class QgsFieldExpressionWidget : QWidget
{
%TypeHeaderCode
#include "qgsfieldexpressionwidget.h"
%End

  public:
    /**
     * @brief QgsFieldExpressionWidget creates a widget with a combo box to display the fields and expression and a button to open the expression dialog
     */
    explicit QgsFieldExpressionWidget( QWidget *parent /TransferThis/ = 0 );

    //! define the title used in the expression dialog
    void setExpressionDialogTitle( const QString& title );

    //! return the title used for the expression dialog
    const QString expressionDialogTitle();

    //! setFilters allows fitering according to the type of field
    void setFilters( QgsFieldProxyModel::Filters filters );

    void setLeftHandButtonStyle( bool isLeft );

    //! currently used filter on list of fields
    QgsFieldProxyModel::Filters filters() const;

    //! set the geometry calculator used in the expression dialog
    void setGeomCalculator( const QgsDistanceArea &da );

    /**
     * @brief currentField returns the currently selected field or expression if allowed
     * @param isExpression determines if the string returned is the name of a field or an expression
     * @param isValid determines if the expression (or field) returned is valid
     */
    QString currentField( bool *isExpression = 0, bool *isValid = 0 ) const;

    /**
      * Return true if the current expression is valid
      */
    bool isValidExpression( QString *expressionError = 0 ) const;

    bool isExpression() const;
    /**
      * Return the current text that is set in the expression area
      */
    QString currentText() const;

    /** Returns the currently selected field or expression. If a field is currently selected, the returned
     * value will be converted to a valid expression referencing this field (ie enclosing the field name with
     * appropriate quotations).
     * @note added in QGIS 2.14
     */
    QString asExpression() const;

    /**
     * Returns the currently selected field or expression. If a field is currently selected, the returned
     * value will be converted to a valid expression referencing this field (ie enclosing the field name with
     * appropriate quotations).
     *
     * Alias for asExpression()
     *
     * @note added in QGIS 3.0
     */
    QString expression() const;

    /**
     * Returns the layer currently associated with the widget.
     * @see setLayer()
     */
    QgsVectorLayer* layer() const;

    /**
     * Register an expression context generator class that will be used to retrieve
     * an expression context for the widget.
     * @param generator A QgsExpressionContextGenerator class that will be used to
     *                  create an expression context when required.
     * @note added in QGIS 3.0
     */
    void registerExpressionContextGenerator( QgsExpressionContextGenerator* generator );

  signals:
    //! the signal is emitted when the currently selected field changes
    void fieldChanged( const QString& fieldName );

    //! fieldChanged signal with indication of the validity of the expression
    void fieldChanged( const QString& fieldName, bool isValid );

//    void returnPressed();

  public slots:
    /**
     * Sets the layer used to display the fields and expression.
     * @see layer()
     */
    void setLayer( QgsMapLayer* layer );

    //! sets the current row in the widget
    void setRow( int row );

    //! sets the current field or expression in the widget
    void setField( const QString &fieldName );

    /**
     * Sets the current expression text and if applicable also the field.
     * Alias for setField.
     *
     * @note Added in QGIS 3.0
     */
    void setExpression( const QString& expression );

  protected slots:
    //! open the expression dialog to edit the current or add a new expression
    void editExpression();

    //! when expression is edited by the user in the line edit, it will be checked for validity
    void expressionEdited( const QString& expression );

    //! when expression has been edited (finished) it will be added to the model
    void expressionEditingFinished();

    void currentFieldChanged();

    /**
     * @brief updateLineEditStyle will re-style (color/font) the line edit depending on content and status
     * @param expression if expression is given it will be evaluated for the given string, otherwise it takes
     * current expression from the model
     */
    void updateLineEditStyle( const QString& expression = QString() );

    bool isExpressionValid( const QString& expressionStr );

  protected:
    void changeEvent( QEvent* event );
};