Add support for Gio async functions.

Fixes hugopl/gtk4.cr#59

README.md

@@ -259,6 +259,20 @@ So if you want to rescue from this specific error you must `rescue e : GLib::Fil
 error in this domain you must `rescue e : GLib::FileError`, and finally if you want to rescue from any GLib errors you do
 `rescue e : GLib::GLibError`.
 
+## Gio Async Pattern
+
+All `*_async` methods with a `*_finish` methods receive a block, the block works as the `Gio::AsyncReadyCallback` and you need
+to call the `*_finish` on the `result`, exceptions are raised by the `*_finish` functions on errors.
+
+Example:
+
+```Crystal
+file = Gio::File.new_for_path("/my/nice/file")
+file.read_async(0, nil) do |obj, result|
+  obj.as(Gio::File).read_finish(result)
+end
+```
+
 ## Raw C Structs
 
 At [binding.yml](BINDING_YML.md) file you can define the strategy used to bind the structs, if set to `auto`it will behave

src/generator/arg_strategy.cr

@@ -23,7 +23,8 @@ module Generator
       CrystalToC
     end
 
-    property? remove_from_declaration : Bool
+    property? remove_from_declaration : Bool = false
+    property? capture_block : Bool = false
     getter method : CallableInfo
     getter arg : ArgInfo
 
@@ -32,6 +33,7 @@ module Generator
     def self.find_strategies(method : CallableInfo, direction : Direction) : Array(ArgStrategy)
       strategies = method.args.map { |arg| ArgStrategy.new(method, arg) }
       {% for plan_class in %w(CallbackArgPlan
+                             AsyncPatternArgPlan
                              TransferFullArgPlan
                              ArrayLengthArgPlan
                              OutArgUsedInReturnPlan
@@ -60,7 +62,6 @@ module Generator
     end
 
     def initialize(@method : CallableInfo, @arg : ArgInfo)
-      @remove_from_declaration = false
     end
 
     def has_implementation? : Bool
@@ -78,12 +79,18 @@ module Generator
     def render_declaration(io : IO)
       return if remove_from_declaration?
 
-      # Default arg declaration
-      null_mark = '?' if arg.nullable?
+      # ⚠️`capture_block` flag is used for GIO async methods, the generator must
+      # create another version of the method with the block capture since `&callback : Proc(Nil)?
+      # trigger a compiler error.
+      #
+      # As the generator needs a refactor to make it easier to add method overloads based on argument
+      # strategies, for now I just set the captured block as non-nilable.
+      null_mark = '?' if arg.nullable? && !capture_block?
       type = to_crystal_type(arg_type, is_arg: true)
       name = to_crystal_arg_decl(arg.name)
+      io << '&' if capture_block?
       io << name << " : " << type << null_mark
-      io << ','
+      io << ',' if !capture_block? # Crystal bug(?) parsing foo(&capture : Proc(Nil),).
     end
 
     def add_implementation(arg_plan : ArgPlan, direction : Direction) : Nil
@@ -446,4 +453,45 @@ module Generator
     def generate_c_to_crystal_implementation(io : IO, strategy : ArgStrategy) : Nil
     end
   end
+
+  struct AsyncPatternArgPlan < ArgPlan
+    def match?(strategy : ArgStrategy, direction : ArgStrategy::Direction) : Bool
+      arg = strategy.arg
+      type_info = arg.type_info
+      callback = type_info.interface
+      return false unless callback.is_a?(CallbackInfo)
+      return false if callback.name != "AsyncReadyCallback" || callback.namespace.name != "Gio"
+
+      idx = strategies.index(strategy)
+      return false if idx.nil? || idx != strategies.size - 2
+
+      user_data_arg = strategies[idx + 1].arg
+      return false unless user_data_arg.type_info.tag.void?
+
+      strategy.capture_block = true
+      strategies[idx + 1].remove_from_declaration = true
+    end
+
+    def generate_crystal_to_c_implementation(io : IO, strategy : ArgStrategy) : Nil
+      arg = strategy.arg
+      type_info = arg.type_info
+      idx = strategies.index(strategy).not_nil!
+
+      callback_var = to_identifier(arg.name)
+      user_data_var = to_identifier(strategies[idx + 1].arg.name)
+      io << user_data_var << " = Box.box(" << callback_var << ")\n"
+      io << callback_var << " = if " << callback_var << ".nil?\n"
+      io << "  Pointer(Void).null\n"
+      io << "else\n"
+      io << "  ->(gobject : Void*, result : Void*, box : Void*) {\n"
+      io << "    unboxed_callback = Box(Gio::AsyncReadyCallback).unbox(box)\n"
+      io << "    GICrystal::ClosureDataManager.deregister(box)\n"
+      io << "    unboxed_callback.call(typeof(self).new(gobject, :none), Gio::AbstractAsyncResult.new(result, :none))\n"
+      io << "  }.pointer\n"
+      io << "end\n"
+    end
+
+    def generate_c_to_crystal_implementation(io : IO, strategy : ArgStrategy) : Nil
+    end
+  end
 end