by the algorithm.

const QgsProcessingParameterDefinition* QgsProcessingAlgorithm::parameterDefinition(const QString& name) const

Returns a matching parameter by name.

Matching is done in a case-insensitive manner, but exact case matches will be preferred.

QgsProcessingParameterDefinitions QgsProcessingAlgorithm::parameterDefinitions() const

Returns an ordered list of parameter definitions utilized by the algorithm.

QVariantMap QgsProcessingAlgorithm::postProcess(QgsProcessingContext& context, QgsProcessingFeedback* feedback)

Should be called in the main thread following the completion of runPrepared().

This method allows the algorithm to perform any required cleanup tasks. The returned variant map includes the results evaluated by the algorithm.

bool QgsProcessingAlgorithm::prepare(const QVariantMap& parameters, QgsProcessingContext& context, QgsProcessingFeedback* feedback)

Prepares the algorithm for execution.

This must be run in the main thread, and allows the algorithm to pre-evaluate input parameters in a thread-safe manner. This must be called before calling runPrepared() (which is safe to do in any thread).

QVariantMap QgsProcessingAlgorithm::preprocessParameters(const QVariantMap& parameters) virtual

Pre-processes a set of parameters, allowing the algorithm to clean their values.

This method is automatically called after users enter parameters, e.g. via the algorithm dialog. This method should NOT be called manually by algorithms.

QVariantMap QgsProcessingAlgorithm::run(const QVariantMap& parameters, QgsProcessingContext& context, QgsProcessingFeedback* feedback, bool* ok = nullptr, const QVariantMap& configuration = QVariantMap()) const

Executes the algorithm using the specified parameters.

This method internally creates a copy of the algorithm before running it, so it is safe to call on algorithms directly retrieved from QgsProcessingRegistry and QgsProcessingProvider.

The context argument specifies the context in which the algorithm is being run.

Algorithm progress should be reported using the supplied feedback object.

If specified, ok will be set to true if algorithm was successfully run.

QVariantMap QgsProcessingAlgorithm::runPrepared(const QVariantMap& parameters, QgsProcessingContext& context, QgsProcessingFeedback* feedback)

Runs the algorithm, which has been prepared by an earlier call to prepare().

This method is safe to call from any thread. Returns true if the algorithm was successfully executed. After runPrepared() has finished, the postProcess() method should be called from the main thread to allow the algorithm to perform any required cleanup tasks and return its final result.

void QgsProcessingAlgorithm::setProvider(QgsProcessingProvider* provider)

Associates this algorithm with its provider.

No transfer of ownership is involved.

QString QgsProcessingAlgorithm::shortDescription() const virtual

Returns an optional translated short description of the algorithm.

This should be at most a single sentence, e.g. "Converts 2D features to 3D by sampling a DEM raster."

QString QgsProcessingAlgorithm::shortHelpString() const virtual

Returns a localised short helper string for the algorithm.

This string should provide a basic description about what the algorithm does and the parameters and outputs associated with it.

QString QgsProcessingAlgorithm::svgIconPath() const virtual

Returns a path to an SVG version of the algorithm's icon.

QStringList QgsProcessingAlgorithm::tags() const virtual

Returns a list of tags which relate to the algorithm, and are used to assist users in searching for suitable algorithms.

These tags should be localised.

bool QgsProcessingAlgorithm::validateInputCrs(const QVariantMap& parameters, QgsProcessingContext& context) const virtual

Checks whether the coordinate reference systems for the specified set of parameters are valid for the algorithm.

For instance, the base implementation performs checks to ensure that all input CRS are equal Returns true if parameters have passed the CRS check.

static QString QgsProcessingAlgorithm::invalidRasterError(const QVariantMap& parameters, const QString& name) protected

Returns a user-friendly string to use as an error when a raster layer input could not be loaded.

The parameters argument should give the algorithms parameter map, and the name should correspond to the invalid source parameter name.

static QString QgsProcessingAlgorithm::invalidSinkError(const QVariantMap& parameters, const QString& name) protected

Returns a user-friendly string to use as an error when a sink parameter could not be created.

The parameters argument should give the algorithms parameter map, and the name should correspond to the invalid source parameter name.

static QString QgsProcessingAlgorithm::invalidSourceError(const QVariantMap& parameters, const QString& name) protected

Returns a user-friendly string to use as an error when a source parameter could not be loaded.

The parameters argument should give the algorithms parameter map, and the name should correspond to the invalid source parameter name.

bool QgsProcessingAlgorithm::addOutput(QgsProcessingOutputDefinition* outputDefinition) protected

Adds an output definition to the algorithm.

Ownership of the definition is transferred to the algorithm. Returns true if the output could be successfully added, or false if the output could not be added (e.g. as a result of a duplicate name).

This should usually be called from a subclass' initAlgorithm() implementation.

Note that in some cases output creation can be automatically performed when calling addParameter(). See the notes in addParameter() for a description of when this occurs.

bool QgsProcessingAlgorithm::addParameter(QgsProcessingParameterDefinition* parameterDefinition, bool createOutput = true) protected

Adds a parameter definition to the algorithm.

Ownership of the definition is transferred to the algorithm. Returns true if parameter could be successfully added, or false if the parameter could not be added (e.g. as a result of a duplicate name).

This should usually be called from a subclass' initAlgorithm() implementation.

If the createOutput argument is true, then a corresponding output definition will also be created (and added to the algorithm) where appropriate. E.g. when adding a QgsProcessingParameterVectorDestination and createOutput is true, then a QgsProcessingOutputVectorLayer output will be created and added to the algorithm. There is no need to call addOutput() to manually add a corresponding output for this vector. If createOutput is false then this automatic output creation will not occur.

QgsProcessingAlgorithm* QgsProcessingAlgorithm::createInstance() const pure virtual protected

Creates a new instance of the algorithm class.

This method should return a 'pristine' instance of the algorithm class.

void QgsProcessingAlgorithm::initAlgorithm(const QVariantMap& configuration = QVariantMap()) pure virtual protected

Initializes the algorithm using the specified configuration.

This should be called directly after creating algorithms and before retrieving any parameterDefinitions() or outputDefinitions().

Subclasses should use their implementations to add all required input parameter and output definitions (which can be dynamically adjusted according to configuration).

Dynamic configuration can be used by algorithms which alter their behavior when used inside processing models. For instance, a "feature router" type algorithm which sends input features to one of any number of outputs sinks based on some preconfigured filter parameters can use the init method to create these outputs based on the specified configuration.

QString QgsProcessingAlgorithm::parameterAsCompatibleSourceLayerPath(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context, const QStringList& compatibleFormats, const QString& preferredFormat = QString("shp"), QgsProcessingFeedback* feedback = nullptr) protected

Evaluates the parameter with matching name to a source vector layer file path of compatible format.

If the parameter is evaluated to an existing layer, and that layer is not of the format listed in the compatibleFormats argument, then the layer will first be exported to a compatible format in a temporary location. The function will then return the path to that temporary file.

compatibleFormats should consist entirely of lowercase file extensions, e.g. 'shp'.

The preferredFormat argument is used to specify to desired file extension to use when a temporary layer export is required.

QgsRectangle QgsProcessingAlgorithm::parameterAsExtent(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context, const QgsCoordinateReferenceSystem& crs = QgsCoordinateReferenceSystem()) const protected

Evaluates the parameter with matching name to a rectangular extent.

If crs is set, and the original coordinate reference system of the parameter can be determined, then the extent will be automatically reprojected so that it is in the specified crs. In this case the extent of the reproject rectangle will be returned.

QgsCoordinateReferenceSystem QgsProcessingAlgorithm::parameterAsExtentCrs(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) protected

Returns the coordinate reference system associated with an extent parameter value.

QgsGeometry QgsProcessingAlgorithm::parameterAsExtentGeometry(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context, const QgsCoordinateReferenceSystem& crs = QgsCoordinateReferenceSystem()) protected

Evaluates the parameter with matching name to a rectangular extent, and returns a geometry covering this extent.

If crs is set, and the original coordinate reference system of the parameter can be determined, then the extent will be automatically reprojected so that it is in the specified crs. Unlike parameterAsExtent(), the reprojected rectangle returned by this function will no longer be a rectangle itself (i.e. this method returns the geometry of the actual reprojected rectangle, while parameterAsExtent() returns just the extent of the reprojected rectangle).

QList<int> QgsProcessingAlgorithm::parameterAsInts(const QVariantMap& parameters, const QString& name, const QgsProcessingContext& context) const protected

Evaluates the parameter with matching name to a list of integer values.

QgsMapLayer* QgsProcessingAlgorithm::parameterAsLayer(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) const protected

Evaluates the parameter with matching name to a map layer.

Layers will either be taken from context's active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

QVariantList QgsProcessingAlgorithm::parameterAsMatrix(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) const protected

Evaluates the parameter with matching name to a matrix/table of values.

Tables are collapsed to a 1 dimensional list.

QgsMeshLayer* QgsProcessingAlgorithm::parameterAsMeshLayer(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) const protected

Evaluates the parameter with matching name to a mesh layer.

Layers will either be taken from context's active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

QgsPointXY QgsProcessingAlgorithm::parameterAsPoint(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context, const QgsCoordinateReferenceSystem& crs = QgsCoordinateReferenceSystem()) const protected

Evaluates the parameter with matching name to a point.

If crs is set then the point will be automatically reprojected so that it is in the specified crs.

QgsCoordinateReferenceSystem QgsProcessingAlgorithm::parameterAsPointCrs(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) protected

Returns the coordinate reference system associated with an point parameter value.

QgsRasterLayer* QgsProcessingAlgorithm::parameterAsRasterLayer(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) const protected

Evaluates the parameter with matching name to a raster layer.

Layers will either be taken from context's active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

QgsFeatureSink* QgsProcessingAlgorithm::parameterAsSink(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context, QString& destinationIdentifier, const QgsFields& fields, QgsWkbTypes::Type geometryType = QgsWkbTypes::NoGeometry, const QgsCoordinateReferenceSystem& crs = QgsCoordinateReferenceSystem(), QgsFeatureSink::SinkFlags sinkFlags = nullptr) const protected

Evaluates the parameter with matching name to a feature sink.

Sinks will either be taken from context's active project, or created from external providers and stored temporarily in the context.

The fields, geometryType and crs parameters dictate the properties of the resulting feature sink.

The destinationIdentifier argument will be set to a string which can be used to retrieve the layer corresponding to the sink, e.g. via calling QgsProcessingUtils::mapLayerFromString().

This function creates a new object and the caller takes responsibility for deleting the returned object.

QgsProcessingFeatureSource* QgsProcessingAlgorithm::parameterAsSource(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) const protected

Evaluates the parameter with matching name to a feature source.

Sources will either be taken from context's active project, or loaded from external sources and stored temporarily in the context.

This function creates a new object and the caller takes responsibility for deleting the returned object.

QgsVectorLayer* QgsProcessingAlgorithm::parameterAsVectorLayer(const QVariantMap& parameters, const QString& name, QgsProcessingContext& context) const protected

Evaluates the parameter with matching name to a vector layer.

Layers will either be taken from context's active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

QVariantMap QgsProcessingAlgorithm::postProcessAlgorithm(QgsProcessingContext& context, QgsProcessingFeedback* feedback) virtual protected

Allows the algorithm to perform any required cleanup tasks.

The returned variant map includes the results evaluated by the algorithm. These may be output layer references, or calculated values such as statistical calculations.

The context argument specifies the context in which the algorithm was run.

Postprocess progress should be reported using the supplied feedback object. Additionally, well-behaved algorithms should periodically check feedback to determine whether the post processing should be canceled and exited early.

postProcessAlgorithm should be used to handle any thread-sensitive cleanup which is required by the algorithm. It will always be called from the same thread that context has thread affinity with. While this will generally be the main thread, it is not guaranteed. For instance, algorithms which are run as a step in a larger model or as a subcomponent of a script-based algorithm will call postProcessAlgorithm from the same thread as that model/script it being executed in.

postProcessAlgorithm will not be called if the prepareAlgorithm() step failed (returned false), or if an exception was raised by the processAlgorithm() step.

bool QgsProcessingAlgorithm::prepareAlgorithm(const QVariantMap& parameters, QgsProcessingContext& context, QgsProcessingFeedback* feedback) virtual protected

Prepares the algorithm to run using the specified parameters.

Algorithms should implement their logic for evaluating parameter values here. The evaluated parameter results should be stored in member variables ready for a call to processAlgorithm().

The context argument specifies the context in which the algorithm is being run.

prepareAlgorithm should be used to handle any thread-sensitive preparation which is required by the algorithm. It will always be called from the same thread that context has thread affinity with. While this will generally be the main thread, it is not guaranteed. For instance, algorithms which are run as a step in a larger model or as a subcomponent of a script-based algorithm will call prepareAlgorithm from the same thread as that model/script it being executed in.

Note that the processAlgorithm step uses a temporary context with affinity for the thread in which the algorithm is executed, making it safe for processAlgorithm implementations to load sources and sinks without issue. Implementing prepareAlgorithm is only required if special thread safe handling is required by the algorithm.

Algorithm preparation progress should be reported using the supplied feedback object. Additionally, well-behaved algorithms should periodically check feedback to determine whether the algorithm should be canceled and exited early.

If the preparation was successful algorithms must return true. If a false value is returned this indicates that the preparation could not be completed, and the algorithm execution will be canceled.

QVariantMap QgsProcessingAlgorithm::processAlgorithm(const QVariantMap& parameters, QgsProcessingContext& context, QgsProcessingFeedback* feedback) pure virtual protected

Runs the algorithm using the specified parameters.

Algorithms should implement their custom processing logic here.

The context argument gives a temporary context with thread affinity matching the thread in which the algorithm is being run. This is a cut-back copy of the context passed to the prepareAlgorithm() and postProcessAlgorithm() steps, but it is generally safe for most algorithms to utilize this context for loading layers and creating sinks. Any loaded layers or sinks created within this temporary context will be transferred back to the main execution context upon successful completion of the processAlgorithm() step.

Algorithm progress should be reported using the supplied feedback object. Additionally, well-behaved algorithms should periodically check feedback to determine whether the algorithm should be canceled and exited early.

This method will not be called if the prepareAlgorithm() step failed (returned false).

c++ implementations of processAlgorithm can throw the QgsProcessingException exception to indicate that a fatal error occurred within the execution. Python based subclasses should raise GeoAlgorithmExecutionException for the same purpose.

bool QgsProcessingAlgorithm::supportInPlaceEdit(const QgsMapLayer* layer) const virtual protected

Checks whether this algorithm supports in-place editing on the given layer Default implementation returns false.