From b29fefa6ec993a4fc82d20e8f994a692fd882103 Mon Sep 17 00:00:00 2001
From: Patrick Wuttke <mewin@mewin.de>
Date: Thu, 19 Sep 2024 00:58:27 +0200
Subject: [PATCH] Began implementing some little widget tree system.

---
 private/sdl_gpu_test/6_ui/app.cpp             |  11 ++
 private/sdl_gpu_test/6_ui/app.hpp             |   7 +-
 private/sdl_gpu_test/SModule                  |   4 +-
 private/sdl_gpu_test/application.cpp          |   1 +
 private/sdl_gpu_test/application.hpp          |   5 +
 private/sdl_gpu_test/gui/label.cpp            |  75 +++++++++
 private/sdl_gpu_test/gui/label.hpp            |  58 +++++++
 .../{6_ui/ui => gui}/ui_renderer.cpp          | 157 ++++++++++++------
 .../{6_ui/ui => gui}/ui_renderer.hpp          |  41 +++--
 private/sdl_gpu_test/gui/widget.cpp           |  67 ++++++++
 private/sdl_gpu_test/gui/widget.hpp           |  76 +++++++++
 11 files changed, 437 insertions(+), 65 deletions(-)
 create mode 100644 private/sdl_gpu_test/gui/label.cpp
 create mode 100644 private/sdl_gpu_test/gui/label.hpp
 rename private/sdl_gpu_test/{6_ui/ui => gui}/ui_renderer.cpp (62%)
 rename private/sdl_gpu_test/{6_ui/ui => gui}/ui_renderer.hpp (54%)
 create mode 100644 private/sdl_gpu_test/gui/widget.cpp
 create mode 100644 private/sdl_gpu_test/gui/widget.hpp

diff --git a/private/sdl_gpu_test/6_ui/app.cpp b/private/sdl_gpu_test/6_ui/app.cpp
index 46d79d9..6958704 100644
--- a/private/sdl_gpu_test/6_ui/app.cpp
+++ b/private/sdl_gpu_test/6_ui/app.cpp
@@ -69,6 +69,15 @@ void UIApp::init(const AppInitArgs& args)
     {
         mGamepad.open(gamepads[0]);
     }
+
+    // init the UI
+    mWidgetTree.init({.renderer = &mUIRenderer});
+    mLabel = mWidgetTree.getRootWidget().emplaceChild<Label>({
+        .text = "Test-Text!\nSecond $f00line$fff?",
+        .color = glm::vec4(0.f, 1.f, 0.f, 1.f),
+        .posX = 100,
+        .posY = 100
+    });
 }
 
 void UIApp::update(const AppUpdateArgs& args)
@@ -76,6 +85,8 @@ void UIApp::update(const AppUpdateArgs& args)
     Application::update(args);
 
     processInput(args);
+    mLabel->setText(std::format("Rotation: {}{}\n$rUI vertices: $f00{}", mRotation > 180.f ? "$00f": "$fff" , mRotation, mNumVertices));
+    mWidgetTree.revalidateWidgets();
 
     // begin rendering
     sdlpp::GPUCommandBuffer cmdBuffer = mDevice.acquireCommandBuffer();
diff --git a/private/sdl_gpu_test/6_ui/app.hpp b/private/sdl_gpu_test/6_ui/app.hpp
index 6f6c6dd..fdf6adf 100644
--- a/private/sdl_gpu_test/6_ui/app.hpp
+++ b/private/sdl_gpu_test/6_ui/app.hpp
@@ -6,8 +6,10 @@
 
 #include <glm/vec2.hpp>
 
-#include "./ui/ui_renderer.hpp"
 #include "../application.hpp"
+#include "../gui/label.hpp"
+#include "../gui/widget.hpp"
+#include "../gui/ui_renderer.hpp"
 #include "../sdlpp/gamepad.hpp"
 
 namespace sdl_gpu_test
@@ -22,6 +24,9 @@ private:
     sdlpp::GPUSampler mSampler;
 
     UIRenderer mUIRenderer;
+    WidgetTree mWidgetTree;
+
+    Label* mLabel;
 
     sdlpp::Gamepad mGamepad;
     Uint32 mNumVertices = 0;
diff --git a/private/sdl_gpu_test/SModule b/private/sdl_gpu_test/SModule
index 7568632..4ac2e07 100644
--- a/private/sdl_gpu_test/SModule
+++ b/private/sdl_gpu_test/SModule
@@ -5,6 +5,9 @@ src_files = Split("""
     main.cpp
     
     application.cpp
+    gui/label.cpp
+    gui/ui_renderer.cpp
+    gui/widget.cpp
     util/bitmap.cpp
     util/font_map.cpp
     util/mesh.cpp
@@ -16,7 +19,6 @@ src_files = Split("""
     4_textured_cube/app.cpp
     5_input/app.cpp
     6_ui/app.cpp
-    6_ui/ui/ui_renderer.cpp
 """)
 
 shader_files = env.Glob("#assets/shaders/glsl/*.frag") \
diff --git a/private/sdl_gpu_test/application.cpp b/private/sdl_gpu_test/application.cpp
index 8206118..2ee2cb9 100644
--- a/private/sdl_gpu_test/application.cpp
+++ b/private/sdl_gpu_test/application.cpp
@@ -76,6 +76,7 @@ void Application::run(std::span<const char*> args)
             .secondsSinceStart = std::chrono::duration_cast<std::chrono::duration<float>>(frameTime - startTime).count(),
             .tickSeconds = std::chrono::duration_cast<std::chrono::duration<float>>(frameTime - lastFrameTime).count()
         });
+        mTaskLoop.tick();
         lastFrameTime = frameTime;
     }
     cleanup({});
diff --git a/private/sdl_gpu_test/application.hpp b/private/sdl_gpu_test/application.hpp
index 13094e4..f271685 100644
--- a/private/sdl_gpu_test/application.hpp
+++ b/private/sdl_gpu_test/application.hpp
@@ -6,6 +6,7 @@
 
 #include <cstring>
 
+#include <mijin/async/coroutine.hpp>
 #include <mijin/virtual_filesystem/stacked.hpp>
 
 #include "./sdlpp/event.hpp"
@@ -36,6 +37,7 @@ protected:
     sdlpp::Window mWindow;
     sdlpp::GPUDevice mDevice;
     mijin::StackedFileSystemAdapter mFileSystem;
+    mijin::SimpleTaskLoop mTaskLoop;
     bool mRunning = true;
 public:
     virtual ~Application() noexcept = default;
@@ -49,6 +51,9 @@ public:
     [[nodiscard]]
     mijin::StackedFileSystemAdapter& getFileSystem() noexcept { return mFileSystem; }
 
+    [[nodiscard]]
+    mijin::SimpleTaskLoop& getTaskLoop() noexcept { return mTaskLoop; }
+
     virtual void init(const AppInitArgs& args);
     virtual void cleanup(const AppCleanupArgs& args);
     virtual void update(const AppUpdateArgs& args);
diff --git a/private/sdl_gpu_test/gui/label.cpp b/private/sdl_gpu_test/gui/label.cpp
new file mode 100644
index 0000000..75faba3
--- /dev/null
+++ b/private/sdl_gpu_test/gui/label.cpp
@@ -0,0 +1,75 @@
+
+#include "./label.hpp"
+
+namespace sdl_gpu_test
+{
+Label::Label(LabelCreateArgs args) : mText(std::move(args.text)), mColor(args.color), mPosX(args.posX), mPosY(args.posY)
+{
+
+}
+
+void Label::setText(std::string text)
+{
+    if (text != mText)
+    {
+        mText = std::move(text);
+        invalidate();
+    }
+}
+
+void Label::setColor(const glm::vec4& color)
+{
+    if (color != mColor)
+    {
+        mColor = color;
+        invalidate();
+    }
+}
+
+void Label::setPosX(int posX)
+{
+    if (posX != mPosX)
+    {
+        mPosX = posX;
+        invalidate();
+    }
+}
+
+void Label::setPosY(int posY)
+{
+    if (posY != mPosY)
+    {
+        mPosY = posY;
+        invalidate();
+    }
+}
+
+void Label::handleEnteredTree()
+{
+    update();
+}
+
+void Label::revalidate()
+{
+    update();
+}
+
+void Label::update()
+{
+    if (mTree == nullptr)
+    {
+        return;
+    }
+
+    if (mPrimitiveID != UIRenderer::UNSET_PRIMITIVE_ID)
+    {
+        mTree->getRenderer().removePrimitive(mPrimitiveID);
+    }
+    mTree->getRenderer().drawText({
+        .x = mPosX,
+        .y = mPosY,
+        .text = mText,
+        .color = mColor
+    }, &mPrimitiveID);
+}
+}
\ No newline at end of file
diff --git a/private/sdl_gpu_test/gui/label.hpp b/private/sdl_gpu_test/gui/label.hpp
new file mode 100644
index 0000000..76ffb6b
--- /dev/null
+++ b/private/sdl_gpu_test/gui/label.hpp
@@ -0,0 +1,58 @@
+
+#pragma once
+
+#if !defined(SDL_GPU_TEST_PRIVATE_SDL_GPU_TEST_GUI_LABEL_HPP_INCLUDED)
+#define SDL_GPU_TEST_PRIVATE_SDL_GPU_TEST_GUI_LABEL_HPP_INCLUDED 1
+
+#include <glm/vec4.hpp>
+
+#include "./widget.hpp"
+
+namespace sdl_gpu_test
+{
+struct LabelCreateArgs
+{
+    std::string text;
+    glm::vec4 color = {1.f, 1.f, 1.f, 1.f};
+    int posX = 0;
+    int posY = 0;
+};
+
+class Label : public Widget
+{
+public:
+    using create_args_t = LabelCreateArgs;
+private:
+    std::string mText;
+    glm::vec4 mColor = {1.f, 1.f, 1.f, 1.f};
+    int mPosX = 0;
+    int mPosY = 0;
+    UIRenderer::primitive_id_t mPrimitiveID = UIRenderer::UNSET_PRIMITIVE_ID;
+public:
+    explicit Label(LabelCreateArgs args);
+
+    [[nodiscard]]
+    const std::string& getText() const noexcept { return mText; }
+
+    [[nodiscard]]
+    const glm::vec4& getColor() const noexcept { return mColor; }
+
+    [[nodiscard]]
+    int getPosX() const noexcept { return mPosX; }
+
+    [[nodiscard]]
+    int getPosY() const noexcept { return mPosY; }
+
+    void setText(std::string text);
+    void setColor(const glm::vec4& color);
+    void setPosX(int posX);
+    void setPosY(int posY);
+
+    void handleEnteredTree() override;
+    void revalidate() override;
+private:
+    void update();
+};
+} // namespace sdl_gpu_test
+
+#endif // !defined(SDL_GPU_TEST_PRIVATE_SDL_GPU_TEST_GUI_LABEL_HPP_INCLUDED)
diff --git a/private/sdl_gpu_test/6_ui/ui/ui_renderer.cpp b/private/sdl_gpu_test/gui/ui_renderer.cpp
similarity index 62%
rename from private/sdl_gpu_test/6_ui/ui/ui_renderer.cpp
rename to private/sdl_gpu_test/gui/ui_renderer.cpp
index 431198f..3df81fe 100644
--- a/private/sdl_gpu_test/6_ui/ui/ui_renderer.cpp
+++ b/private/sdl_gpu_test/gui/ui_renderer.cpp
@@ -72,7 +72,7 @@ void UIRenderer::init(Application& application)
             .offset = offsetof(UIVertex, color)
         }
     };
-    mUIPipeline.create(device, {
+    mPipeline.create(device, {
         .vertexShader = vertexShader,
         .fragmentShader = fragmentShader,
         .vertexInputState = {
@@ -92,34 +92,24 @@ void UIRenderer::init(Application& application)
         .textureHeight = 256
     });
 
-    // render some test text
-    drawText({
-        .x = 100,
-        .y = 100,
-        .text = "Test-Text!\nSecond $f00line$fff?"
-    });
-
-    // create UI vertex buffer
-    mUIVertexBuffer.create(device, {
-        .usage = {.vertex = true},
-        .size = static_cast<Uint32>(mUIVertices.size() * sizeof(UIVertex))
-    });
-    application.uploadVertexData(mUIVertexBuffer, std::span(mUIVertices));
-
     // create texture and sampler
     sdlpp::GPUTextureCreateArgs textureArgs = {
         .format = sdlpp::GPUTextureFormat::R8G8B8A8_UNORM_SRGB,
         .usage = {.sampler = true}
     };
-    mUITexture = application.loadTexture("bitmaps/ui.png", textureArgs);
+    mTexture = application.loadTexture("bitmaps/ui.png", textureArgs);
 
-    mUISampler.create(device, {});
+    mSampler.create(device, {});
 }
 
 void UIRenderer::render(const UIRendererRenderArgs& args)
 {
-    if (!mUIVertices.empty())
+    if (!mVertices.empty())
     {
+        if (mVerticesDirty)
+        {
+            uploadVertices();
+        }
         const UIVertexShaderParameters uiVertexShaderParameters = {
             .windowSize = {
                 static_cast<float>(args.targetTextureWidth), static_cast<float>(args.targetTextureHeight)
@@ -137,19 +127,24 @@ void UIRenderer::render(const UIRendererRenderArgs& args)
         static const glm::vec4 WHITE(1.f, 1.f, 1.f, 1.f);
         args.cmdBuffer->pushFragmentUniformData(0, std::span(&WHITE, 1));
         args.cmdBuffer->pushVertexUniformData(0, std::span(&uiVertexShaderParameters, 1));
-        renderPass.bindFragmentSampler({.texture = mUITexture, .sampler = mUISampler});
-        renderPass.bindGraphicsPipeline(mUIPipeline);
-        renderPass.bindVertexBuffer({.buffer = mUIVertexBuffer});
-        renderPass.drawPrimitives({.numVertices = static_cast<Uint32>(mUIVertices.size())});
+        renderPass.bindFragmentSampler({.texture = mTexture, .sampler = mSampler});
+        renderPass.bindGraphicsPipeline(mPipeline);
+        renderPass.bindVertexBuffer({.buffer = mVertexBuffer});
+        renderPass.drawPrimitives({.numVertices = static_cast<Uint32>(mVertices.size())});
         renderPass.end();
     }
 }
 
-void UIRenderer::drawText(const DrawTextArgs& args)
+void UIRenderer::drawText(const DrawTextArgs& args, primitive_id_t* outPrimitiveId)
 {
     glm::vec4 color = args.color;
-    unsigned posX = args.x;
-    unsigned posY = args.y;
+    int posX = args.x;
+    int posY = args.y;
+
+    if (outPrimitiveId != nullptr && *outPrimitiveId == UNSET_PRIMITIVE_ID)
+    {
+        *outPrimitiveId = mNextOwner++;
+    }
 
     for (std::size_t pos = 0; pos < args.text.size(); ++pos)
     {
@@ -167,20 +162,32 @@ void UIRenderer::drawText(const DrawTextArgs& args)
                 break;
             }
             chr = args.text[pos];
-            if (pos < args.text.size() - 2
-                && mijin::isHexadecimalChar(chr)
-                && mijin::isHexadecimalChar(args.text[pos + 1])
-                && mijin::isHexadecimalChar(args.text[pos + 2]))
+            switch (chr)
             {
-                unsigned num = 0;
-                (void) mijin::toNumber(args.text.substr(pos, 3), num, 16);
-                const std::uint8_t red = static_cast<std::uint8_t>(num >> 8);
-                const std::uint8_t green = static_cast<std::uint8_t>((num >> 4) & 0xF);
-                const std::uint8_t blue = static_cast<std::uint8_t>(num & 0xF);
-                color.r = static_cast<float>(red) / 15.f;
-                color.g = static_cast<float>(green) / 15.f;
-                color.b = static_cast<float>(blue) / 15.f;
-                pos += 2;
+                case 'r':
+                    color = args.color;
+                    break;
+                default:
+                    if (pos < args.text.size() - 2
+                        && mijin::isHexadecimalChar(chr)
+                        && mijin::isHexadecimalChar(args.text[pos + 1])
+                        && mijin::isHexadecimalChar(args.text[pos + 2]))
+                    {
+                        unsigned num = 0;
+                        (void) mijin::toNumber(args.text.substr(pos, 3), num, 16);
+                        const std::uint8_t red = static_cast<std::uint8_t>(num >> 8);
+                        const std::uint8_t green = static_cast<std::uint8_t>((num >> 4) & 0xF);
+                        const std::uint8_t blue = static_cast<std::uint8_t>(num & 0xF);
+                        color.r = static_cast<float>(red) / 15.f;
+                        color.g = static_cast<float>(green) / 15.f;
+                        color.b = static_cast<float>(blue) / 15.f;
+                        pos += 2;
+                    }
+                    else
+                    {
+                        // what?
+                    }
+                    break;
             }
             break;
         case '\\':
@@ -197,14 +204,19 @@ void UIRenderer::drawText(const DrawTextArgs& args)
                 .y = posY,
                 .chr = chr,
                 .color = color
-            });
+            }, outPrimitiveId);
             break;
         }
     }
 }
 
-unsigned UIRenderer::drawChar(const DrawCharArgs& args)
+unsigned UIRenderer::drawChar(const DrawCharArgs& args, primitive_id_t* outPrimitiveId)
 {
+    if (outPrimitiveId != nullptr && *outPrimitiveId == UNSET_PRIMITIVE_ID)
+    {
+        *outPrimitiveId = mNextOwner++;
+    }
+
     const UVFontMapEntry& entry = mFontMap.entries[args.chr < 0 ? '_' : args.chr]; // TODO: more chars
     const glm::vec2 topLeft = {args.x + entry.xOffset, args.y + entry.yOffset};
     drawQuadInternal({
@@ -213,12 +225,31 @@ unsigned UIRenderer::drawChar(const DrawCharArgs& args)
         .uvTopLeft = {entry.uvX, entry.uvY},
         .uvBottomRight = {entry.uvX + entry.uvWidth, entry.uvY + entry.uvHeight},
         .color = args.color
-    });
+    }, outPrimitiveId == nullptr ? UNSET_PRIMITIVE_ID : *outPrimitiveId);
 
     return entry.xAdvance;
 }
 
-void UIRenderer::drawQuadInternal(const DrawQuadInternalArgs& args)
+bool UIRenderer::removePrimitive(primitive_id_t primitiveId)
+{
+    bool didRemove = false;
+    for (int triangleIdx = static_cast<int>(mTriangleOwners.size()) - 1; triangleIdx >= 0; --triangleIdx)
+    {
+        if (mTriangleOwners[triangleIdx] == primitiveId)
+        {
+            mTriangleOwners.erase(mTriangleOwners.begin() + triangleIdx);
+            mVertices.erase(mVertices.begin() + 3 * triangleIdx, mVertices.begin() + 3 * (triangleIdx + 1));
+            didRemove = true;
+        }
+    }
+    if (didRemove)
+    {
+        mVerticesDirty = true;
+    }
+    return didRemove;
+}
+
+void UIRenderer::drawQuadInternal(const DrawQuadInternalArgs& args, primitive_id_t primitiveId)
 {
     const UIVertex topLeft = {
         .pos = args.topLeft,
@@ -241,11 +272,41 @@ void UIRenderer::drawQuadInternal(const DrawQuadInternalArgs& args)
         .color = args.color
     };
 
-    mUIVertices.push_back(topLeft);
-    mUIVertices.push_back(bottomLeft);
-    mUIVertices.push_back(topRight);
-    mUIVertices.push_back(bottomRight);
-    mUIVertices.push_back(topRight);
-    mUIVertices.push_back(bottomLeft);
+    addTriangleInternal(topLeft, bottomLeft, topRight, primitiveId);
+    addTriangleInternal(bottomRight, topRight, bottomLeft, primitiveId);
+
+    mVerticesDirty = true;
+}
+
+void UIRenderer::addTriangleInternal(const UIVertex& v0, const UIVertex& v1, const UIVertex& v2, UIRenderer::primitive_id_t primitiveId)
+{
+    mVertices.push_back(v0);
+    mVertices.push_back(v1);
+    mVertices.push_back(v2);
+    mTriangleOwners.push_back(primitiveId);
+}
+
+void UIRenderer::uploadVertices()
+{
+    if (mVertexBufferSize < mVertices.size())
+    {
+        if (mVertexBufferSize > 0)
+        {
+            mVertexBuffer.destroy();
+        }
+
+        mVertexBuffer.create(mApplication->getDevice(), {
+            .usage = {.vertex = true},
+            .size = static_cast<Uint32>(mVertices.size() * sizeof(UIVertex))
+        });
+
+        mVertexBufferSize = mVertices.size();
+    }
+
+    if (!mVertices.empty())
+    {
+        mApplication->uploadVertexData(mVertexBuffer, std::span(mVertices));
+    }
+    mVerticesDirty = false;
 }
 }
diff --git a/private/sdl_gpu_test/6_ui/ui/ui_renderer.hpp b/private/sdl_gpu_test/gui/ui_renderer.hpp
similarity index 54%
rename from private/sdl_gpu_test/6_ui/ui/ui_renderer.hpp
rename to private/sdl_gpu_test/gui/ui_renderer.hpp
index d97aea0..1a456f0 100644
--- a/private/sdl_gpu_test/6_ui/ui/ui_renderer.hpp
+++ b/private/sdl_gpu_test/gui/ui_renderer.hpp
@@ -6,9 +6,9 @@
 
 #include <glm/vec2.hpp>
 #include <glm/vec4.hpp>
-#include "../../application.hpp"
-#include "../../sdlpp/gpu.hpp"
-#include "../../util/font_map.hpp"
+#include "../application.hpp"
+#include "../sdlpp/gpu.hpp"
+#include "../util/font_map.hpp"
 
 namespace sdl_gpu_test::inline app6
 {
@@ -21,16 +21,16 @@ struct UIVertex
 
 struct DrawTextArgs
 {
-    unsigned x = 0;
-    unsigned y = 0;
+    int x = 0;
+    int y = 0;
     std::string_view text;
     glm::vec4 color = glm::vec4(1.f);
 };
 
 struct DrawCharArgs
 {
-    unsigned x = 0;
-    unsigned y = 0;
+    int x = 0;
+    int y = 0;
     char chr;
     glm::vec4 color = glm::vec4(1.f);
 };
@@ -45,20 +45,29 @@ struct UIRendererRenderArgs
 
 class UIRenderer
 {
+public:
+    using primitive_id_t = std::uint64_t;
+
+    static constexpr std::uint64_t UNSET_PRIMITIVE_ID = 0;
 private:
     Application* mApplication = nullptr;
-    sdlpp::GPUBuffer mUIVertexBuffer;
-    sdlpp::GPUGraphicsPipeline mUIPipeline;
-    std::vector<UIVertex> mUIVertices;
-    sdlpp::GPUTexture mUITexture;
-    sdlpp::GPUSampler mUISampler;
+    sdlpp::GPUBuffer mVertexBuffer;
+    sdlpp::GPUGraphicsPipeline mPipeline;
+    sdlpp::GPUTexture mTexture;
+    sdlpp::GPUSampler mSampler;
+    std::vector<UIVertex> mVertices;
+    std::vector<primitive_id_t> mTriangleOwners;
+    primitive_id_t mNextOwner = 1;
     UVFontMap mFontMap;
+    bool mVerticesDirty = true;
+    std::size_t mVertexBufferSize = 0;
 public:
     void init(Application& application);
     void render(const UIRendererRenderArgs& args);
 
-    void drawText(const DrawTextArgs& args);
-    unsigned drawChar(const DrawCharArgs& args);
+    void drawText(const DrawTextArgs& args, primitive_id_t* outPrimitiveId = nullptr);
+    unsigned drawChar(const DrawCharArgs& args, primitive_id_t* outPrimitiveId = nullptr);
+    bool removePrimitive(primitive_id_t primitiveId);
 private:
     struct DrawQuadInternalArgs
     {
@@ -68,7 +77,9 @@ private:
         glm::vec2 uvBottomRight;
         glm::vec4 color = glm::vec4(1.f);
     };
-    void drawQuadInternal(const DrawQuadInternalArgs& args);
+    void drawQuadInternal(const DrawQuadInternalArgs& args, primitive_id_t primitiveId);
+    void addTriangleInternal(const UIVertex& v0, const UIVertex& v1, const UIVertex& v2, primitive_id_t primitiveId);
+    void uploadVertices();
 };
 } // namespace sdl_gpu_test::inline app6
 
diff --git a/private/sdl_gpu_test/gui/widget.cpp b/private/sdl_gpu_test/gui/widget.cpp
new file mode 100644
index 0000000..4bd26e1
--- /dev/null
+++ b/private/sdl_gpu_test/gui/widget.cpp
@@ -0,0 +1,67 @@
+
+#include "./widget.hpp"
+
+#include <mijin/debug/assert.hpp>
+
+namespace sdl_gpu_test
+{
+void Widget::enterTree(WidgetTree* tree, ParentWidget* parent)
+{
+    MIJIN_ASSERT(mTree == nullptr && mParent == nullptr, "Widget is already in a tree.");
+    MIJIN_ASSERT_FATAL(tree != nullptr, "Missing argument: tree");
+
+    mTree = tree;
+    mParent = parent;
+    handleEnteredTree();
+}
+
+void Widget::invalidate()
+{
+    if (mTree != nullptr)
+    {
+        mTree->invalidateWidget(this);
+    }
+}
+
+void ParentWidget::handleEnteredTree()
+{
+    for (widget_ptr_t& child : mChildren)
+    {
+        child->enterTree(mTree, this);
+    }
+}
+
+Widget* ParentWidget::addChild(widget_ptr_t&& child)
+{
+    mChildren.push_back(std::move(child));
+
+    if (mTree != nullptr)
+    {
+        mChildren.back()->enterTree(mTree, this);
+    }
+
+    return mChildren.back().get();
+}
+
+void WidgetTree::init(const WidgetTreeInitArgs& args)
+{
+    MIJIN_ASSERT_FATAL(args.renderer != nullptr, "Missing argument: renderer.");
+    mRenderer = args.renderer;
+
+    mRootWidget.enterTree(this, nullptr);
+}
+
+void WidgetTree::revalidateWidgets()
+{
+    for (Widget* widget : mInvalidWidgets)
+    {
+        widget->revalidate();
+    }
+    mInvalidWidgets.clear();
+}
+
+void WidgetTree::invalidateWidget(Widget* widget) noexcept
+{
+    mInvalidWidgets.insert(widget);
+}
+}
\ No newline at end of file
diff --git a/private/sdl_gpu_test/gui/widget.hpp b/private/sdl_gpu_test/gui/widget.hpp
new file mode 100644
index 0000000..2d86a52
--- /dev/null
+++ b/private/sdl_gpu_test/gui/widget.hpp
@@ -0,0 +1,76 @@
+
+#pragma once
+
+#if !defined(SDL_GPU_TEST_PRIVATE_SDL_GPU_TEST_GUI_WIDGET_HPP_INCLUDED)
+#define SDL_GPU_TEST_PRIVATE_SDL_GPU_TEST_GUI_WIDGET_HPP_INCLUDED 1
+
+#include <memory>
+#include <vector>
+#include <unordered_set>
+
+#include "./ui_renderer.hpp"
+
+namespace sdl_gpu_test
+{
+class Widget
+{
+protected:
+    class WidgetTree* mTree = nullptr;
+    class ParentWidget* mParent = nullptr;
+public:
+    virtual ~Widget() noexcept = default;
+
+    virtual void handleEnteredTree() {}
+    virtual void revalidate() {}
+
+    void invalidate();
+private:
+    void enterTree(class WidgetTree* tree, class ParentWidget* parent);
+
+    friend class ParentWidget;
+    friend class WidgetTree;
+};
+using widget_ptr_t = std::unique_ptr<Widget>;
+
+class ParentWidget : public Widget
+{
+private:
+    std::vector<widget_ptr_t> mChildren;
+public:
+    void handleEnteredTree() override;
+
+    Widget* addChild(widget_ptr_t&& child);
+
+    template<typename TChild>
+    TChild* emplaceChild(typename TChild::create_args_t args)
+    {
+        return static_cast<TChild*>(addChild(std::make_unique<TChild>(args)));
+    }
+};
+
+struct WidgetTreeInitArgs
+{
+    UIRenderer* renderer = nullptr;
+};
+
+class WidgetTree
+{
+private:
+    UIRenderer* mRenderer = nullptr;
+    ParentWidget mRootWidget;
+    std::unordered_set<Widget*> mInvalidWidgets;
+public:
+    void init(const WidgetTreeInitArgs& args);
+    void revalidateWidgets();
+
+    [[nodiscard]]
+    UIRenderer& getRenderer() const noexcept { return *mRenderer; }
+
+    [[nodiscard]]
+    ParentWidget& getRootWidget() noexcept { return mRootWidget; }
+
+    void invalidateWidget(Widget* widget) noexcept;
+};
+} // namespace sdl_gpu_test
+
+#endif // !defined(SDL_GPU_TEST_PRIVATE_SDL_GPU_TEST_GUI_WIDGET_HPP_INCLUDED)