Kaleidoscope/doc/plugin-api-internals.md

Kaleidoscope Plugin API Internals

This document attempts to explain how the plugin system works internally, behind the scenes. We feel this is necessary, because there are some unorthodox solutions in play, which make the code incredibly hard to untangle. This is an unavoidable side-effect of employing a system that lets us use non-virtual functions, and save large amounts of RAM thereby. It would be a lot simpler without this feature, but alas, saving hundreds of bytes of RAM is something we felt is worth the complexity.

Lets start at the top:

KALEIDOSCOPE_INIT_PLUGINS

#define KALEIDOSCOPE_INIT_PLUGINS(...) _KALEIDOSCOPE_INIT_PLUGINS(__VA_ARGS__)

So far so good, this is pretty simple. The reason we use an indirection here is because this is in Kaleidoscope.h, and we do not want the complexity of the _KALEIDOSCOPE_INIT_PLUGINS macro here, nor do we want to move the macro to another header, because it belongs to Kaleidoscope.h.

_KALEIDOSCOPE_INIT_PLUGINS

#define _KALEIDOSCOPE_INIT_PLUGINS(...)                                       \
  namespace kaleidoscope_internal {                                           \
  struct EventDispatcher {                                                    \
                                                                              \
    template<typename EventHandler__, typename... Args__ >                    \
    static kaleidoscope::EventHandlerResult apply(Args__&&... hook_args) {    \
                                                                              \
      kaleidoscope::EventHandlerResult result;                                \
      MAP(_INLINE_EVENT_HANDLER_FOR_PLUGIN, __VA_ARGS__)                      \
                                                                              \
      return result;                                                          \
    }                                                                         \
  };                                                                          \
                                                                              \
  }                                                                           \
  _FOR_EACH_EVENT_HANDLER(_REGISTER_EVENT_HANDLER)

This is where things get interesting. This macro does two things:

• It creates kaleidoscope_internal::EventDispatcher, a class with a single method, apply. This is a templated method, the template argument is the method apply will call. Thus, EventDispatcher::template apply<foo> will resolve to a function that calls the foo method of each plugin we listed for KALEIDOSCOPE_INIT_PLUGINS. We'll see in a bit how this happens.

• The other part creates overrides for the Kaleidoscope::Hooks:: family of functions. These are wrappers around EventDispatcher::template apply<foo>. We have these so higher level code would not need to care about the implementation details, so that it can invoke the hooks as if they were ordinary functions.

_FOR_EACH_EVENT_HANDLER(_REGISTER_EVENT_HANDLER)

Lets look at _FOR_EACH_EVENT_HANDLER and _REGISTER_EVENT_HANDLER first, because that's easier to explain, and does not lead down another rabbit hole.

_REGISTER_EVENT_HANDLER

#define _REGISTER_EVENT_HANDLER(                                          \
    HOOK_NAME, SHOULD_ABORT_ON_CONSUMED_EVENT, SIGNATURE, ARGS_LIST)      \
                                                                          \
  namespace kaleidoscope_internal {                                       \
                                                                          \
   struct EventHandler_##HOOK_NAME {                                      \
                                                                          \
      static bool shouldAbortOnConsumedEvent() {                          \
        return SHOULD_ABORT_ON_CONSUMED_EVENT;                            \
      }                                                                   \
                                                                          \
      template<typename Plugin__, typename... Args__>                     \
      static kaleidoscope::EventHandlerResult                             \
        call(Plugin__ &plugin, Args__&&... hook_args) {                   \
         _VALIDATE_EVENT_HANDLER_SIGNATURE(HOOK_NAME, Plugin__)           \
         return plugin.HOOK_NAME(hook_args...);                           \
      }                                                                   \
    };                                                                    \
                                                                          \
   } 	                                                                  \
                                                                          \
   namespace kaleidoscope {                                               \
                                                                          \
     EventHandlerResult Hooks::HOOK_NAME SIGNATURE {                      \
        return kaleidoscope_internal::EventDispatcher::template           \
        apply<kaleidoscope_internal::EventHandler_ ## HOOK_NAME>          \
             ARGS_LIST;                                                   \
      }                                                                   \
                                                                          \
   }

This looks big and scary, but in practice, it isn't all that bad. Nevertheless, this is where the magic happens!

We create two things: EventHandler_SomeThing and Hooks::SomeThing, the latter being a wrapper around the first, that uses EventDispatcher::template apply<> discussed above.

Lets take onSetup as an example! For that, the above expands to:

namespace kaleidoscope_internal {

struct EventHandler_onSetup {
  static bool shouldAbortOnConsumedEvent() {
    return false;
  }

  template<typename Plugin__, typename... Args__>
  static kaleidoscope::EventHandlerResult
  call(Plugin__ &plugin, Args__&&... hook_args) {
    return plugin.onSetup(hook_args...);
  }
};

}

namespace kaleidoscope {

EventHandlerResult Hooks::onSetup() {
  return kaleidoscope_internal::EventDispatcher::template
         apply<kaleidoscope_internal::EventHandler_onSetup>();
}

}

This still looks scary... but please read a bit further, and it will all make sense!

_FOR_EACH_EVENT_HANDLER

This just evaluates its argument for each event handler supported by Kaleidoscope core. Very simple macro expansion, which we will not expand here, because that would take up a lot of space, and they all look the same (see above).

EventDispatcher

namespace kaleidoscope_internal {
struct EventDispatcher {
  template<typename EventHandler__, typename... Args__ >
  static kaleidoscope::EventHandlerResult apply(Args__&&... hook_args) {

    kaleidoscope::EventHandlerResult result;
    MAP(_INLINE_EVENT_HANDLER_FOR_PLUGIN, __VA_ARGS__)

    return result;
  }
};

This is where the other part of the magic happens, and we need to understand this to be able to make sense of _REGISTER_EVENT_HANDLER above.

_INLINE_EVENT_HANDLER_FOR_PLUGIN

This in isolation, is not very interesting, and is closely tied to EventDispatcher. The definition is here so we can look at it while we learn the details of EventDispatcher below.

#define _INLINE_EVENT_HANDLER_FOR_PLUGIN(PLUGIN)                     \
                                                                     \
   result = EventHandler__::call(PLUGIN, hook_args...);              \
                                                                     \
   if (EventHandler__::shouldAbortOnConsumedEvent() &&               \
       result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) { \
      return result;                                                 \
   }

Back to EventDispatcher...

The EventDispatcher structure has a single method: apply<>, which needs an event handler as its template argument. What the macro does, is call the event handler given in the template argument for each and every initialised plugin. It's best explained with an example! Lets use two plugins, SomePlugin and ExampleEffect:

namespace kaleidoscope_internal {
struct EventDispatcher {
  template<typename EventHandler__, typename... Args__ >
  static kaleidoscope::EventHandlerResult apply(Args__&&... hook_args) {

    kaleidoscope::EventHandlerResult result;

    result = EventHandler__::call(SomePlugin);
    if (EventHandler__::shouldAbortOnConsumedEvent() &&
        result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
        return result;
    }

    result = EventHandler__::call(ExampleEffect);
    if (EventHandler__::shouldAbortOnConsumedEvent() &&
        result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
        return result;
    }

    return result;
  }
};

See? It's unrolled! But how do we get from here to - say - calling the onSetup() method of the plugins? Why, by way of EventDispatcher::template apply<EventHandler_onSetup>! Lets see what happens when we do a call like that:

namespace kaleidoscope_internal {
struct EventDispatcher {
  template<typename EventHandler_onSetup, typename... Args__ >
  static kaleidoscope::EventHandlerResult apply(Args__&&... hook_args) {

    kaleidoscope::EventHandlerResult result;

    result = EventHandler_onSetup::call(SomePlugin);
    if (EventHandler_onSetup::shouldAbortOnConsumedEvent() &&
        result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
        return result;
    }

    result = EventHandler_onSetup::call(ExampleEffect);
    if (EventHandler_onSetup::shouldAbortOnConsumedEvent() &&
        result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
        return result;
    }

    return result;
  }
};

Because we call EventHandler_onSetup::call with the plugin as the first argument, and because call is also a templated function, where the first argument is templated, we get a method that is polymorphic on its first argument. Meaning, for each and every plugin, we'll have a matching EventHandler_onSetup::call, that is tied to that plugin. This is the magic that lets us use non-virtual methods!

Exploring what the compiler does

Because all hooks are called via kaleidoscope::Hooks::NAME, let's explore how the compiler will optimize the code for onSetup, assuming we use two plugins, SomePlugin and ExampleEffect.

Our entry point is this:

return kaleidoscope::Hooks::onSetup();

_REGISTER_EVENT_HANDLER created Hooks::onSetup() for us:

EventHandlerResult kaleidoscope::Hooks::onSetup() {
  return kaleidoscope_internal::EventDispatcher::template
         apply<kaleidoscope_internal::EventHandler_onSetup>();
}

If we inline the call to EventDispatcher::template apply<>, we end up with the following:

EventHandlerResult kaleidoscope::Hooks::onSetup() {
  kaleidoscope::EventHandlerResult result;

  result = EventHandler_onSetup::call(SomePlugin);
  if (EventHandler_onSetup::shouldAbortOnConsumedEvent() &&
    result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
    return result;
  }

  result = EventHandler_onSetup::call(ExampleEffect);
  if (EventHandler_onSetup::shouldAbortOnConsumedEvent() &&
    result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
    return result;
  }

  return result;
}

This is starting to look human readable, isn't it? But we can go further, because EventHandler::onSetup::call and EventHandler_onSetup::shouldAbortOnConsumedEvent are evaluated at compile-time too!

EventHandlerResult kaleidoscope::Hooks::onSetup() {
  kaleidoscope::EventHandlerResult result;

  result = SomePlugin.onSetup();
  if (false &&
    result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
    return result;
  }

  result = ExampleEffect.onSetup();
  if (false &&
    result == kaleidoscope::EventHandlerResult::EVENT_CONSUMED) {
    return result;
  }

  return result;
}

Which in turn, may be optimized further to something like the following:

EventHandlerResult kaleidoscope::Hooks::onSetup() {
  kaleidoscope::EventHandlerResult result;

  result = SomePlugin.onSetup();
  result = ExampleEffect.onSetup();

  return result;
}

And this, is the end of the magic. This is roughly how much the code gets transformed at compile time, so that at run-time, none of this indirection is present.

Summary

As we can see, there is a lot going on behind the scenes, and a combination of template meta programming and pre-processor macros is used to accomplish our goal. But following the code path like we did above allows us to see what the compiler sees (more or less), and inlining all the things that are done compile-time gives us the final code, which is pretty simple by that point.