[layouts] Nicer detection of rendering in a view

In compositions, a flag must be explicitly set to indicate
whether the render occuring is for "previews" (i.e.
rendering in a graphics view) or outputs (i.e. rendering
to a image/pdf/other destination device)

This isn't nice api.

So we avoid this by checking the paint device type when
an item is being rendered.

python/core/layout/qgslayoutitem.sip

@@ -589,6 +589,14 @@ class QgsLayoutItem : QgsLayoutObject, QGraphicsRectItem, QgsLayoutUndoObjectInt
  Draws the background for the item.
 %End
 
+    bool isPreviewRender( QPainter *painter ) const;
+%Docstring
+ Returns true if the render to the specified ``painter`` is a preview render,
+ i.e. is being rendered inside a QGraphicsView widget as opposed to a destination
+ device (such as an image).
+ :rtype: bool
+%End
+
     virtual void setFixedSize( const QgsLayoutSize &size );
 %Docstring
  Sets a fixed ``size`` for the layout item, which prevents it from being freely

src/core/layout/qgslayoutitem.cpp

@@ -224,7 +224,7 @@ void QgsLayoutItem::setParentGroup( QgsLayoutItemGroup *group )
 
 void QgsLayoutItem::paint( QPainter *painter, const QStyleOptionGraphicsItem *itemStyle, QWidget * )
 {
-  if ( !painter || !painter->device() || !shouldDrawItem() )
+  if ( !painter || !painter->device() || !shouldDrawItem( painter ) )
   {
     return;
   }
@@ -431,15 +431,13 @@ bool QgsLayoutItem::shouldBlockUndoCommands() const
   return !mLayout || mLayout != scene() || mBlockUndoCommands;
 }
 
-bool QgsLayoutItem::shouldDrawItem() const
+bool QgsLayoutItem::shouldDrawItem( QPainter *painter ) const
 {
-#if 0 //TODO
+  if ( isPreviewRender( painter ) )
-  if ( ( mLayout && mLayout->plotStyle() == QgsComposition::Preview ) || !mLayout )
   {
-    //preview mode or no composition, so OK to draw item
+    //preview mode so OK to draw item
     return true;
   }
-#endif
 
   //exporting layout, so check if item is excluded from exports
   return !mEvaluatedExcludeFromExports;
@@ -769,6 +767,31 @@ void QgsLayoutItem::drawBackground( QgsRenderContext &context )
   p->restore();
 }
 
+bool QgsLayoutItem::isPreviewRender( QPainter *painter ) const
+{
+  if ( !painter || !painter->device() )
+    return false;
+
+  // if rendering to a QGraphicsView, we are in preview mode
+  QPaintDevice *device = painter->device();
+  if ( dynamic_cast< QPixmap * >( device ) )
+    return true;
+
+  QObject *obj = dynamic_cast< QObject *>( device );
+  if ( !obj )
+    return false;
+
+  const QMetaObject *mo = obj->metaObject();
+  while ( mo )
+  {
+    if ( mo->className() == QStringLiteral( "QGraphicsView" ) )
+      return true;
+
+    mo = mo->superClass();
+  }
+  return false;
+}
+
 void QgsLayoutItem::setFixedSize( const QgsLayoutSize &size )
 {
   mFixedSize = size;

src/core/layout/qgslayoutitem.h

@@ -583,6 +583,13 @@ class CORE_EXPORT QgsLayoutItem : public QgsLayoutObject, public QGraphicsRectIt
      */
     virtual void drawBackground( QgsRenderContext &context );
 
+    /**
+     * Returns true if the render to the specified \a painter is a preview render,
+     * i.e. is being rendered inside a QGraphicsView widget as opposed to a destination
+     * device (such as an image).
+     */
+    bool isPreviewRender( QPainter *painter ) const;
+
     /**
      * Sets a fixed \a size for the layout item, which prevents it from being freely
      * resized. Set an empty size if item can be freely resized.
@@ -770,10 +777,12 @@ class CORE_EXPORT QgsLayoutItem : public QgsLayoutObject, public QGraphicsRectIt
 
     /**
      * Returns whether the item should be drawn in the current context
+     * and to the given \a painter.
      */
-    bool shouldDrawItem() const;
+    bool shouldDrawItem( QPainter *painter ) const;
 
     friend class TestQgsLayoutItem;
+    friend class TestQgsLayoutView;
     friend class QgsLayoutItemGroup;
 };
 

tests/src/gui/testqgslayoutview.cpp

@@ -23,9 +23,12 @@
 #include "qgslayoutitemregistry.h"
 #include "qgslayoutitemguiregistry.h"
 #include "qgslayoutitemwidget.h"
+#include "qgslayoutitemshape.h"
 #include "qgsproject.h"
 #include "qgsgui.h"
 #include <QtTest/QSignalSpy>
+#include <QSvgGenerator>
+#include <QPrinter>
 
 class TestQgsLayoutView: public QObject
 {
@@ -40,6 +43,7 @@ class TestQgsLayoutView: public QObject
     void events();
     void guiRegistry();
     void rubberBand();
+    void isPreviewRender();
 
   private:
 
@@ -320,5 +324,88 @@ void TestQgsLayoutView::rubberBand()
   QCOMPARE( band.pen().color(), QColor( 0, 255, 0 ) );
 }
 
+class TestViewItem : public QgsLayoutItem
+{
+    Q_OBJECT
+
+  public:
+
+    TestViewItem( QgsLayout *layout ) : QgsLayoutItem( layout )
+    {
+    }
+
+    //implement pure virtual methods
+    int type() const override { return QgsLayoutItemRegistry::LayoutItem + 101; }
+    QString stringType() const override { return QStringLiteral( "TestItemType" ); }
+    bool mDrawn = false;
+    bool mPreview = false;
+
+  protected:
+    void paint( QPainter *painter, const QStyleOptionGraphicsItem *, QWidget * ) override
+    {
+      mDrawn = true;
+      mPreview = isPreviewRender( painter );
+    }
+    void draw( QgsRenderContext &, const QStyleOptionGraphicsItem * ) override
+    {
+
+    }
+
+};
+
+void TestQgsLayoutView::isPreviewRender()
+{
+  // test if items can detect whether a preview render is occurring
+  QgsProject p;
+  QgsLayoutView *view = new QgsLayoutView();
+  QgsLayout *layout = new QgsLayout( &p );
+  view->setCurrentLayout( layout );
+
+  TestViewItem *item = new TestViewItem( layout );
+  layout->addLayoutItem( item );
+  item->attemptMove( QgsLayoutPoint( 0, 0 ) );
+  item->attemptResize( QgsLayoutSize( 100, 100 ) );
+  layout->updateBounds();
+
+
+  // render to image
+  QVERIFY( !item->isPreviewRender( nullptr ) );
+  QImage im = QImage( 250, 250, QImage::Format_RGB32 );
+  QPainter painter;
+  QVERIFY( painter.begin( &im ) );
+  QVERIFY( !item->isPreviewRender( &painter ) );
+  painter.end();
+
+  // render to svg
+  QSvgGenerator generator;
+  generator.setFileName( QDir::tempPath() + "/layout_text.svg" );
+  QVERIFY( painter.begin( &generator ) );
+  QVERIFY( !item->isPreviewRender( &painter ) );
+  painter.end();
+
+  // render to pdf
+  QPrinter printer;
+  printer.setOutputFileName( QString() );
+  printer.setOutputFormat( QPrinter::PdfFormat );
+  printer.setOutputFileName( QDir::tempPath() + "/layout_text.pdf" );
+  QVERIFY( painter.begin( &printer ) );
+  QVERIFY( !item->isPreviewRender( &painter ) );
+  painter.end();
+
+  // render in view - kinda gross!
+  item->mDrawn = false;
+  item->mPreview = false;
+  view->show();
+  view->zoomFull();
+  while ( !item->mDrawn )
+  {
+    QApplication::processEvents();
+  }
+
+  QVERIFY( item->mDrawn );
+  QVERIFY( item->mPreview );
+
+}
+
 QGSTEST_MAIN( TestQgsLayoutView )
 #include "testqgslayoutview.moc"