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Dolce Progress and A Word on Mixins

Posted by Aldacron, in D, Derelict, Dolce 11 June 2011 · 520 views

D Derelict Dolce
My time has been limited lately, but the core of Dolce continues to evolve. I think it's converging into something fairly decent. I'm still not completely happy with it, but I believe I'm most of the way there. As usual, I'm getting wild ideas for new projects. Like a renewed interest in building a 2D library from scratch in D. Or a more generic framework with pluggable backends for Allegro, SDL, SFML, and GLFW. You know, like I explicitly said at the beginning I had no interest in making. This time, though, I refuse to let myself get distracted. So I'm digging my heels in.

Recently, I've added event handling, mapping keycodes or mouse button ids to delegates. The first pass is usable, but it can be improved a good deal. D's delegates are great for this sort of thing. But delegates aren't what I want to talk about today.

Until now, my test app has only been using the keyboard. With the inclusion of the event handler, I added mouse support. While testing it out, I kept getting a puzzling result. Allegro has no constants for mouse buttons. They are numbered from 1 to whatever the maximum number is (which you can find out via al_get_mouse_num_buttons()). But my event handler kept getting button 0 passed to it, no matter which button I pushed. al_get_mouse_num_buttons was correctly reporting 3 (I dumped my 8-button gaming mouse some time ago). After several minutes of digging around, I started to suspect my DerelictAllegro binding. Sure enough, I had left out 4 fields from the struct declaration. I patched that up and all is well.

Looking at the binding source reminded me of a D feature that would be worth mentioning here. In my last post, I briefly mentioned D's string mixins. There is another type of useful mixin in D that I've made use of in the Allegro binding: the template mixin.

Allegro's event structs are all declared with the following macro at the top:

#define _AL_EVENT_HEADER(srctype)                	\
   ALLEGRO_EVENT_TYPE type;                      	\
   srctype *source;                              	\
   double timestamp;

So the mouse event structure looks like this:

typedef struct ALLEGRO_MOUSE_EVENT
{
   _AL_EVENT_HEADER(struct ALLEGRO_MOUSE)
   struct ALLEGRO_DISPLAY *display;
   int x,  y,  z, w;
   int dx, dy, dz, dw;
   unsigned int button;
   float pressure;
} ALLEGRO_MOUSE_EVENT;

D has no #defines, and no preprocessor at all. But this C idiom can easily be replicated with template mixins. Here's what I did in the DerelictAllegro binding:

// First, declare the template that you intend to use as a mixin.
template _AL_EVENT_HEADER(T)
{
	ALLEGRO_EVENT_TYPE type;
	T* source;
	double timestamp;
}

// Then, mix it in.
struct ALLEGRO_MOUSE_EVENT
{
	mixin _AL_EVENT_HEADER!(ALLEGRO_MOUSE);
	ALLEGRO_DISPLAY* display;
	int x, y, z, w;
	int dx, dy, dz, dw;
	uint button;
	float pressure;
}

Simple. String mixins could also be used here.

 // You would want to use a compile time function with a signature something like this
string allegroEventHeader(string type) { ... }

// Then call it like so:
mixin(allegroEventHeader("ALLEGRO_MOUSE"));

Personally, I prefer template mixins in situations like this.

Take a look at the template mixin documentation over at the shiny, new d-programming-language.org website for more info.




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