[api] Add constructor for QgsSpatialIndex which allows for a callback

function during bulk index load from a feature iterator

Allows single-iteration of a source for dual purposes simultaneously,
e.g. doing other feature-based operations while still gaining the full
advantage of the bulk loaded spatial index without having to do multiple
feature iterations

python/core/auto_generated/qgsspatialindex.sip.in

@@ -63,6 +63,7 @@ that of the spatial index construction.
 .. versionadded:: 2.8
 %End
 
+
     explicit QgsSpatialIndex( const QgsFeatureSource &source, QgsFeedback *feedback = 0, QgsSpatialIndex::Flags flags = 0 );
 %Docstring
 Constructor - creates R-tree and bulk loads it with features from the source.

src/core/qgsspatialindex.cpp

@@ -163,10 +163,12 @@ class QgsFeatureIteratorDataStream : public IDataStream
 {
   public:
     //! constructor - needs to load all data to a vector for later access when bulk loading
-    explicit QgsFeatureIteratorDataStream( const QgsFeatureIterator &fi, QgsFeedback *feedback = nullptr, QgsSpatialIndex::Flags flags = nullptr )
+    explicit QgsFeatureIteratorDataStream( const QgsFeatureIterator &fi, QgsFeedback *feedback = nullptr, QgsSpatialIndex::Flags flags = nullptr,
+                                           const std::function< bool( const QgsFeature & ) > *callback = nullptr )
       : mFi( fi )
       , mFeedback( feedback )
       , mFlags( flags )
+      , mCallback( callback )
     {
       readNextEntry();
     }
@@ -207,6 +209,15 @@ class QgsFeatureIteratorDataStream : public IDataStream
       QgsFeatureId id;
       while ( mFi.nextFeature( f ) )
       {
+        if ( mCallback )
+        {
+          bool res = ( *mCallback )( f );
+          if ( !res )
+          {
+            mNextData = nullptr;
+            return;
+          }
+        }
         if ( QgsSpatialIndex::featureInfo( f, r, id ) )
         {
           mNextData = new RTree::Data( 0, nullptr, r, id );
@@ -222,6 +233,7 @@ class QgsFeatureIteratorDataStream : public IDataStream
     RTree::Data *mNextData = nullptr;
     QgsFeedback *mFeedback = nullptr;
     QgsSpatialIndex::Flags mFlags = nullptr;
+    const std::function< bool( const QgsFeature & ) > *mCallback = nullptr;
 
 };
 
@@ -253,10 +265,11 @@ class QgsSpatialIndexData : public QSharedData
      * of \a feedback is not transferred, and callers must take care that the lifetime of feedback exceeds
      * that of the spatial index construction.
      */
-    explicit QgsSpatialIndexData( const QgsFeatureIterator &fi, QgsFeedback *feedback = nullptr, QgsSpatialIndex::Flags flags = nullptr )
+    explicit QgsSpatialIndexData( const QgsFeatureIterator &fi, QgsFeedback *feedback = nullptr, QgsSpatialIndex::Flags flags = nullptr,
+                                  const std::function< bool( const QgsFeature & ) > *callback = nullptr )
       : mFlags( flags )
     {
-      QgsFeatureIteratorDataStream fids( fi, feedback, mFlags );
+      QgsFeatureIteratorDataStream fids( fi, feedback, mFlags, callback );
       initTree( &fids );
       if ( flags & QgsSpatialIndex::FlagStoreFeatureGeometries )
         mGeometries = fids.geometries;
@@ -335,6 +348,11 @@ QgsSpatialIndex::QgsSpatialIndex( const QgsFeatureIterator &fi, QgsFeedback *fee
   d = new QgsSpatialIndexData( fi, feedback, flags );
 }
 
+QgsSpatialIndex::QgsSpatialIndex( const QgsFeatureIterator &fi, const std::function< bool( const QgsFeature & )> &callback, QgsSpatialIndex::Flags flags )
+{
+  d = new QgsSpatialIndexData( fi, nullptr, flags, &callback );
+}
+
 QgsSpatialIndex::QgsSpatialIndex( const QgsFeatureSource &source, QgsFeedback *feedback, QgsSpatialIndex::Flags flags )
 {
   d = new QgsSpatialIndexData( source.getFeatures( QgsFeatureRequest().setNoAttributes() ), feedback, flags );

src/core/qgsspatialindex.h

@@ -95,6 +95,23 @@ class CORE_EXPORT QgsSpatialIndex : public QgsFeatureSink
      */
     explicit QgsSpatialIndex( const QgsFeatureIterator &fi, QgsFeedback *feedback = nullptr, QgsSpatialIndex::Flags flags = nullptr );
 
+#ifndef SIP_RUN
+
+    /**
+     * Constructor - creates R-tree and bulk loads it with features from the iterator.
+     * This is much faster approach than creating an empty index and then inserting features one by one.
+     *
+     * This construct and bulk load variant allows for a \a callback function to be specified, which is
+     * called for each added feature in turn. It allows for bulk spatial index load along with other feature
+     * based operations on a single iteration through a feature source. If \a callback returns FALSE, the
+     * load and iteration is canceled.
+     *
+     * \note Not available in Python bindings
+     * \since QGIS 2.12
+     */
+    explicit QgsSpatialIndex( const QgsFeatureIterator &fi, const std::function< bool( const QgsFeature & ) > &callback, QgsSpatialIndex::Flags flags = nullptr );
+#endif
+
     /**
      * Constructor - creates R-tree and bulk loads it with features from the source.
      * This is much faster approach than creating an empty index and then inserting features one by one.

tests/src/core/testqgsspatialindex.cpp

@@ -234,6 +234,51 @@ class TestQgsSpatialIndex : public QObject
       delete indexInsert;
     }
 
+    void bulkLoadWithCallback()
+    {
+      std::unique_ptr< QgsVectorLayer > vl = qgis::make_unique< QgsVectorLayer >( QStringLiteral( "Point" ), QStringLiteral( "x" ), QStringLiteral( "memory" ) );
+      QList< QgsFeatureId >  addedIds;
+      for ( int i = 0; i < 10; ++i )
+      {
+        QgsFeature f( i );
+        QgsGeometry g = QgsGeometry::fromPointXY( QgsPointXY( i, 1 ) );
+        f.setGeometry( g );
+        vl->dataProvider()->addFeature( f );
+        addedIds.append( f.id() );
+      }
+      QCOMPARE( vl->featureCount(), 10L );
+
+      QgsFeatureIds ids;
+      QgsSpatialIndex i( vl->getFeatures(), [ & ]( const QgsFeature & f )->bool
+      {
+        ids.insert( f.id() );
+        return true;
+      } );
+
+      QCOMPARE( ids.size(), 10 );
+      for ( int i = 0; i < 10; ++i )
+        QVERIFY( ids.contains( addedIds.at( i ) ) );
+
+      QList<QgsFeatureId> res = i.intersects( QgsRectangle( 1.5, 0, 3.5, 10 ) );
+      QCOMPARE( res.size(), 2 );
+      QVERIFY( res.contains( addedIds.at( 2 ) ) );
+      QVERIFY( res.contains( addedIds.at( 3 ) ) );
+
+      // try canceling
+      ids.clear();
+      QgsSpatialIndex i2( vl->getFeatures(), [ & ]( const QgsFeature & f )->bool
+      {
+        ids.insert( f.id() );
+        return false;
+      } );
+
+      QCOMPARE( ids.size(), 1 );
+      QVERIFY( ids.contains( addedIds.at( 0 ) ) );
+
+      res = i2.intersects( QgsRectangle( 1.5, 0, 3.5, 10 ) );
+      QVERIFY( res.isEmpty() );
+    }
+
     void testRetrieveGeometries()
     {
       QgsVectorLayer *vl = new QgsVectorLayer( QStringLiteral( "LineString" ), QStringLiteral( "x" ), QStringLiteral( "memory" ) );