[processing] Add method to attempt to reattach model children to linked algorithms

Refs #19857

Author: nyalldawson

python/core/auto_generated/processing/models/qgsprocessingmodelchildalgorithm.sip.in

@@ -71,14 +71,32 @@ Returns the underlying child algorithm's ID.
 .. seealso:: :py:func:`setAlgorithmId`
 %End
 
-    void setAlgorithmId( const QString &algorithmId );
+    bool setAlgorithmId( const QString &algorithmId );
 %Docstring
 Sets the underlying child algorithm's ID. This
 should be set to an existing QgsProcessingAlgorithm algorithm ID.
 
+Returns true if the algorithm was successfully set.
+
+.. seealso:: :py:func:`reattach`
+
 .. seealso:: :py:func:`algorithm`
 
 .. seealso:: :py:func:`algorithmId`
+%End
+
+    bool reattach() const;
+%Docstring
+Attempts to re-attach the child to the algorithm specified by ``algorithmId``().
+
+This can be run to relink the child to algorithms from providers which were not
+originally available for the model to link to.
+
+Returns true if the algorithm was successfully reattached.
+
+.. seealso:: :py:func:`algorithm`
+
+.. seealso:: :py:func:`setAlgorithmId`
 %End
 
     QVariantMap configuration() const;
@@ -110,6 +128,8 @@ to adjust their behavior at run time according to some user configuration.
 Returns the underlying child algorithm, or a None
 if a matching algorithm is not available.
 
+.. seealso:: :py:func:`reattach`
+
 .. seealso:: :py:func:`algorithmId`
 %End
 

src/core/processing/models/qgsprocessingmodelchildalgorithm.cpp

@@ -222,10 +222,16 @@ void QgsProcessingModelChildAlgorithm::generateChildId( const QgsProcessingModel
   mId = id;
 }
 
-void QgsProcessingModelChildAlgorithm::setAlgorithmId( const QString &algorithmId )
+bool QgsProcessingModelChildAlgorithm::setAlgorithmId( const QString &algorithmId )
 {
   mAlgorithmId = algorithmId;
   mAlgorithm.reset( QgsApplication::processingRegistry()->createAlgorithmById( mAlgorithmId, mConfiguration ) );
+  return static_cast< bool >( mAlgorithm.get() );
+}
+
+bool QgsProcessingModelChildAlgorithm::reattach() const
+{
+  return const_cast< QgsProcessingModelChildAlgorithm * >( this )->setAlgorithmId( mAlgorithmId );
 }
 
 ///@endcond

src/core/processing/models/qgsprocessingmodelchildalgorithm.h

@@ -82,10 +82,27 @@ class CORE_EXPORT QgsProcessingModelChildAlgorithm : public QgsProcessingModelCo
     /**
      * Sets the underlying child algorithm's ID. This
      * should be set to an existing QgsProcessingAlgorithm algorithm ID.
+     *
+     * Returns true if the algorithm was successfully set.
+     *
+     * \see reattach()
      * \see algorithm()
      * \see algorithmId()
      */
-    void setAlgorithmId( const QString &algorithmId );
+    bool setAlgorithmId( const QString &algorithmId );
+
+    /**
+     * Attempts to re-attach the child to the algorithm specified by \a algorithmId().
+     *
+     * This can be run to relink the child to algorithms from providers which were not
+     * originally available for the model to link to.
+     *
+     * Returns true if the algorithm was successfully reattached.
+     *
+     * \see algorithm()
+     * \see setAlgorithmId()
+     */
+    bool reattach() const;
 
     /**
      * Returns the child algorithm's configuration map.
@@ -114,6 +131,7 @@ class CORE_EXPORT QgsProcessingModelChildAlgorithm : public QgsProcessingModelCo
     /**
      * Returns the underlying child algorithm, or a nullptr
      * if a matching algorithm is not available.
+     * \see reattach()
      * \see algorithmId()
      */
     const QgsProcessingAlgorithm *algorithm() const;
@@ -289,6 +307,8 @@ class CORE_EXPORT QgsProcessingModelChildAlgorithm : public QgsProcessingModelCo
     //! Whether list of outputs should be collapsed in the graphical modeler
     bool mOutputsCollapsed = true;
 
+    friend class TestQgsProcessing;
+
 };
 
 ///@endcond

tests/src/analysis/testqgsprocessing.cpp

@@ -5539,15 +5539,21 @@ void TestQgsProcessing::modelerAlgorithm()
            QgsProcessingModelChildParameterSource::fromStaticValue( QStringLiteral( "b" ) ) );
 
 
-
-
-
   QgsProcessingModelChildAlgorithm child( QStringLiteral( "some_id" ) );
   QCOMPARE( child.algorithmId(), QStringLiteral( "some_id" ) );
   QVERIFY( !child.algorithm() );
-  child.setAlgorithmId( QStringLiteral( "native:centroids" ) );
+  QVERIFY( !child.setAlgorithmId( QStringLiteral( "blah" ) ) );
+  QVERIFY( !child.reattach() );
+  QVERIFY( child.setAlgorithmId( QStringLiteral( "native:centroids" ) ) );
   QVERIFY( child.algorithm() );
   QCOMPARE( child.algorithm()->id(), QStringLiteral( "native:centroids" ) );
+  // bit of a hack -- but try to simulate an algorithm not originally available!
+  child.mAlgorithm.reset();
+  QVERIFY( !child.algorithm() );
+  QVERIFY( child.reattach() );
+  QVERIFY( child.algorithm() );
+  QCOMPARE( child.algorithm()->id(), QStringLiteral( "native:centroids" ) );
+
   QVariantMap myConfig;
   myConfig.insert( QStringLiteral( "some_key" ), 11 );
   child.setConfiguration( myConfig );