Add Support for Composite Templates

[Blquinn]

Composite templates are quite convenient when you use UI files. It would be cool to add them!

https://blogs.gnome.org/tvb/2013/04/09/announcing-composite-widget-templates/

[MikeWey]

I agree, i'll try and investigate when i get some time.

[Blquinn]

@MikeWey I did some d exploration because I wanted to see if we could implement something similar to what PyGobject or Vala has in terms of succinctness with these templates. It's a really nice experience to be able to define the children, the resource path for the ui file and callbacks with attributes.

Best solution I could find was with template mixins and user defined attributes.

import std.stdio;

struct GtkTemplate
{
    string ui_resource;
}

struct GtkChild;
struct GtkCallback;
    
public static GtkTemplate getGtkTemplate(T)() {
    GtkTemplate res;
    static foreach (attr; __traits(getAttributes, T)) {
        static if (is(typeof(attr) == GtkTemplate)) {
            res = attr;
        }
    }
    return res;
}

mixin template GtkTemplateMixin()
{
	private const void initializeTemplate() 
	{
          import std.traits;
  
          const GtkTemplate res = getGtkTemplate!(typeof(this));
          static if (res.ui_resource == "") {
              pragma(msg, "error: widgets using the GtkTemplateMixin must define the @GtkTemplate attribute.");
          }
          static assert(res.ui_resource != "");
  
          static foreach (member; __traits(derivedMembers, typeof(this)))
          {
              static if (hasUDA!(__traits(getMember, this, member), GtkChild)) 
              {
                  // TODO: call gtk_widget_class_bind_child
                  pragma(msg, member, " is a GtkChild");
              }
  
              else static if (hasUDA!(__traits(getMember, this, member), GtkCallback)) 
              {
                  // TODO: call gtk_widget_class_bind_callback
                  pragma(msg, member, " is a GtkCallback");
              }
          }
	}
}

/////////////////////////////////////
// What the user sees.

@GtkTemplate("/the/ui/file.ui")
class MyWidget
{
	@GtkChild string object_property;

	this()
	{
		mixin GtkTemplateMixin;
		initializeTemplate();
	}

	@GtkCallback
	private void foo() {
	}
}

void main()
{
	auto w = new MyWidget();
}