Allow a QgsFeedback object to be associated with a render context,

for cancelation check support While there is the existing QgsRenderContext::renderingStopped() check, this doesn't play well with other parts of QGIS which utilise a QgsFeedback object for cancelation support. By adding support for a proper QgsFeedback object in render contexts then it becomes trivial to pass this on to feature requests, expression contexts, and other potentially expensive functions so that these can all be canceled responsively too. (At some stage in the future we should consider deprecating QgsRenderContext::rendereringStopped(), but that's not done here)
qgis · Sep 24, 2021 · d838ce7 · d838ce7
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diff --git a/python/core/auto_generated/qgsrendercontext.sip.in b/python/core/auto_generated/qgsrendercontext.sip.in
@@ -294,7 +294,36 @@ Returns the targeted DPI for rendering.
 Returns ``True`` if the rendering operation has been stopped and any ongoing
 rendering should be canceled immediately.
 
+.. note::
+
+   Since QGIS 3.22 the :py:func:`~QgsRenderContext.feedback` member exists as an alternative means of cancelation support.
+
 .. seealso:: :py:func:`setRenderingStopped`
+
+.. seealso:: :py:func:`feedback`
+%End
+
+    void setFeedback( QgsFeedback *feedback );
+%Docstring
+Attach a ``feedback`` object that can be queried regularly during rendering to check
+if rendering should be canceled.
+
+Ownership of ``feedback`` is NOT transferred, and the caller must take care that it exists
+for the lifetime of the render context.
+
+.. seealso:: :py:func:`feedback`
+
+.. versionadded:: 3.22
+%End
+
+    QgsFeedback *feedback() const;
+%Docstring
+Returns the feedback object that can be queried regularly during rendering to check
+if rendering should be canceled, if set. Maybe be ``None``.
+
+.. seealso:: :py:func:`setFeedback`
+
+.. versionadded:: 3.22
 %End
 
     bool forceVectorOutput() const;
@@ -436,7 +465,15 @@ Sets whether edit markers should be drawn during the render operation.
 Sets whether the rendering operation has been ``stopped`` and any ongoing
 rendering should be canceled immediately.
 
+.. note::
+
+   Since QGIS 3.22 the :py:func:`~QgsRenderContext.feedback` member exists as an alternative means of cancelation support.
+
 .. seealso:: :py:func:`renderingStopped`
+
+.. seealso:: :py:func:`feedback`
+
+.. seealso:: :py:func:`setFeedback`
 %End
 
     void setDistanceArea( const QgsDistanceArea &distanceArea );

diff --git a/src/core/qgsrendercontext.cpp b/src/core/qgsrendercontext.cpp
@@ -50,6 +50,7 @@ QgsRenderContext::QgsRenderContext( const QgsRenderContext &rh )
   , mOriginalMapExtent( rh.mOriginalMapExtent )
   , mMapToPixel( rh.mMapToPixel )
   , mRenderingStopped( rh.mRenderingStopped )
+  , mFeedback( rh.mFeedback )
   , mScaleFactor( rh.mScaleFactor )
   , mDpiTarget( rh.mDpiTarget )
   , mRendererScale( rh.mRendererScale )
@@ -89,6 +90,7 @@ QgsRenderContext &QgsRenderContext::operator=( const QgsRenderContext &rh )
   mOriginalMapExtent = rh.mOriginalMapExtent;
   mMapToPixel = rh.mMapToPixel;
   mRenderingStopped = rh.mRenderingStopped;
+  mFeedback = rh.mFeedback;
   mScaleFactor = rh.mScaleFactor;
   mDpiTarget = rh.mDpiTarget;
   mRendererScale = rh.mRendererScale;
@@ -177,6 +179,16 @@ void QgsRenderContext::setTransformContext( const QgsCoordinateTransformContext
 #endif
 }
 
+void QgsRenderContext::setFeedback( QgsFeedback *feedback )
+{
+  mFeedback = feedback;
+}
+
+QgsFeedback *QgsRenderContext::feedback() const
+{
+  return mFeedback;
+}
+
 void QgsRenderContext::setFlags( QgsRenderContext::Flags flags )
 {
   mFlags = flags;

diff --git a/src/core/qgsrendercontext.h b/src/core/qgsrendercontext.h
@@ -344,10 +344,36 @@ class CORE_EXPORT QgsRenderContext : public QgsTemporalRangeObject
      * Returns TRUE if the rendering operation has been stopped and any ongoing
      * rendering should be canceled immediately.
      *
+     * \note Since QGIS 3.22 the feedback() member exists as an alternative means of cancelation support.
+     *
      * \see setRenderingStopped()
+     * \see feedback()
      */
     bool renderingStopped() const {return mRenderingStopped;}
 
+    /**
+     * Attach a \a feedback object that can be queried regularly during rendering to check
+     * if rendering should be canceled.
+     *
+     * Ownership of \a feedback is NOT transferred, and the caller must take care that it exists
+     * for the lifetime of the render context.
+     *
+     * \see feedback()
+     *
+     * \since QGIS 3.22
+     */
+    void setFeedback( QgsFeedback *feedback );
+
+    /**
+     * Returns the feedback object that can be queried regularly during rendering to check
+     * if rendering should be canceled, if set. Maybe be NULLPTR.
+     *
+     * \see setFeedback()
+     *
+     * \since QGIS 3.22
+     */
+    QgsFeedback *feedback() const;
+
     /**
      * Returns TRUE if rendering operations should use vector operations instead
      * of any faster raster shortcuts.
@@ -482,7 +508,11 @@ class CORE_EXPORT QgsRenderContext : public QgsTemporalRangeObject
      * Sets whether the rendering operation has been \a stopped and any ongoing
      * rendering should be canceled immediately.
      *
+     * \note Since QGIS 3.22 the feedback() member exists as an alternative means of cancelation support.
+     *
      * \see renderingStopped()
+     * \see feedback()
+     * \see setFeedback()
      */
     void setRenderingStopped( bool stopped ) {mRenderingStopped = stopped;}
 
@@ -975,6 +1005,9 @@ class CORE_EXPORT QgsRenderContext : public QgsTemporalRangeObject
     //! True if the rendering has been canceled
     bool mRenderingStopped = false;
 
+    //! Optional feedback object, as an alternative for mRenderingStopped for cancelation support
+    QgsFeedback *mFeedback = nullptr;
+
     //! Factor to scale line widths and point marker sizes
     double mScaleFactor = 1.0;