[FEATURE] Background saving of vector layers

Switches the vector layer save operation to a background task
using the task manager framework. Allows layers to saved without
blocking the interface.

python/core/core.sip

@@ -149,6 +149,7 @@
 %Include qgsunittypes.sip
 %Include qgsvectordataprovider.sip
 %Include qgsvectorfilewriter.sip
+%Include qgsvectorfilewritertask.sip
 %Include qgsvectorlayer.sip
 %Include qgsvectorlayercache.sip
 %Include qgsvectorlayereditbuffer.sip

python/core/qgstaskmanager.sip

@@ -78,14 +78,7 @@ class QgsTask : QObject
      */
     double progress() const;
 
-    /**
-     * Notifies the task that it should terminate. Calling this is not guaranteed
-     * to immediately end the task, rather it sets the isCanceled() flag which
-     * task subclasses can check and terminate their operations at an appropriate
-     * time. Any subtasks owned by this task will also be canceled.
-     * @see isCanceled()
-     */
-    void cancel();
+    virtual void cancel();
 
     /**
      * Places the task on hold. If the task in not queued

python/core/qgsvectorfilewriter.sip

@@ -97,6 +97,7 @@ class QgsVectorFileWriter
       ErrProjection,
       ErrFeatureWriteFailed,
       ErrInvalidLayer,
+      Canceled,
     };
 
     enum SymbologyExport
@@ -323,6 +324,8 @@ class QgsVectorFileWriter
 
         /** Field value converter */
         QgsVectorFileWriter::FieldValueConverter* fieldValueConverter;
+
+        QgsFeedback* feedback;
     };
 
     /** Writes a layer out to a vector file.

python/core/qgsvectorfilewritertask.sip

@@ -0,0 +1,25 @@
+class QgsVectorFileWriterTask : QgsTask
+{
+%TypeHeaderCode
+#include <qgsvectorfilewritertask.h>
+%End
+
+  public:
+
+    QgsVectorFileWriterTask( QgsVectorLayer* layer,
+                             const QString& fileName,
+                             const QgsVectorFileWriter::SaveVectorOptions& options );
+
+    virtual void cancel();
+
+  signals:
+
+    void writeComplete( const QString& newFilename );
+    void errorOccurred( int error, const QString& errorMessage );
+
+  protected:
+
+    virtual bool run();
+    virtual void finished( bool result );
+};
+

src/app/qgisapp.cpp

@@ -250,6 +250,7 @@
 #include "qgsvectorlayerjoininfo.h"
 #include "qgsvectorlayerutils.h"
 #include "qgshelp.h"
+#include "qgsvectorfilewritertask.h"
 
 #include "qgssublayersdialog.h"
 #include "ogr/qgsopenvectorlayerdialog.h"
@@ -6477,12 +6478,6 @@ void QgisApp::saveAsVectorFileGeneral( QgsVectorLayer* vlayer, bool symbologyOpt
       }
     }
 
-    // ok if the file existed it should be deleted now so we can continue...
-    QApplication::setOverrideCursor( Qt::WaitCursor );
-
-    QgsVectorFileWriter::WriterError error;
-    QString errorMessage;
-    QString newFilename;
     QgsRectangle filterExtent = dialog->filterExtent();
     QgisAppFieldValueConverter converter( vlayer, dialog->attributesAsDisplayedValues() );
     QgisAppFieldValueConverter* converterPtr = nullptr;
@@ -6510,32 +6505,41 @@ void QgisApp::saveAsVectorFileGeneral( QgsVectorLayer* vlayer, bool symbologyOpt
     options.attributes = dialog->selectedAttributes();
     options.fieldValueConverter = converterPtr;
 
-    error = QgsVectorFileWriter::writeAsVectorFormat(
-              vlayer, vectorFilename, options, &newFilename, &errorMessage );
+    bool addToCanvas = dialog->addToCanvas();
+    QString layerName = dialog->layername();
+    QgsVectorFileWriterTask* writerTask = new QgsVectorFileWriterTask( vlayer, vectorFilename, options );
 
-    QApplication::restoreOverrideCursor();
-
-    if ( error == QgsVectorFileWriter::NoError )
+    // when writer is successful:
+    connect( writerTask, &QgsVectorFileWriterTask::writeComplete, this, [this, addToCanvas, layerName, encoding, vectorFilename, vlayer]( const QString& newFilename )
     {
-      if ( dialog->addToCanvas() )
+      if ( addToCanvas )
       {
         QString uri( newFilename );
-        if ( !dialog->layername().isEmpty() )
-          uri += "|layername=" + dialog->layername();
-        addVectorLayers( QStringList( uri ), encoding, QStringLiteral( "file" ) );
+        if ( !layerName.isEmpty() )
+          uri += "|layername=" + layerName;
+        this->addVectorLayers( QStringList( uri ), encoding, QStringLiteral( "file" ) );
       }
-      emit layerSavedAs( vlayer, vectorFilename );
-      messageBar()->pushMessage( tr( "Saving done" ),
-                                 tr( "Export to vector file has been completed" ),
-                                 QgsMessageBar::INFO, messageTimeout() );
+      this->emit layerSavedAs( vlayer, vectorFilename );
+      this->messageBar()->pushMessage( tr( "Saving done" ),
+                                       tr( "Export to vector file has been completed" ),
+                                       QgsMessageBar::INFO, messageTimeout() );
     }
-    else
+           );
+
+    // when an error occurs:
+    connect( writerTask, &QgsVectorFileWriterTask::errorOccurred, this, [=]( int error, const QString & errorMessage )
     {
-      QgsMessageViewer *m = new QgsMessageViewer( nullptr );
-      m->setWindowTitle( tr( "Save error" ) );
-      m->setMessageAsPlainText( tr( "Export to vector file failed.\nError: %1" ).arg( errorMessage ) );
-      m->exec();
+      if ( error != QgsVectorFileWriter::Canceled )
+      {
+        QgsMessageViewer *m = new QgsMessageViewer( nullptr );
+        m->setWindowTitle( tr( "Save error" ) );
+        m->setMessageAsPlainText( tr( "Export to vector file failed.\nError: %1" ).arg( errorMessage ) );
+        m->exec();
+      }
     }
+           );
+
+    QgsApplication::taskManager()->addTask( writerTask );
   }
 
   delete dialog;

src/core/CMakeLists.txt

@@ -237,6 +237,7 @@ SET(QGIS_CORE_SRCS
   qgsunittypes.cpp
   qgsvectordataprovider.cpp
   qgsvectorfilewriter.cpp
+  qgsvectorfilewritertask.cpp
   qgsvectorlayer.cpp
   qgsvectorlayercache.cpp
   qgsvectorlayerdiagramprovider.cpp
@@ -528,6 +529,7 @@ SET(QGIS_CORE_MOC_HDRS
   qgstransactiongroup.h
   qgsunittypes.h
   qgsvectordataprovider.h
+  qgsvectorfilewritertask.h
   qgsvectorlayercache.h
   qgsvectorlayereditbuffer.h
   qgsvectorlayereditpassthrough.h

src/core/qgstaskmanager.h

@@ -116,9 +116,11 @@ class CORE_EXPORT QgsTask : public QObject
      * to immediately end the task, rather it sets the isCanceled() flag which
      * task subclasses can check and terminate their operations at an appropriate
      * time. Any subtasks owned by this task will also be canceled.
+     * Derived classes must ensure that the base class implementation is called
+     * from any overridden version.
      * @see isCanceled()
      */
-    void cancel();
+    virtual void cancel();
 
     /**
      * Places the task on hold. If the task in not queued

src/core/qgsvectorfilewriter.cpp

@@ -2426,6 +2426,12 @@ QgsVectorFileWriter::writeAsVectorFormat( QgsVectorLayer* layer,
   // write all features
   while ( fit.nextFeature( fet ) )
   {
+    if ( options.feedback && options.feedback->isCanceled() )
+    {
+      delete writer;
+      return Canceled;
+    }
+
     if ( shallTransform )
     {
       try
@@ -3085,3 +3091,4 @@ bool QgsVectorFileWriter::areThereNewFieldsToCreate( const QString& datasetName,
   OGR_DS_Destroy( hDS );
   return ret;
 }
+

src/core/qgsvectorfilewriter.h

@@ -21,7 +21,10 @@
 
 #include "qgis_core.h"
 #include "qgsfields.h"
+#include "qgsfeedback.h"
 #include "qgssymbol.h"
+#include "qgstaskmanager.h"
+#include "qgsvectorlayer.h"
 #include <ogr_api.h>
 
 #include <QPair>
@@ -164,6 +167,7 @@ class CORE_EXPORT QgsVectorFileWriter
       ErrProjection,
       ErrFeatureWriteFailed,
       ErrInvalidLayer,
+      Canceled, //!< Writing was interrupted by manual cancelation
     };
 
     enum SymbologyExport
@@ -391,6 +395,9 @@ class CORE_EXPORT QgsVectorFileWriter
 
         //! Field value converter
         FieldValueConverter* fieldValueConverter;
+
+        //! Optional feedback object allowing cancelation of layer save
+        QgsFeedback* feedback = nullptr;
     };
 
     /** Writes a layer out to a vector file.
@@ -616,4 +623,5 @@ class CORE_EXPORT QgsVectorFileWriter
 
 Q_DECLARE_OPERATORS_FOR_FLAGS( QgsVectorFileWriter::EditionCapabilities )
 
+
 #endif

src/core/qgsvectorfilewritertask.cpp

@@ -0,0 +1,57 @@
+/***************************************************************************
+                          qgsvectorfilewritertask.cpp
+                          ---------------------------
+    begin                : Feb 2017
+    copyright            : (C) 2017 by Nyall Dawson
+    email                : nyall dot dawson at gmail dot com
+ ***************************************************************************/
+
+/***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "qgsvectorfilewritertask.h"
+
+
+QgsVectorFileWriterTask::QgsVectorFileWriterTask( QgsVectorLayer* layer, const QString& fileName, const QgsVectorFileWriter::SaveVectorOptions& options )
+    : QgsTask( tr( "Saving %1 " ).arg( fileName ), QgsTask::CanCancel )
+    , mLayer( layer )
+    , mDestFileName( fileName )
+    , mOptions( options )
+{
+  if ( !mOptions.feedback )
+  {
+    mOwnedFeedback.reset( new QgsFeedback() );
+    mOptions.feedback = mOwnedFeedback.get();
+  }
+}
+
+void QgsVectorFileWriterTask::cancel()
+{
+  mOptions.feedback->cancel();
+  QgsTask::cancel();
+}
+
+bool QgsVectorFileWriterTask::run()
+{
+  if ( !mLayer )
+    return false;
+
+
+  mError = QgsVectorFileWriter::writeAsVectorFormat(
+             mLayer, mDestFileName, mOptions, &mNewFilename, &mErrorMessage );
+  return mError == QgsVectorFileWriter::NoError;
+}
+
+void QgsVectorFileWriterTask::finished( bool result )
+{
+  if ( result )
+    emit writeComplete( mNewFilename );
+  else
+    emit errorOccurred( mError, mErrorMessage );
+}

src/core/qgsvectorfilewritertask.h

@@ -0,0 +1,85 @@
+/***************************************************************************
+                          qgsvectorfilewritertask.h
+                          -------------------------
+    begin                : Feb 2017
+    copyright            : (C) 2017 by Nyall Dawson
+    email                : nyall dot dawson at gmail dot com
+ ***************************************************************************/
+
+/***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#ifndef QGSVECTORFILEWRITERTASK_H
+#define QGSVECTORFILEWRITERTASK_H
+
+#include "qgis_core.h"
+#include "qgsvectorfilewriter.h"
+#include "qgstaskmanager.h"
+#include "qgsvectorlayer.h"
+
+/**
+ * \class QgsVectorFileWriterTask
+ * \ingroup core
+ * QgsTask task which performs a QgsVectorFileWriter layer saving operation as a background
+ * task. This can be used to save a vector layer out to a file without blocking the
+ * QGIS interface.
+ * \note Added in QGIS 3.0
+ */
+class CORE_EXPORT QgsVectorFileWriterTask : public QgsTask
+{
+    Q_OBJECT
+
+  public:
+
+    /**
+     * Constructor for QgsVectorFileWriterTask. Takes a source \a layer, destination \a fileName
+     * and save \a options.
+     */
+    QgsVectorFileWriterTask( QgsVectorLayer* layer,
+                             const QString& fileName,
+                             const QgsVectorFileWriter::SaveVectorOptions& options );
+
+    virtual void cancel() override;
+
+  signals:
+
+    /**
+     * Emitted when writing the layer is successfully completed. The \a newFilename
+     * parameter indicates the file path for the written file.
+     */
+    void writeComplete( const QString& newFilename );
+
+    /**
+     * Emitted when an error occurs which prevented the file being written (or if
+     * the task is canceled). The writing \a error and \a errorMessage will be reported.
+     */
+    void errorOccurred( int error, const QString& errorMessage );
+
+  protected:
+
+    virtual bool run() override;
+    virtual void finished( bool result ) override;
+
+  private:
+
+    QPointer< QgsVectorLayer > mLayer = nullptr;
+
+    QString mDestFileName;
+
+    std::unique_ptr< QgsFeedback > mOwnedFeedback;
+
+    QgsVectorFileWriter::WriterError mError = QgsVectorFileWriter::NoError;
+
+    QString mNewFilename;
+    QString mErrorMessage;
+
+    QgsVectorFileWriter::SaveVectorOptions mOptions;
+};
+
+#endif