Add layout util to retrieve map layer from project by string

Considers both layer id (preferred) and layer names

python/core/layout/qgslayoututils.sip.in

@@ -223,6 +223,17 @@ If the string was correctly decoded, ``ok`` will be set to true.
     static double scaleFactorFromItemStyle( const QStyleOptionGraphicsItem *style );
 %Docstring
 Extracts the scale factor from an item ``style``.
+%End
+
+    static QgsMapLayer *mapLayerFromString( const QString &string, QgsProject *project );
+%Docstring
+Resolves a ``string`` into a map layer from a given ``project``. Attempts different
+forms of layer matching such as matching by layer id or layer name.
+
+Layer names are matched using a case-insensitive check, ONLY if an exact case
+match was not found.
+
+.. versionadded:: 3.2
 %End
 
 };

src/core/layout/qgslayoututils.cpp

@@ -395,6 +395,27 @@ double QgsLayoutUtils::scaleFactorFromItemStyle( const QStyleOptionGraphicsItem
   return !qgsDoubleNear( style->matrix.m11(), 0.0 ) ? style->matrix.m11() : style->matrix.m12();
 }
 
+QgsMapLayer *QgsLayoutUtils::mapLayerFromString( const QString &string, QgsProject *project )
+{
+  // Maybe it's a layer id?
+  if ( QgsMapLayer *ml = project->mapLayer( string ) )
+    return ml;
+
+  // Still nothing? Check for layer name
+  if ( QgsMapLayer *ml = project->mapLayersByName( string ).value( 0 ) )
+    return ml;
+
+  // Still nothing? Check for layer name, case-insensitive
+  const auto layers = project->mapLayers();
+  for ( auto it = layers.constBegin(); it != layers.constEnd(); ++it )
+  {
+    if ( it.value()->name().compare( string, Qt::CaseInsensitive ) == 0 )
+      return it.value();
+  }
+
+  return nullptr;
+}
+
 double QgsLayoutUtils::pointsToMM( const double pointSize )
 {
   //conversion to mm based on 1 point = 1/72 inch

src/core/layout/qgslayoututils.h

@@ -211,6 +211,17 @@ class CORE_EXPORT QgsLayoutUtils
      */
     static double scaleFactorFromItemStyle( const QStyleOptionGraphicsItem *style );
 
+    /**
+     * Resolves a \a string into a map layer from a given \a project. Attempts different
+     * forms of layer matching such as matching by layer id or layer name.
+     *
+     * Layer names are matched using a case-insensitive check, ONLY if an exact case
+     * match was not found.
+     *
+     * \since QGIS 3.2
+     */
+    static QgsMapLayer *mapLayerFromString( const QString &string, QgsProject *project );
+
   private:
 
     //! Scale factor for upscaling fontsize and downscaling painter

tests/src/core/testqgslayoututils.cpp

@@ -54,6 +54,7 @@ class TestQgsLayoutUtils: public QObject
     void largestRotatedRect(); //test largest rotated rect helper function
     void decodePaperOrientation();
     void scaleFactorFromItemStyle();
+    void mapLayerFromString();
 
   private:
 
@@ -638,6 +639,28 @@ void TestQgsLayoutUtils::scaleFactorFromItemStyle()
   QCOMPARE( QgsLayoutUtils::scaleFactorFromItemStyle( &style ), 2.0 );
 }
 
+void TestQgsLayoutUtils::mapLayerFromString()
+{
+  // add some layers to a project
+  QgsVectorLayer *l1 = new QgsVectorLayer( QStringLiteral( "Point?field=col1:integer&field=col2:integer&field=col3:integer" ), QStringLiteral( "layer 1" ), QStringLiteral( "memory" ) );
+  QgsVectorLayer *l2 = new QgsVectorLayer( QStringLiteral( "Point?field=col1:integer&field=col2:integer&field=col3:integer" ), QStringLiteral( "layer 2" ), QStringLiteral( "memory" ) );
+  QgsVectorLayer *l2a = new QgsVectorLayer( QStringLiteral( "Point?field=col1:integer&field=col2:integer&field=col3:integer" ), QStringLiteral( "LAYER 2" ), QStringLiteral( "memory" ) );
+  QgsProject p;
+  p.addMapLayer( l1 );
+  p.addMapLayer( l2 );
+  p.addMapLayer( l2a );
+
+  QCOMPARE( QgsLayoutUtils::mapLayerFromString( "layer 1", &p ), l1 );
+  QCOMPARE( QgsLayoutUtils::mapLayerFromString( "LAYER 1", &p ), l1 );
+  QCOMPARE( QgsLayoutUtils::mapLayerFromString( "layer 2", &p ), l2 );
+  QCOMPARE( QgsLayoutUtils::mapLayerFromString( "LAYER 2", &p ), l2a );
+  QCOMPARE( QgsLayoutUtils::mapLayerFromString( l1->id(), &p ), l1 );
+  QCOMPARE( QgsLayoutUtils::mapLayerFromString( l2->id(), &p ), l2 );
+  QCOMPARE( QgsLayoutUtils::mapLayerFromString( l2a->id(), &p ), l2a );
+  QVERIFY( !QgsLayoutUtils::mapLayerFromString( "none", &p ) );
+
+}
+
 bool TestQgsLayoutUtils::renderCheck( const QString &testName, QImage &image, int mismatchCount )
 {
   mReport += "<h2>" + testName + "</h2>\n";