Allow processing plugins to load their own parameters

python/core/processing/qgsprocessingprovider.sip.in

@@ -204,6 +204,17 @@ Returns the matching algorithm by ``name``, or a None if no matching
 algorithm is contained by this provider.
 
 .. seealso:: :py:func:`algorithms`
+%End
+
+    virtual QgsProcessingParameterDefinition *createParameter( const QString &type, const QString &name ) /Factory/;
+%Docstring
+Returns a new parameter from of the given type and name.
+This should be reimplemented by providers that implement custom types.
+If the provider does not implement the parameter with ``type``, a None will be returned.
+
+By default, this returns a None.
+
+.. versionadded:: 3.2
 %End
 
   signals:

src/core/processing/qgsprocessingparameters.cpp

@@ -25,6 +25,7 @@
 #include "qgssettings.h"
 #include "qgsvectorfilewriter.h"
 #include "qgsreferencedgeometry.h"
+#include "qgsprocessingregistry.h"
 #include <functional>
 
 bool QgsProcessingParameters::isDynamic( const QVariantMap &parameters, const QString &name )
@@ -1079,6 +1080,21 @@ QgsProcessingParameterDefinition *QgsProcessingParameters::parameterFromVariantM
     def.reset( new QgsProcessingParameterFolderDestination( name ) );
   else if ( type == QgsProcessingParameterBand::typeName() )
     def.reset( new QgsProcessingParameterBand( name ) );
+  else
+  {
+    const QList<QgsProcessingProvider *> providers = QgsApplication::instance()->processingRegistry()->providers();
+
+    for ( QgsProcessingProvider *provider : providers )
+    {
+      QgsProcessingParameterDefinition *param = provider->createParameter( type, name );
+
+      if ( param )
+      {
+        def.reset( param );
+        break;
+      }
+    }
+  }
 
   if ( !def )
     return nullptr;

src/core/processing/qgsprocessingprovider.cpp

@@ -74,6 +74,13 @@ const QgsProcessingAlgorithm *QgsProcessingProvider::algorithm( const QString &n
   return mAlgorithms.value( name );
 }
 
+QgsProcessingParameterDefinition *QgsProcessingProvider::createParameter( const QString &type, const QString &name )
+{
+  Q_UNUSED( type )
+  Q_UNUSED( name )
+  return nullptr;
+}
+
 bool QgsProcessingProvider::addAlgorithm( QgsProcessingAlgorithm *algorithm )
 {
   if ( !algorithm )

src/core/processing/qgsprocessingprovider.h

@@ -203,6 +203,17 @@ class CORE_EXPORT QgsProcessingProvider : public QObject
      */
     const QgsProcessingAlgorithm *algorithm( const QString &name ) const;
 
+    /**
+     * Returns a new parameter from of the given type and name.
+     * This should be reimplemented by providers that implement custom types.
+     * If the provider does not implement the parameter with \a type, a nullptr will be returned.
+     *
+     * By default, this returns a nullptr.
+     *
+     * \since QGIS 3.2
+     */
+    virtual QgsProcessingParameterDefinition *createParameter( const QString &type, const QString &name ) SIP_FACTORY;
+
   signals:
 
     /**