Check for empty field name. Fixes #4914

Author: NathanW2

src/app/qgsfieldcalculator.cpp

@@ -36,8 +36,7 @@ QgsFieldCalculator::QgsFieldCalculator( QgsVectorLayer* vl )
   populateFields();
   populateOutputFieldTypes();
 
-  QPushButton* okbutton = mButtonBox->button( QDialogButtonBox::Ok );
-  connect( builder, SIGNAL( expressionParsed( bool ) ), okbutton, SLOT( setEnabled( bool ) ) );
+  connect( builder, SIGNAL( expressionParsed( bool ) ), this, SLOT( setOkButtonState() ) );
 
   //default values for field width and precision
   mOuputFieldWidthSpinBox->setValue( 10 );
@@ -269,7 +268,19 @@ void QgsFieldCalculator::populateFields()
 
 void QgsFieldCalculator::setOkButtonState()
 {
-  bool okEnabled = ( !mOutputFieldNameLineEdit->text().isEmpty() || mUpdateExistingGroupBox->isChecked() ) && builder->isExpressionValid();
+  QPushButton* okButton = mButtonBox->button( QDialogButtonBox::Ok );
+  okButton->setToolTip("");
 
-  mButtonBox->button( QDialogButtonBox::Ok )->setEnabled( okEnabled );
+  bool emptyFieldName = mOutputFieldNameLineEdit->text().isEmpty();
+  bool expressionVaild = builder->isExpressionValid();
+
+  if ( emptyFieldName )
+      okButton->setToolTip( tr("Please enter a field name") );
+
+  if ( !expressionVaild )
+      okButton->setToolTip( okButton->toolTip() + tr("\n The expression is invaild see (more info) for details") );
+
+  bool okEnabled = ( !emptyFieldName || mUpdateExistingGroupBox->isChecked() ) && expressionVaild;
+
+  okButton->setEnabled( okEnabled );
 }

src/app/qgsfieldcalculator.h

@@ -41,6 +41,10 @@ class QgsFieldCalculator: public QDialog, private Ui::QgsFieldCalculatorBase
 
     void on_mButtonBox_helpRequested() { QgsContextHelp::run( metaObject()->className() ); }
 
+  private slots:
+    /**Sets the ok button enabled / disabled*/
+    void setOkButtonState();
+
   private:
     //default constructor forbidden
     QgsFieldCalculator();
@@ -49,9 +53,6 @@ class QgsFieldCalculator: public QDialog, private Ui::QgsFieldCalculatorBase
     /**Inserts the types supported by the provider into the combo box*/
     void populateOutputFieldTypes();
 
-    /**Sets the ok button enabled / disabled*/
-    void setOkButtonState();
-
     QgsVectorLayer* mVectorLayer;
     /**Key: field name, Value: field index*/
     QMap<QString, int> mFieldMap;