QgsFieldExpressionWidget class

The QgsFieldExpressionWidget class reates a widget to choose fields and edit expressions It contains a combo boxto display the fields and expression and a button to open the expression dialog.

Contents

The combo box is editable, allowing expressions to be edited inline. The validity of the expression is checked live on key press, invalid expressions are displayed in red. The expression will be added to the model (and the fieldChanged signals emitted) only when editing in the line edit is finished (focus lost, enter key pressed).

Constructors, destructors, conversion operators

QgsFieldExpressionWidget(QWidget* parent = nullptr) explicit
QgsFieldExpressionWidget creates a widget with a combo box to display the fields and expression and a button to open the expression dialog.

Public functions

auto allowEvalErrors() const -> bool
Allow accepting expressions with evaluation errors.
void appendScope(QgsExpressionContextScope* scope)
Appends a scope to the current expression context.
auto asExpression() const -> QString
Returns the currently selected field or expression.
auto currentField(bool* isExpression = nullptr, bool* isValid = nullptr) const -> QString
currentField returns the currently selected field or expression if allowed
auto currentText() const -> QString
Returns the current text that is set in the expression area.
auto expression() const -> QString
Returns the currently selected field or expression.
auto expressionDialogTitle() -> const QString
Returns the title used for the expression dialog.
auto filters() const -> QgsFieldProxyModel::Filters
currently used filter on list of fields
auto isExpression() const -> bool
If the content is not just a simple field this method will return true.
auto isValidExpression(QString* expressionError = nullptr) const -> bool
Returns true if the current expression is valid.
auto layer() const -> QgsVectorLayer*
Returns the layer currently associated with the widget.
void registerExpressionContextGenerator(const QgsExpressionContextGenerator* generator)
Register an expression context generator class that will be used to retrieve an expression context for the widget.
void setAllowEvalErrors(bool allowEvalErrors)
Allow accepting expressions with evaluation errors.
void setExpressionDialogTitle(const QString& title)
define the title used in the expression dialog
void setFilters(QgsFieldProxyModel::Filters filters)
setFilters allows fitering according to the type of field
void setGeomCalculator(const QgsDistanceArea& da)
Sets the geometry calculator used in the expression dialog.

Signals

void allowEvalErrorsChanged()
Allow accepting expressions with evaluation errors.
void fieldChanged(const QString& fieldName)
the signal is emitted when the currently selected field changes
void fieldChanged(const QString& fieldName, bool isValid)
fieldChanged signal with indication of the validity of the expression

Public slots

void setExpression(const QString& expression)
Sets the current expression text and if applicable also the field.
void setField(const QString& fieldName)
sets the current field or expression in the widget
void setLayer(QgsMapLayer* layer)
Sets the layer used to display the fields and expression.
void setRow(int row)
sets the current row in the widget

Protected slots

void editExpression()
open the expression dialog to edit the current or add a new expression
void expressionEdited(const QString& expression)
when expression is edited by the user in the line edit, it will be checked for validity
void expressionEditingFinished()
when expression has been edited (finished) it will be added to the model
void updateLineEditStyle(const QString& expression = QString())
updateLineEditStyle will re-style (color/font) the line edit depending on content and status

Function documentation

bool QgsFieldExpressionWidget::allowEvalErrors() const

Allow accepting expressions with evaluation errors.

This can be useful when we are not able to provide an expression context of which we are sure it's completely populated.

void QgsFieldExpressionWidget::appendScope(QgsExpressionContextScope* scope)

Appends a scope to the current expression context.

Parameters
scope The scope to add.

QString QgsFieldExpressionWidget::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).

QString QgsFieldExpressionWidget::currentField(bool* isExpression = nullptr, bool* isValid = nullptr) const

currentField returns the currently selected field or expression if allowed

Parameters
isExpression determines if the string returned is the name of a field or an expression
isValid determines if the expression (or field) returned is valid

QString QgsFieldExpressionWidget::expression() 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()

QgsVectorLayer* QgsFieldExpressionWidget::layer() const

Returns the layer currently associated with the widget.

void QgsFieldExpressionWidget::registerExpressionContextGenerator(const QgsExpressionContextGenerator* generator)

Register an expression context generator class that will be used to retrieve an expression context for the widget.

Parameters
generator A QgsExpressionContextGenerator class that will be used to create an expression context when required.

void QgsFieldExpressionWidget::setAllowEvalErrors(bool allowEvalErrors)

Allow accepting expressions with evaluation errors.

This can be useful when we are not able to provide an expression context of which we are sure it's completely populated.

void QgsFieldExpressionWidget::allowEvalErrorsChanged() signal

Allow accepting expressions with evaluation errors.

This can be useful when we are not able to provide an expression context of which we are sure it's completely populated.

void QgsFieldExpressionWidget::setExpression(const QString& expression) public slot

Sets the current expression text and if applicable also the field.

Alias for setField.

void QgsFieldExpressionWidget::setLayer(QgsMapLayer* layer) public slot

Sets the layer used to display the fields and expression.

void QgsFieldExpressionWidget::updateLineEditStyle(const QString& expression = QString()) protected slot

updateLineEditStyle will re-style (color/font) the line edit depending on content and status

Parameters
expression if expression is given it will be evaluated for the given string, otherwise it takes current expression from the model