Add method to create a QgsDataDefined from a string.

Automatically analyses to determine whether the string refers to
a column or is an expression.

Author: vmora

python/core/qgsdatadefined.sip

@@ -23,20 +23,27 @@ class QgsDataDefined
                     const QString& field = QString() );
 
     /**
-     * Construct a new data defined object, analyse the expression to determine
-     * if it's a simple field
-     *
+     * Construct a new data defined object, analysing the expression to determine
+     * if it's a simple field reference or an expression.
      * @param expression can be null
      */
-    QgsDataDefined( const QgsExpression * expression );
+    explicit QgsDataDefined( const QgsExpression * expression );
+
+    /**
+     * Construct a new data defined object, analysing the string to determine
+     * if it's a simple field reference or an expression
+     * @param string field reference or an expression, can be empty
+     * @note added in QGIS 2.9
+     */
+    explicit QgsDataDefined( const QString& string );
 
     /**
      * Copy constructor. Note that copies of data defined objects with expressions
      * will not be prepared.
      */
     QgsDataDefined( const QgsDataDefined& other );
 
-    ~QgsDataDefined();
+    virtual ~QgsDataDefined();
 
     /**Returns whether the data defined container is set to all the default
      * values, ie, disabled, with empty expression and no assigned field

src/core/qgsdatadefined.cpp

@@ -35,7 +35,7 @@ QgsDataDefined::QgsDataDefined( bool active,
 
 QgsDataDefined::QgsDataDefined( const QgsExpression * expression )
     : mActive( bool( expression ) )
-    , mUseExpression( expression && expression->rootNode() && !dynamic_cast<const QgsExpression::NodeColumnRef*>( expression->rootNode() ) )
+    , mUseExpression( expression && ! expression->isField() )
     , mExpressionString( mUseExpression ? expression->expression() : "" )
     , mField( !mUseExpression ? ( expression ? expression->expression() : "" ) : "" )
 {
@@ -55,6 +55,17 @@ QgsDataDefined::QgsDataDefined( const QgsDataDefined &other )
 
 }
 
+QgsDataDefined::QgsDataDefined( const QString & string )
+{
+  QgsExpression expression( string );
+  mActive = expression.rootNode();
+  mUseExpression = mActive && ! expression.isField();
+  mExpressionString = mUseExpression ? expression.expression() : QString();
+  mField = expression.isField() ? expression.rootNode()->dump() : QString();
+  mExpression = 0;
+  mExpressionPrepared = false;
+}
+
 QgsDataDefined::~QgsDataDefined()
 {
   delete mExpression;

src/core/qgsdatadefined.h

@@ -45,20 +45,27 @@ class CORE_EXPORT QgsDataDefined
                     const QString& field = QString() );
 
     /**
-     * Construct a new data defined object, analyse the expression to determine
-     * if it's a simple field
-     *
+     * Construct a new data defined object, analysing the expression to determine
+     * if it's a simple field reference or an expression.
      * @param expression can be null
      */
     explicit QgsDataDefined( const QgsExpression * expression );
 
+    /**
+     * Construct a new data defined object, analysing the string to determine
+     * if it's a simple field reference or an expression
+     * @param string field reference or an expression, can be empty
+     * @note added in QGIS 2.9
+     */
+    explicit QgsDataDefined( const QString& string );
+
     /**
      * Copy constructor. Note that copies of data defined objects with expressions
      * will not be prepared.
      */
     QgsDataDefined( const QgsDataDefined& other );
 
-    ~QgsDataDefined();
+    virtual ~QgsDataDefined();
 
     /**Returns whether the data defined container is set to all the default
      * values, ie, disabled, with empty expression and no assigned field

tests/src/core/testqgsdatadefined.cpp

@@ -70,6 +70,22 @@ void TestQgsDataDefined::create()
   QVERIFY( dd->useExpression() );
   QCOMPARE( dd->expressionString(), QString( "exp" ) );
   QCOMPARE( dd->field(), QString( "field" ) );
+
+  //test with string constructor
+  QScopedPointer<QgsDataDefined> stringConstructorField( new QgsDataDefined( QString( "\"col1\"" ) ) );
+  QVERIFY( stringConstructorField->isActive() );
+  QVERIFY( ! stringConstructorField->useExpression() );
+  QVERIFY( stringConstructorField->expressionString().isEmpty() );
+  QCOMPARE( stringConstructorField->field(), QString( "col1" ) );
+
+  QScopedPointer<QgsDataDefined> stringConstructorExp( new QgsDataDefined( QString( "1 + 2" ) ) );
+  QVERIFY( stringConstructorExp->isActive() );
+  QVERIFY( stringConstructorExp->useExpression() );
+  QCOMPARE( stringConstructorExp->expressionString(), QString( "1 + 2" ) );
+  QVERIFY( stringConstructorExp->field().isEmpty() );
+
+  QScopedPointer<QgsDataDefined> stringConstructorEmpty( new QgsDataDefined( QString( "" ) ) );
+  QVERIFY( ! stringConstructorEmpty->isActive() );
 }
 
 void TestQgsDataDefined::copy()