ENH: Add direct support for lambda command to Object class

Overload AddObserver with std::function argument to allow direct usage
of lambdas without the explicit addition of a new Command class.

Modules/Core/Common/include/itkCommand.h

@@ -20,6 +20,7 @@
 
 #include "itkObject.h"
 #include "itkObjectFactory.h"
+#include <functional>
 
 namespace itk
 {

Modules/Core/Common/include/itkObject.h

@@ -33,6 +33,8 @@
 #include "itkMetaDataDictionary.h"
 #include "itkSingletonMacro.h"
 
+#include <functional>
+
 namespace itk
 {
 // Forward reference because of private implementation
@@ -153,10 +155,29 @@ class ITKCommon_EXPORT Object : public LightObject
    * different objects  */
   unsigned long
   AddObserver(const EventObject & event, Command *);
-
   unsigned long
   AddObserver(const EventObject & event, Command *) const;
 
+  /** \brief A convenient method to add an C++ lambda function as an observer.
+   *
+   * A FunctionCommand object in implicitly create to hold function, and
+   * passed to this object as a standard observer. The command will be invoked
+   * for the specified event.
+   *
+   * The function or lambda \b cannot captured this object as SmartPoint because
+   * a circular dependency is generated which causing memory leaks.
+   * Sample usage:
+   *  \code
+   *    auto &objRef = *o.GetPointer();
+   *    o->AddObserver(itk::AnyEvent(), [&objRef](const itk::EventObject &event)
+   *    { std::cout << "Object: " << objRef.GetNameOfClass() << " Event: " << event << std::endl; });
+   *  \endcode
+   *
+   *  \see FunctionCommand
+   */
+  unsigned long
+  AddObserver(const EventObject & event, std::function<void(const EventObject &)> function) const;
+
   /** Get the command associated with the given tag.  NOTE: This returns
    * a pointer to a Command, but it is safe to assign this to a
    * Command::Pointer.  Since Command inherits from LightObject, at this

Modules/Core/Common/src/itkObject.cxx

@@ -503,6 +503,15 @@ Object ::AddObserver(const EventObject & event, Command * cmd) const
   return this->m_SubjectImplementation->AddObserver(event, cmd);
 }
 
+unsigned long
+Object::AddObserver(const EventObject & event, std::function<void(const EventObject &)> function) const
+{
+  auto cmd = FunctionCommand::New();
+  cmd->SetCallback(std::move(function));
+  return this->AddObserver(event, cmd);
+}
+
+
 Command *
 Object ::GetCommand(unsigned long tag)
 {

Modules/Core/Common/test/itkCommandObserverObjectTest.cxx

@@ -283,6 +283,28 @@ testDeleteEventThrow()
   return EXIT_SUCCESS;
 }
 
+int
+testLambdaCommand()
+{
+  // check the case where an exception in thrown in the DeleteEvent
+  itk::Object::Pointer o = itk::Object::New();
+
+  int cnt = 0;
+  o->AddObserver(itk::AnyEvent(), [&cnt](const itk::EventObject &) { ++cnt; });
+
+  auto & objRef = *o.GetPointer();
+  o->AddObserver(itk::AnyEvent(), [&objRef](const itk::EventObject & event) {
+    std::cout << "Object: " << objRef.GetNameOfClass() << " Event: " << event << std::endl;
+  });
+
+  o->InvokeEvent(itk::AnyEvent());
+
+  ITK_TEST_EXPECT_EQUAL(1, cnt);
+
+  return EXIT_SUCCESS;
+}
+
+
 } // end namespace
 
 
@@ -295,6 +317,7 @@ itkCommandObserverObjectTest(int, char *[])
   ret &= (testCommandConstObject() == EXIT_SUCCESS);
   ret &= (testCommandRecursiveObject() == EXIT_SUCCESS);
   ret &= (testDeleteEventThrow() == EXIT_SUCCESS);
+  ret &= (testLambdaCommand() == EXIT_SUCCESS);
 
   return ret ? EXIT_SUCCESS : EXIT_FAILURE;
 }