[FEATURE][locator] Add quick calculator (expression evaluator) to

locator bar

Allows evaluation of simple expressions (well, actually ANY QGIS
expression... so you could use aggregates and the like if you
wanted!) by entering "= " followed by an expression into the
locator bar. If a valid expression is entered, the user is given
an option to copy the result to the clipboard

E.g. entering

"= 10/3" gives an entry "Copy '3.3333333' to clipboard".

Inspired by the same feature in Qt Creator 4.6

src/app/locator/qgsinbuiltlocatorfilters.cpp

@@ -26,6 +26,7 @@
 #include "qgslayoutmanager.h"
 #include "qgsmapcanvas.h"
 #include <QToolButton>
+#include <QClipboard>
 
 QgsLayerTreeLocatorFilter::QgsLayerTreeLocatorFilter( QObject *parent )
   : QgsLocatorFilter( parent )
@@ -277,3 +278,40 @@ void QgsActiveLayerFeaturesLocatorFilter::triggerResult( const QgsLocatorResult
 
   QgisApp::instance()->mapCanvas()->zoomToFeatureIds( layer, QgsFeatureIds() << id );
 }
+
+//
+// QgsExpressionCalculatorLocatorFilter
+//
+QgsExpressionCalculatorLocatorFilter::QgsExpressionCalculatorLocatorFilter( QObject *parent )
+  : QgsLocatorFilter( parent )
+{
+  setUseWithoutPrefix( false );
+}
+
+void QgsExpressionCalculatorLocatorFilter::fetchResults( const QString &string, const QgsLocatorContext &, QgsFeedback * )
+{
+  QgsExpressionContext context;
+  context << QgsExpressionContextUtils::globalScope()
+          << QgsExpressionContextUtils::projectScope( QgsProject::instance() );
+
+  QString error;
+  if ( QgsExpression::checkExpression( string, &context, error ) )
+  {
+    QgsExpression exp( string );
+    QString resultString = exp.evaluate( &context ).toString();
+    if ( !resultString.isEmpty() )
+    {
+      QgsLocatorResult result;
+      result.filter = this;
+      result.displayString = tr( "Copy “%1” to clipboard" ).arg( resultString );
+      result.userData = resultString;
+      result.score = 1;
+      emit resultFetched( result );
+    }
+  }
+}
+
+void QgsExpressionCalculatorLocatorFilter::triggerResult( const QgsLocatorResult &result )
+{
+  QApplication::clipboard()->setText( result.userData.toString() );
+}

src/app/locator/qgsinbuiltlocatorfilters.h

@@ -112,6 +112,22 @@ class QgsActiveLayerFeaturesLocatorFilter : public QgsLocatorFilter
     QIcon mLayerIcon;
 };
 
+class QgsExpressionCalculatorLocatorFilter : public QgsLocatorFilter
+{
+    Q_OBJECT
+
+  public:
+
+    QgsExpressionCalculatorLocatorFilter( QObject *parent = nullptr );
+    QString name() const override { return QStringLiteral( "calculator" ); }
+    QString displayName() const override { return tr( "Calculator" ); }
+    Priority priority() const override { return Highest; }
+    QString prefix() const override { return QStringLiteral( "=" ); }
+
+    void fetchResults( const QString &string, const QgsLocatorContext &context, QgsFeedback *feedback ) override;
+    void triggerResult( const QgsLocatorResult &result ) override;
+};
+
 
 #endif // QGSINBUILTLOCATORFILTERS_H
 

src/app/qgisapp.cpp

@@ -2936,6 +2936,7 @@ void QgisApp::createStatusBar()
 
   mLocatorWidget->locator()->registerFilter( new QgsActionLocatorFilter( actionObjects ) );
   mLocatorWidget->locator()->registerFilter( new QgsActiveLayerFeaturesLocatorFilter() );
+  mLocatorWidget->locator()->registerFilter( new QgsExpressionCalculatorLocatorFilter() );
 }
 
 void QgisApp::setIconSizes( int size )

src/core/locator/qgslocator.cpp

@@ -61,7 +61,7 @@ void QgsLocator::registerFilter( QgsLocatorFilter *filter )
   {
     if ( filter->name() == QStringLiteral( "actions" ) || filter->name() == QStringLiteral( "processing_alg" )
          || filter->name() == QStringLiteral( "layertree" ) || filter->name() == QStringLiteral( "layouts" )
-         || filter->name() == QStringLiteral( "features" ) )
+         || filter->name() == QStringLiteral( "features" ) || filter->name() == QStringLiteral( "calculator" ) )
     {
       //inbuilt filter, no prefix check
       mPrefixedFilters.insert( filter->prefix(), filter );