add callback step

Author: UnixY2K

README.md

@@ -67,4 +67,56 @@ after the dll is compiled, you can use it from the C# project.
 
 ## Multiplatform Support
 
-the SDK is only available for Windows, IOS and Android, for this example exists `csharp-cross-platform.csproj` and `source.cs`.
+the SDK (C#) is only available for Windows, IOS and Android, for this example exists `csharp-cross-platform.csproj` and `source.cs`.
+
+## AppCenter Callbacks
+
+you may need to call the SDK from the C++ code, for example when you want to send a crash report or log an event.
+
+- in the `C++ project` setup a function that saves the C# callback function pointer into c++ so it can be called from the c++ code , you can check it on `AppCenter.cpp`.
+  ```c++
+  // the C function pointers
+  void (*trackEventCallback)(const char *str) = nullptr;
+  void (*trackEventExtraCallback)(const char *str, const char *data) = nullptr;
+  // this functions will be called from another method in the c++ code
+  void setTrackEventCallback(void (*callback)(const char *str)) {
+      trackEventCallback = callback;
+  }
+  void setTrackEventCallback(void (*callback)(const char *str,
+                                              const char *data)) {
+      trackEventExtraCallback = callback;
+  }
+  ```
+- once you have this functions you can call them directly or wrap them like this:
+  ```c++
+  void trackEvent(std::string event) {
+	    if (trackEventCallback) {
+	        trackEventCallback(event.c_str());
+	    } else {
+	        std::cerr << "Interop::AppCenter::trackEventCallback is nullptr\n";
+	    }
+  }
+
+  void trackEvent(std::string event, Properties properties) {
+      if (trackEventExtraCallback) {
+          trackEventExtraCallback(event.c_str(), toJson(properties).c_str());
+      } else {
+          std::cerr << "Interop::AppCenter::trackEventExtraCallback is nullptr\n";
+      }
+  }
+  ```
+- on `AppCenter.hpp` you can define them.
+- finally define `setupAppCenterCallbacks` in the `C++ project` like this:
+  ```c++
+  extern "C"{
+  // this will setup the callbacks needed by the C++ code
+  // in order to call the AppCenter.trackEvent() function
+  myAppAPI void setupAppCenterCallbacks(
+    void (*trackEventCallback)(const char *str),
+    void (*trackEventExtraCallback)(const char *str, const char *data)) {
+	    Interop::AppCenter::setTrackEventCallback(trackEventCallback);
+	    Interop::AppCenter::setTrackEventCallback(trackEventExtraCallback);
+  }
+  }
+  ```
+- on the `C# project` setup a class like the one on `AppCenter.cs` of this project.