[FEATURE] Add ability to create user defined Python (and C++) express…

…ion functions

python/core/qgsexpression.sip

@@ -117,29 +117,33 @@ class QgsExpression
     // static const char* BinaryOgcOperatorText[];
     // static const char* UnaryOgcOperatorText[];
 
-    struct FunctionDef /NoDefaultCtors/
+    class Function
     {
-      //FunctionDef( QString fnname, int params, FcnEval fcn, QString group, QString helpText = QString(), bool usesGeometry = false );
-      /** The name of the function. */
-      QString mName;
-      /** The number of parameters this function takes. */
-      int mParams;
-      /** Pointer to function. */
-      //FcnEval mFcn;
-      /** Does this function use a geometry object. */
-      bool mUsesGeometry;
-      /** The group the function belongs to. */
-      QString mGroup;
-      /** The help text for the function. */
-      QString mHelpText;
+      public:
+        Function( QString fnname, int params, QString group, QString helpText = QString(), bool usesGeometry = false );
+        /** The name of the function. */
+        QString name();
+        /** The number of parameters this function takes. */
+        int params();
+        /** Does this function use a geometry object. */
+        bool usesgeometry();
+        /** The group the function belongs to. */
+        QString group();
+        /** The help text for the function. */
+        QString helptext();
+
+        virtual QVariant func(const QVariantList& values, QgsFeature* f, QgsExpression* parent) = 0;
     };
 
-    static const QList<QgsExpression::FunctionDef> &BuiltinFunctions();
+    static const QList<QgsExpression::Function *> &Functions();
+
+    static bool registerFunction(Function* function);
+    static bool unregisterFunction(QString name);
 
     // tells whether the identifier is a name of existing function
     static bool isFunctionName( QString name );
 
-    // return index of the function in BuiltinFunctions array
+    // return index of the function in Functions array
     static int functionIndex( QString name );
 
     /**  Returns the number of functions defined in the parser
@@ -150,7 +154,7 @@ class QgsExpression
     /**
      * Returns a list of special Column definitions
      */
-    static QList<QgsExpression::FunctionDef> specialColumns();
+    static QList<QgsExpression::Function *> specialColumns();
 
     //! return quoted column reference (in double quotes)
     static QString quotedColumnRef( QString name );

python/utils.py

@@ -28,8 +28,9 @@
 
 """
 
-from PyQt4.QtCore import QCoreApplication,QLocale
-from qgis.core import QGis
+from PyQt4.QtCore import QCoreApplication,QLocale, QString
+from qgis.core import QGis, QgsExpression
+from string import Template
 import sys
 import traceback
 import glob
@@ -371,6 +372,59 @@ def closeProjectMacro():
     mod.closeProject()
 
 
+def qgsfunction(args, group, **kwargs):
+  """
+  Decorator function used to define a user expression function.
+
+  Custom functions should take (values, feature, parent) as args, 
+  they can also shortcut naming feature and parent args by using *args 
+  if they are not needed in the function.
+
+  Functions should return a value compatible with QVariant
+
+  Eval errors can be raised using parent.setEvalErrorString()
+
+  Functions must be unregistered when no longer needed using 
+  QgsExpression.unregisterFunction
+
+  Example:
+    @qgsfunction(2, 'test'):
+    def add(values, feature, parent):
+      pass
+
+    Will create and register a function in QgsExpression called 'add' in the
+    'test' group that takes two arguments.
+
+    or not using feature and parent:
+
+    @qgsfunction(2, 'test'):
+    def add(values, *args):
+      pass
+  """
+  helptemplate = Template("""<h3>$name function</h3><br>$doc""")
+  class QgsExpressionFunction(QgsExpression.Function):
+    def __init__(self, name, args, group, helptext=''):
+      QgsExpression.Function.__init__(self, name, args, group, QString(helptext))
+
+    def func(self, values, feature, parent):
+      pass
+
+  def wrapper(func):
+    name = kwargs.get('name', func.__name__)
+    help = func.__doc__ or ''
+    help = help.strip()
+    if args == 0 and not name[0] == '$':
+      name = '${0}'.format(name)
+    func.__name__ = name
+    help = helptemplate.safe_substitute(name=name, doc=help)
+    f = QgsExpressionFunction(name, args, group, help)
+    f.func = func
+    register = kwargs.get('register', True)
+    if register:
+      QgsExpression.registerFunction(f)
+    return f
+  return wrapper
+
 #######################
 # IMPORT wrapper