Let Crystal user objects be created from g_gobject_new calls.

This is a breaking change, since it now requires all GObject subclasses
to have a constructor without arguments.

What changed:

There are 2 thread local variables to inform if the object is being created
in C land or Crystal land, we store the objects instance pointers there.

When the object is created in C land, the Crystal instance is created on
GObject instance_init method.

When the object is created in Crystal land, the GObject instance_init method
creates no Crystal instance.

This also fixes the use case of tryign to use a Crystal defined GObject property
before the Wrapper gets fully initialized.

Fixes hugopl/gtk4.cr#69

spec/c_born_crystal_objects_spec.cr

@@ -0,0 +1,26 @@
+require "./spec_helper"
+
+private class UserObj < GObject::Object
+  @[GObject::Property]
+  property crystal_prop = ""
+  getter crystal_attr : Int32 = 42
+
+  def initialize(@hey = "")
+    super()
+  end
+end
+
+describe "Crystal GObjects" do
+  it "can born in C land" do
+    puts "\n\n\n"
+    ptr = LibGObject.g_object_new(UserObj.g_type, "crystal_prop", "value", Pointer(Void).null)
+    user_obj = UserObj.new(ptr, :none)
+    user_obj.crystal_prop.should eq("value")
+    user_obj.crystal_attr.should eq(42)
+  end
+
+  it "can born in Crystal land" do
+    puts "\n\n\n"
+    user_obj = UserObj.new
+  end
+end

spec/gc_spec.cr

@@ -3,6 +3,11 @@ require "./spec_helper"
 private class GCResistantObj < GObject::Object
   property moto : String
 
+  def initialize
+    super
+    @moto = ""
+  end
+
   def initialize(@moto)
     super()
   end

spec/inheritance_spec.cr

@@ -10,9 +10,18 @@ private class UserObjectWithCtor < GObject::Object
   def initialize(@string : String)
     super()
   end
+
+  def initialize
+    super
+    @string = ""
+  end
 end
 
 private class UserSubject < Test::Subject
+  def initialize
+    super
+  end
+
   def initialize(string : String)
     super(string: string)
   end

src/bindings/g_object/binding.yml

@@ -279,3 +279,4 @@ types:
 execute_callback:
  - g_signal_emitv
  - g_closure_invoke
+ - g_object_newv

src/bindings/g_object/object.cr

@@ -128,6 +128,30 @@ module GObject
           {% end %}
         end
 
+        # :nodoc:
+        #
+        # GObject instance initialization, this can be called when a GObject is created from Crystal `MyObj.new` or by
+        # C `g_object_new(MyObject.g_type)`, in both cases some tasks are always done here:
+        #
+        # - INSTANCE_QDATA_KEY is always set here, so Crystal properties can be fetched.
+        # - Floating refs are sank
+        #
+        # So, if the object is created from C
+        #
+        # - Set `GICrystal.g_object_being_created` with the C object instance pointer.
+        # - Call the Crystal object constructor, that will look the flag set above and use it instead of call `g_object_new`.
+        #
+        # If the object is created from Crystal
+        #
+        # - Set `GICrystal.crystal_object_being_created` with the Crystal object instance pointer.
+        # - Use the Crystal object instead of calling the Crystal object constructor.
+        def self._instance_init(instance : Pointer(LibGObject::TypeInstance), type : Pointer(LibGObject::TypeClass)) : Nil
+          GICrystal.g_object_being_created = instance.as(Void*)
+          crystal_instance = GICrystal.crystal_object_being_created || {{ @type }}.new.as(Void*)
+          GICrystal.crystal_object_being_created = Pointer(Void).null
+          GICrystal.g_object_being_created = Pointer(Void).null
+        end
+
         # :nodoc:
         def self._g_toggle_notify(object : Void*, _gobject : Void*, is_last_ref : Int32) : Nil
           return if object.null?
@@ -422,10 +446,6 @@ module GObject
           {% end %}
         end
 
-        # :nodoc:
-        def self._instance_init(instance : Pointer(LibGObject::TypeInstance), type : Pointer(LibGObject::TypeClass)) : Nil
-        end
-
         # :nodoc:
         def self._install_ifaces
           {% verbatim do %}
@@ -452,7 +472,6 @@ module GObject
         # This specific implementation turns a normal reference into a toggle reference.
         private def _after_init : Nil
           # Set toggle ref to protect the crystal object from the garbage collector while in C.
-
           self.class._g_toggle_notify(self.as(Void*), @pointer, 0)
           LibGObject.g_object_add_toggle_ref(@pointer, G_TOGGLE_NOTIFY__, self.as(Void*))
           LibGObject.g_object_unref(@pointer)
@@ -604,9 +623,15 @@ module GObject
     end
 
     def initialize
-      @pointer = LibGObject.g_object_newv(self.class.g_type, 0, Pointer(LibGObject::Parameter).null)
+      GICrystal.crystal_object_being_created = Pointer(Void).new(object_id)
+
+      g_object = GICrystal.g_object_being_created
+      @pointer = g_object || LibGObject.g_object_newv(self.class.g_type, 0, Pointer(LibGObject::Parameter).null)
+      GICrystal.g_object_being_created = Pointer(Void).null
+
+      LibGObject.g_object_set_qdata(self, GICrystal::INSTANCE_QDATA_KEY, self.as(Void*))
       LibGObject.g_object_ref_sink(self) if LibGObject.g_object_is_floating(self) == 1
-      LibGObject.g_object_set_qdata(self, GICrystal::INSTANCE_QDATA_KEY, Pointer(Void).new(object_id))
+
       self._after_init
     end
 

src/generator/method_gen.cr

@@ -87,7 +87,8 @@ module Generator
 
     private def method_return_type_declaration : String
       if @method.flags.constructor?
-        return @method.may_return_null? ? ": self?" : ": self"
+        type = to_crystal_type(object.as(RegisteredTypeInfo))
+        return @method.may_return_null? ? ": #{type}?" : ": #{type}"
       end
 
       return_type = method_return_type

src/gi-crystal.cr

@@ -119,6 +119,15 @@ module GICrystal
     end
   end
 
+  # When creating an user defined GObject from C, the GObject instance is stored here, so the Crystal
+  # constructor uses it instead of call `g_object_new`
+  @[ThreadLocal]
+  class_property g_object_being_created : Pointer(Void) = Pointer(Void).null
+  # When creating an user defined GObject from Crystal, the Crystal instance is stored here, so the
+  # GObject `instance_init` doesn't instantiate another Crystal object.
+  @[ThreadLocal]
+  class_property crystal_object_being_created : Pointer(Void) = Pointer(Void).null
+
   # This declare the `new` method on a instance of type *type*, *qdata_get_func* (g_object_get_qdata) is used
   # to fetch a possible already existing Crystal object.
   #