apprt/gtk-ng: extract common methods into a mixin

Every GObject class we're ever going to make has the same handful of
methods. This adds *just enough* noise to be annoying. This commit
extracts the common ones so far into a central mixin. Since Zig is
removing `usingnamespace` and has no other mixin mechanism, we must
forward the decls, but this is still cleaner imo than what we did
before.

I suspect longer term we can probably abstract more of the `zig-gobject`
boilerplate into a higher level abstraction but I'm not confident doing
that yet across the 4 classes we have so far.

src/apprt/gtk-ng/class.zig

@@ -24,3 +24,45 @@ pub fn unrefLater(obj: anytype) void {
         }
     }).callback, obj.as(gobject.Object));
 }
+
+/// Common methods for all GObject classes we create.
+pub fn Common(
+    comptime Self: type,
+    comptime Private: ?type,
+) type {
+    return struct {
+        /// Upcast our type to a parent type or interface. This will fail at
+        /// compile time if the cast isn't 100% safe. For unsafe casts,
+        /// use `gobject.ext.cast` instead. We don't have a helper for that
+        /// because its uncommon and unsafe behavior should be noisier.
+        pub fn as(self: *Self, comptime T: type) *T {
+            return gobject.ext.as(T, self);
+        }
+
+        /// Increase the reference count of the object.
+        pub fn ref(self: *Self) *Self {
+            return @ptrCast(@alignCast(gobject.Object.ref(self.as(gobject.Object))));
+        }
+
+        /// Decrease the reference count of the object.
+        pub fn unref(self: *Self) void {
+            gobject.Object.unref(self.as(gobject.Object));
+        }
+
+        /// Access the private data of the object. This should be forwarded
+        /// via a non-pub const usually.
+        pub const private = if (Private) |P| (struct {
+            fn private(self: *Self) *P {
+                return gobject.ext.impl_helpers.getPrivate(
+                    self,
+                    P,
+                    P.offset,
+                );
+            }
+        }).private else {};
+    };
+}
+
+test {
+    @import("std").testing.refAllDecls(@This());
+}

src/apprt/gtk-ng/class/application.zig

@@ -20,6 +20,7 @@ const CoreConfig = configpkg.Config;
 const adw_version = @import("../adw_version.zig");
 const gtk_version = @import("../gtk_version.zig");
 const ApprtApp = @import("../App.zig");
+const Common = @import("../class.zig").Common;
 const WeakRef = @import("../weak_ref.zig").WeakRef;
 const Config = @import("config.zig").Config;
 const Window = @import("window.zig").Window;
@@ -84,7 +85,7 @@ pub const Application = extern struct {
         /// outside of our own lifecycle and that's okay.
         config_errors_dialog: WeakRef(ConfigErrorsDialog) = .{},
 
-        var offset: c_int = 0;
+        pub var offset: c_int = 0;
     };
 
     /// Creates a new Application instance.
@@ -699,21 +700,11 @@ pub const Application = extern struct {
         return self.private().core_app.alloc;
     }
 
-    pub fn as(app: *Self, comptime T: type) *T {
-        return gobject.ext.as(T, app);
-    }
-
-    pub fn unref(self: *Self) void {
-        gobject.Object.unref(self.as(gobject.Object));
-    }
-
-    fn private(self: *Self) *Private {
-        return gobject.ext.impl_helpers.getPrivate(
-            self,
-            Private,
-            Private.offset,
-        );
-    }
+    const C = Common(Self, Private);
+    pub const as = C.as;
+    pub const ref = C.ref;
+    pub const unref = C.unref;
+    const private = C.private;
 
     pub const Class = extern struct {
         parent_class: Parent.Class,

src/apprt/gtk-ng/class/config.zig

@@ -9,6 +9,7 @@ const configpkg = @import("../../../config.zig");
 const CoreConfig = configpkg.Config;
 
 const unrefLater = @import("../class.zig").unrefLater;
+const Common = @import("../class.zig").Common;
 
 const log = std.log.scoped(.gtk_ghostty_config);
 
@@ -66,7 +67,7 @@ pub const Config = extern struct {
     const Private = struct {
         config: CoreConfig,
 
-        var offset: c_int = 0;
+        pub var offset: c_int = 0;
     };
 
     /// Create a new GhosttyConfig from a loaded configuration.
@@ -141,25 +142,11 @@ pub const Config = extern struct {
         );
     }
 
-    pub fn as(self: *Self, comptime T: type) *T {
-        return gobject.ext.as(T, self);
-    }
-
-    pub fn ref(self: *Self) *Self {
-        return @ptrCast(@alignCast(gobject.Object.ref(self.as(gobject.Object))));
-    }
-
-    pub fn unref(self: *Self) void {
-        gobject.Object.unref(self.as(gobject.Object));
-    }
-
-    fn private(self: *Self) *Private {
-        return gobject.ext.impl_helpers.getPrivate(
-            self,
-            Private,
-            Private.offset,
-        );
-    }
+    const C = Common(Self, Private);
+    pub const as = C.as;
+    pub const ref = C.ref;
+    pub const unref = C.unref;
+    const private = C.private;
 
     pub const Class = extern struct {
         parent_class: Parent.Class,

src/apprt/gtk-ng/class/config_errors_dialog.zig

@@ -5,6 +5,7 @@ const gtk = @import("gtk");
 
 const gresource = @import("../build/gresource.zig");
 const adw_version = @import("../adw_version.zig");
+const Common = @import("../class.zig").Common;
 const Config = @import("config.zig").Config;
 
 const log = std.log.scoped(.gtk_ghostty_config_errors_dialog);
@@ -58,7 +59,7 @@ pub const ConfigErrorsDialog = extern struct {
 
     const Private = struct {
         config: ?*Config,
-        var offset: c_int = 0;
+        pub var offset: c_int = 0;
     };
 
     pub fn new(config: *Config) *Self {
@@ -129,25 +130,11 @@ pub const ConfigErrorsDialog = extern struct {
         priv.config = config;
     }
 
-    pub fn as(win: *Self, comptime T: type) *T {
-        return gobject.ext.as(T, win);
-    }
-
-    pub fn ref(self: *Self) *Self {
-        return @ptrCast(@alignCast(gobject.Object.ref(self.as(gobject.Object))));
-    }
-
-    pub fn unref(self: *Self) void {
-        gobject.Object.unref(self.as(gobject.Object));
-    }
-
-    fn private(self: *Self) *Private {
-        return gobject.ext.impl_helpers.getPrivate(
-            self,
-            Private,
-            Private.offset,
-        );
-    }
+    const C = Common(Self, Private);
+    pub const as = C.as;
+    pub const ref = C.ref;
+    pub const unref = C.unref;
+    const private = C.private;
 
     pub const Class = extern struct {
         parent_class: Parent.Class,

src/apprt/gtk-ng/class/window.zig

@@ -4,6 +4,7 @@ const gobject = @import("gobject");
 const gtk = @import("gtk");
 
 const gresource = @import("../build/gresource.zig");
+const Common = @import("../class.zig").Common;
 const Application = @import("application.zig").Application;
 
 const log = std.log.scoped(.gtk_ghostty_window);
@@ -22,7 +23,7 @@ pub const Window = extern struct {
 
     const Private = struct {
         _todo: u8,
-        var offset: c_int = 0;
+        pub var offset: c_int = 0;
     };
 
     pub fn new(app: *Application) *Self {
@@ -45,17 +46,11 @@ pub const Window = extern struct {
         );
     }
 
-    pub fn as(self: *Self, comptime T: type) *T {
-        return gobject.ext.as(T, self);
-    }
-
-    fn private(self: *Self) *Private {
-        return gobject.ext.impl_helpers.getPrivate(
-            self,
-            Private,
-            Private.offset,
-        );
-    }
+    const C = Common(Self, Private);
+    pub const as = C.as;
+    pub const ref = C.ref;
+    pub const unref = C.unref;
+    const private = C.private;
 
     pub const Class = extern struct {
         parent_class: Parent.Class,