From dec066fa5cf5cf9466399426e937862d796f146e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?kleines=20Filmr=C3=B6llchen?= <filmroellchen@serenityos.org>
Date: Thu, 25 Jan 2024 14:11:38 +0100
Subject: [PATCH] GMLCompiler: Add common pattern for a user-defined fallible
 initializer

Many widget classes need to run substantial initialization code after
they have been setup from GML. With this change, an
initialize_fallibles() function is called if available, allowing the
initialization to be invoked from the GML setup automatically. This
means that the GML-generated creation function can now be used directly
for many more cases, and reduces code duplication.
---
 Base/usr/share/man/man5/GML/Usage.md                | 13 +++++++++++++
 .../Lagom/Tools/CodeGenerators/GMLCompiler/main.cpp |  2 ++
 Userland/Libraries/LibGUI/Widget.h                  |  9 +++++++++
 3 files changed, 24 insertions(+)

diff --git a/Base/usr/share/man/man5/GML/Usage.md b/Base/usr/share/man/man5/GML/Usage.md
index d9d176b6bf..9d23f0e8e9 100644
--- a/Base/usr/share/man/man5/GML/Usage.md
+++ b/Base/usr/share/man/man5/GML/Usage.md
@@ -50,3 +50,16 @@ From there, you can use `find_descendant_of_type_named` to select widgets from y
 // MyApp::Widget::foo_bar
 m_mem_add_button = *find_descendant_of_type_named<GUI::Button>("mem_add_button");
 ```
+
+### `initialize` Pattern
+
+Initialization, like adding models, attaching callbacks, etc., should be done in a member function with the signature:
+
+```cpp
+// MyApp::Widget
+ErrorOr<void> initialize();
+```
+
+This initializer function, if it exists, will automatically be called after the structure of your widget was set up by the auto-generated GML code.
+
+The only case where this function cannot be used is when your initialization requires additional parameters, like a GUI window. You may still consider moving as much initialization to the canonical function as possible.
diff --git a/Meta/Lagom/Tools/CodeGenerators/GMLCompiler/main.cpp b/Meta/Lagom/Tools/CodeGenerators/GMLCompiler/main.cpp
index c45b1a7e41..c66ca158b4 100644
--- a/Meta/Lagom/Tools/CodeGenerators/GMLCompiler/main.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/GMLCompiler/main.cpp
@@ -334,6 +334,8 @@ static ErrorOr<void> generate_loader_for_object(GUI::GML::Object const& gml_obje
         return {};
     }));
 
+    TRY(append(generator, "TRY(::GUI::initialize(*@object_name@));"));
+
     generator.append(TRY(String::repeated(' ', (indentation - 1) * 4)).bytes_as_string_view());
     generator.appendln("}");
 
diff --git a/Userland/Libraries/LibGUI/Widget.h b/Userland/Libraries/LibGUI/Widget.h
index 642128c9b4..5fc9606a1c 100644
--- a/Userland/Libraries/LibGUI/Widget.h
+++ b/Userland/Libraries/LibGUI/Widget.h
@@ -71,6 +71,15 @@ enum class AllowCallback {
     Yes
 };
 
+template<typename T>
+ALWAYS_INLINE ErrorOr<void> initialize(T& object)
+{
+    if constexpr (requires { { object.initialize() } -> SameAs<ErrorOr<void>>; })
+        return object.initialize();
+    else
+        return {};
+}
+
 class Widget : public GUI::Object {
     C_OBJECT(Widget)
 public: