[feature] New function "load_layer"

This function (available only in Processing expressions for now),
allows loading a map layer via a source string and provider name.

It is designed to allow use of the expression functions which
directly reference map layers (such as the aggregate functions)
with a hardcoded layer path, eg. then permitting these functions
to be used outside of a project (such as via the qgis_process tool)

python/core/auto_generated/expression/qgsexpressioncontextutils.sip.in

@@ -388,6 +388,7 @@ Creates a new scope which contains functions relating to mesh layer element ``el
 };
 
 
+
 /************************************************************************
  * This file has been generated automatically from                      *
  *                                                                      *

resources/function_help/json/load_layer

@@ -0,0 +1,19 @@
+{
+  "name": "load_layer",
+  "type": "function",
+  "groups": ["Map Layers"],
+  "description": "Loads a layer by source URI and provider name.",
+  "arguments": [{
+    "arg": "uri",
+    "description": "layer source URI string"
+  },
+  { 
+    "arg": "provider",
+    "description": "layer data provider name"
+  }],
+  "examples": [{
+    "expression": "layer_property(load_layer('c:/data/roads.shp', 'ogr'), 'feature_count')",
+    "returns": "count of features from the c:/data/roads.shp vector layer"
+  }],
+  "tags": ["layer", "vector", "raster", "mesh", "point", "cloud"]
+}

src/core/expression/qgsexpressioncontextutils.cpp

@@ -35,6 +35,9 @@
 #include "qgstriangularmesh.h"
 #include "qgsvectortileutils.h"
 #include "qgsmeshlayer.h"
+#include "qgsexpressionnodeimpl.h"
+#include "qgsproviderregistry.h"
+#include "qgsmaplayerfactory.h"
 
 QgsExpressionContextScope *QgsExpressionContextUtils::globalScope()
 {
@@ -917,6 +920,7 @@ void QgsExpressionContextUtils::registerContextFunctions()
   QgsExpression::registerFunction( new GetProcessingParameterValue( QVariantMap() ) );
   QgsExpression::registerFunction( new GetCurrentFormFieldValue( ) );
   QgsExpression::registerFunction( new GetCurrentParentFormFieldValue( ) );
+  QgsExpression::registerFunction( new LoadLayerFunction( ) );
 }
 
 bool QgsScopedExpressionFunction::usesGeometry( const QgsExpressionNodeFunction *node ) const
@@ -1296,4 +1300,110 @@ QgsExpressionContextScope *QgsExpressionContextUtils::meshExpressionScope( QgsMe
 
   return scope.release();
 }
+
+
+QVariant LoadLayerFunction::func( const QVariantList &, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * )
+{
+  parent->setEvalErrorString( QObject::tr( "Invalid arguments for load_layer function" ) );
+  return QVariant();
+}
+
+bool LoadLayerFunction::isStatic( const QgsExpressionNodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const
+{
+  if ( node->args()->count() > 1 )
+  {
+    if ( !context )
+      return false;
+
+    QPointer< QgsMapLayerStore > store( context->loadedLayerStore() );
+    if ( !store )
+    {
+      parent->setEvalErrorString( QObject::tr( "load_layer cannot be used in this context" ) );
+      return false;
+    }
+
+    QgsExpressionNode *uriNode = node->args()->at( 0 );
+    QgsExpressionNode *providerNode = node->args()->at( 1 );
+    if ( !uriNode->isStatic( parent, context ) )
+    {
+      parent->setEvalErrorString( QObject::tr( "load_layer requires a static value for the uri argument" ) );
+      return false;
+    }
+    if ( !providerNode->isStatic( parent, context ) )
+    {
+      parent->setEvalErrorString( QObject::tr( "load_layer requires a static value for the provider argument" ) );
+      return false;
+    }
+
+    const QString uri = uriNode->eval( parent, context ).toString();
+    if ( uri.isEmpty() )
+    {
+      parent->setEvalErrorString( QObject::tr( "Invalid uri argument for load_layer" ) );
+      return false;
+    }
+
+    const QString providerKey = providerNode->eval( parent, context ).toString();
+    if ( providerKey.isEmpty() )
+    {
+      parent->setEvalErrorString( QObject::tr( "Invalid provider argument for load_layer" ) );
+      return false;
+    }
+
+    const QgsCoordinateTransformContext transformContext = context->variable( QStringLiteral( "_project_transform_context" ) ).value<QgsCoordinateTransformContext>();
+
+    bool res = false;
+    auto loadLayer = [ uri, providerKey, store, node, parent, &res, &transformContext ]
+    {
+      QgsProviderMetadata *metadata = QgsProviderRegistry::instance()->providerMetadata( providerKey );
+      if ( !metadata )
+      {
+        parent->setEvalErrorString( QObject::tr( "Invalid provider argument for load_layer" ) );
+        return;
+      }
+
+      if ( metadata->supportedLayerTypes().empty() )
+      {
+        parent->setEvalErrorString( QObject::tr( "Cannot use %1 provider for load_layer" ).arg( providerKey ) );
+        return;
+      }
+
+      QgsMapLayerFactory::LayerOptions layerOptions( transformContext );
+      layerOptions.loadAllStoredStyles = false;
+      layerOptions.loadDefaultStyle = false;
+
+      QgsMapLayer *layer = QgsMapLayerFactory::createLayer( uri, uri, metadata->supportedLayerTypes().value( 0 ), layerOptions, providerKey );
+      if ( !layer )
+      {
+        parent->setEvalErrorString( QObject::tr( "Could not load_layer with uri: %1" ).arg( uri ) );
+        return;
+      }
+      if ( !layer->isValid() )
+      {
+        delete layer;
+        parent->setEvalErrorString( QObject::tr( "Could not load_layer with uri: %1" ).arg( uri ) );
+        return;
+      }
+
+      store->addMapLayer( layer );
+
+      node->setCachedStaticValue( QVariant::fromValue( QgsWeakMapLayerPointer( layer ) ) );
+      res = true;
+    };
+
+    // Make sure we load the layer on the thread where the store lives
+    if ( QThread::currentThread() == store->thread() )
+      loadLayer();
+    else
+      QMetaObject::invokeMethod( store, loadLayer, Qt::BlockingQueuedConnection );
+
+    return res;
+  }
+  return false;
+}
+
+QgsScopedExpressionFunction *LoadLayerFunction::clone() const
+{
+  return new LoadLayerFunction();
+}
 ///@endcond
+

src/core/expression/qgsexpressioncontextutils.h

@@ -359,6 +359,24 @@ class CORE_EXPORT QgsExpressionContextUtils
 
 };
 
+///@cond PRIVATE
+#ifndef SIP_RUN
+class LoadLayerFunction : public QgsScopedExpressionFunction
+{
+  public:
+    LoadLayerFunction()
+      : QgsScopedExpressionFunction( QStringLiteral( "load_layer" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "uri" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "provider" ) ), QStringLiteral( "Map Layers" ) )
+    {}
+
+    QVariant func( const QVariantList &, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * ) override;
+    bool isStatic( const QgsExpressionNodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const override;
+
+    QgsScopedExpressionFunction *clone() const override;
+
+};
+#endif
+///@endcond
+
 #ifndef SIP_RUN
 
 /**

src/core/qgsexpressioncontext.cpp

@@ -17,6 +17,7 @@
 #include "qgsxmlutils.h"
 #include "qgsexpression.h"
 #include "qgsmaplayerstore.h"
+#include "qgsexpressioncontextutils.h"
 
 const QString QgsExpressionContext::EXPR_FIELDS( QStringLiteral( "_fields_" ) );
 const QString QgsExpressionContext::EXPR_ORIGINAL_VALUE( QStringLiteral( "value" ) );
@@ -295,9 +296,15 @@ bool QgsExpressionContextScope::writeXml( QDomElement &element, QDomDocument &do
 // QgsExpressionContext
 //
 
+QgsExpressionContext::QgsExpressionContext()
+{
+  mLoadLayerFunction = std::make_unique< LoadLayerFunction >();
+}
+
 QgsExpressionContext::QgsExpressionContext( const QList<QgsExpressionContextScope *> &scopes )
   : mStack( scopes )
 {
+  mLoadLayerFunction = std::make_unique< LoadLayerFunction >();
 }
 
 QgsExpressionContext::QgsExpressionContext( const QgsExpressionContext &other ) : mStack{}
@@ -311,6 +318,7 @@ QgsExpressionContext::QgsExpressionContext( const QgsExpressionContext &other )
   mCachedValues = other.mCachedValues;
   mFeedback = other.mFeedback;
   mDestinationStore = other.mDestinationStore;
+  mLoadLayerFunction = std::make_unique< LoadLayerFunction >();
 }
 
 QgsExpressionContext &QgsExpressionContext::operator=( QgsExpressionContext &&other ) noexcept
@@ -533,8 +541,10 @@ QString QgsExpressionContext::description( const QString &name ) const
 
 bool QgsExpressionContext::hasFunction( const QString &name ) const
 {
-  const auto constMStack = mStack;
-  for ( const QgsExpressionContextScope *scope : constMStack )
+  if ( name.compare( QLatin1String( "load_layer" ) ) == 0 && mDestinationStore )
+    return true;
+
+  for ( const QgsExpressionContextScope *scope : mStack )
   {
     if ( scope->hasFunction( name ) )
       return true;
@@ -551,6 +561,10 @@ QStringList QgsExpressionContext::functionNames() const
     for ( const QString &name : functionNames )
       result.insert( name );
   }
+
+  if ( mDestinationStore )
+    result.insert( QStringLiteral( "load_layer" ) );
+
   QStringList listResult( result.constBegin(), result.constEnd() );
   listResult.sort();
   return listResult;
@@ -558,6 +572,11 @@ QStringList QgsExpressionContext::functionNames() const
 
 QgsExpressionFunction *QgsExpressionContext::function( const QString &name ) const
 {
+  if ( name.compare( QLatin1String( "load_layer" ) ) == 0 && mDestinationStore )
+  {
+    return mLoadLayerFunction.get();
+  }
+
   //iterate through stack backwards, so that higher priority variables take precedence
   QList< QgsExpressionContextScope * >::const_iterator it = mStack.constEnd();
   while ( it != mStack.constBegin() )

src/core/qgsexpressioncontext.h

@@ -29,6 +29,7 @@
 
 class QgsReadWriteContext;
 class QgsMapLayerStore;
+class LoadLayerFunction;
 
 /**
  * \ingroup core
@@ -481,7 +482,7 @@ class CORE_EXPORT QgsExpressionContext
   public:
 
     //! Constructor for QgsExpressionContext
-    QgsExpressionContext() = default;
+    QgsExpressionContext();
 
     /**
      * Initializes the context with given list of scopes.
@@ -936,6 +937,7 @@ class CORE_EXPORT QgsExpressionContext
 
     QgsFeedback *mFeedback = nullptr;
 
+    std::unique_ptr< LoadLayerFunction > mLoadLayerFunction;
     QPointer< QgsMapLayerStore > mDestinationStore;
 
     // Cache is mutable because we want to be able to add cached values to const contexts

tests/src/core/testqgsexpression.cpp

@@ -5650,6 +5650,66 @@ class TestQgsExpression: public QObject
       QCOMPARE( QgsExpressionUtils::toLocalizedString( QString( "hello world" ) ), QStringLiteral( "hello world" ) );
     }
 
+    void testLoadLayer()
+    {
+      QgsExpressionContext context;
+      QgsMapLayerStore store;
+
+      // load_layer is only available when a destination store is set
+      QVERIFY( !context.hasFunction( QStringLiteral( "load_layer" ) ) );
+      QVERIFY( !context.functionNames().contains( QStringLiteral( "load_layer" ) ) );
+      QVERIFY( !context.function( QStringLiteral( "load_layer" ) ) );
+
+      context.setLoadedLayerStore( &store );
+      QVERIFY( context.hasFunction( QStringLiteral( "load_layer" ) ) );
+      QVERIFY( context.functionNames().contains( QStringLiteral( "load_layer" ) ) );
+      QVERIFY( context.function( QStringLiteral( "load_layer" ) ) );
+
+      const QString pointsFileName = QStringLiteral( TEST_DATA_DIR ) + '/' + "points.shp";
+      QgsExpression exp( QStringLiteral( "layer_property(load_layer('%1', 'ogr'), 'feature_count')" ).arg( pointsFileName ) );
+      QVERIFY( exp.prepare( &context ) );
+      QVERIFY( !exp.hasEvalError() );
+      QCOMPARE( exp.evaluate( &context ).toInt(), 17 );
+
+      // non-static arguments are not allowed
+      QgsFields fields;
+      fields.append( QgsField( QStringLiteral( "first_field" ), QVariant::Int ) );
+
+      QgsFeature f( fields );
+      f.setAttributes( QgsAttributes() << 11 );
+      context.setFields( fields );
+      context.setFeature( f );
+
+      exp = QgsExpression( QStringLiteral( "layer_property(load_layer('%1' || \"first_field\", 'ogr'), 'feature_count')" ).arg( pointsFileName ) );
+      QVERIFY( exp.prepare( &context ) );
+      QVERIFY( exp.hasEvalError() );
+      QCOMPARE( exp.evalErrorString(), QStringLiteral( "load_layer requires a static value for the uri argument" ) );
+
+      exp = QgsExpression( QStringLiteral( "layer_property(load_layer('%1', 'ogr' || \"first_field\"), 'feature_count')" ).arg( pointsFileName ) );
+      QVERIFY( exp.prepare( &context ) );
+      QVERIFY( exp.hasEvalError() );
+      QCOMPARE( exp.evalErrorString(), QStringLiteral( "load_layer requires a static value for the provider argument" ) );
+
+      // invalid provider
+      exp = QgsExpression( QStringLiteral( "layer_property(load_layer('%1', 'magic'), 'feature_count')" ).arg( pointsFileName ) );
+      QVERIFY( exp.prepare( &context ) );
+      QVERIFY( exp.hasEvalError() );
+      QCOMPARE( exp.evalErrorString(), QStringLiteral( "Invalid provider argument for load_layer" ) );
+
+      // invalid uri
+      exp = QgsExpression( QStringLiteral( "layer_property(load_layer('nope', 'ogr'), 'feature_count')" ) );
+      QVERIFY( exp.prepare( &context ) );
+      QVERIFY( exp.hasEvalError() );
+      QCOMPARE( exp.evalErrorString(), QStringLiteral( "Could not load_layer with uri: nope" ) );
+
+      // raster layer
+      const QString rasterFileName = QStringLiteral( TEST_DATA_DIR ) + '/' + "tenbytenraster.asc";
+      exp = QgsExpression( QStringLiteral( "layer_property(load_layer('%1', 'gdal'), 'type')" ).arg( rasterFileName ) );
+      QVERIFY( exp.prepare( &context ) );
+      QVERIFY( !exp.hasEvalError() );
+      QCOMPARE( exp.evaluate( &context ).toString(), QStringLiteral( "Raster" ) );
+    }
+
 };
 
 QGSTEST_MAIN( TestQgsExpression )