[Escher] Add a magic number to app as poor man's RTTI

Change-Id: If7e907f1187b6b6bda33f89aee4a5dfbd922ed6d

escher/include/escher/app.h

@@ -15,10 +15,12 @@
 
 class App : public Responder {
 public:
+  constexpr static uint8_t Magic = 0xA8;
   App();
   void setWindow(Window * window);
   void focus(Responder * responder);
   void processEvent(Ion::Events::Event event);
+  uint8_t m_magic; // Poor man's RTTI
 protected:
   virtual ViewController * rootViewController() = 0;
 private:

escher/src/app.cpp

@@ -5,6 +5,7 @@ extern "C" {
 }
 
 App::App() :
+  m_magic(Magic),
   Responder(nullptr),
   m_focusedResponder(nullptr)
 {

escher/src/responder.cpp

@@ -1,4 +1,6 @@
 #include <escher/responder.h>
+#include <escher/app.h>
+#include <assert.h>
 
 Responder::Responder(Responder * parentResponder) :
   m_parentResponder(parentResponder)
@@ -29,5 +31,7 @@ App * Responder::app() {
   /* If we used RTTI we could use a dynamic_cast, which would be a lot more
    * safe, as such:
    * return dynamic_cast<App *>(rootResponder); */
-  return (App *)rootResponder;
+   App * result = (App *)rootResponder;
+   assert(result->m_magic == App::Magic); // Poor man's RTTI
+  return result;
 }