From 183f7c9830c8d2d6e0f1654e4394a9d613ecb4ad Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Wed, 2 Oct 2019 20:24:29 +0200
Subject: [PATCH] LibGUI: Add GLazyWidget, a convenience widget for
 lazily-built UI's

Here's how you can use this to speed up startup time:

    auto widget = GLazyWidget::construct();
    widget->on_first_show = [](auto& self) {
        self.set_layout(...);
        ...
    };

Basically, it allows you to delay building the widget subtree until
it's shown for the first time.
---
 Libraries/LibGUI/GLazyWidget.cpp | 20 ++++++++++++++++++++
 Libraries/LibGUI/GLazyWidget.h   | 19 +++++++++++++++++++
 Libraries/LibGUI/Makefile        |  1 +
 3 files changed, 40 insertions(+)
 create mode 100644 Libraries/LibGUI/GLazyWidget.cpp
 create mode 100644 Libraries/LibGUI/GLazyWidget.h

diff --git a/Libraries/LibGUI/GLazyWidget.cpp b/Libraries/LibGUI/GLazyWidget.cpp
new file mode 100644
index 0000000000..e8eed96d0e
--- /dev/null
+++ b/Libraries/LibGUI/GLazyWidget.cpp
@@ -0,0 +1,20 @@
+#include <LibGUI/GLazyWidget.h>
+
+GLazyWidget::GLazyWidget(GWidget* parent)
+    : GWidget(parent)
+{
+}
+
+GLazyWidget::~GLazyWidget()
+{
+}
+
+void GLazyWidget::show_event(GShowEvent&)
+{
+    if (m_has_been_shown)
+        return;
+    m_has_been_shown = true;
+
+    ASSERT(on_first_show);
+    on_first_show(*this);
+}
diff --git a/Libraries/LibGUI/GLazyWidget.h b/Libraries/LibGUI/GLazyWidget.h
new file mode 100644
index 0000000000..69bc235508
--- /dev/null
+++ b/Libraries/LibGUI/GLazyWidget.h
@@ -0,0 +1,19 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GLazyWidget : public GWidget {
+    C_OBJECT(GLazyWidget)
+public:
+    virtual ~GLazyWidget() override;
+
+    Function<void(GLazyWidget&)> on_first_show;
+
+protected:
+    explicit GLazyWidget(GWidget* parent = nullptr);
+
+private:
+    virtual void show_event(GShowEvent&) override;
+
+    bool m_has_been_shown { false };
+};
diff --git a/Libraries/LibGUI/Makefile b/Libraries/LibGUI/Makefile
index ad806e4108..24d3d70ad7 100644
--- a/Libraries/LibGUI/Makefile
+++ b/Libraries/LibGUI/Makefile
@@ -55,6 +55,7 @@ OBJS = \
     GJsonArrayModel.o \
     GAboutDialog.o \
     GModelSelection.o \
+    GLazyWidget.o \
     GWindow.o
 
 LIBRARY = libgui.a