Stabilize XCUI schema compiler and phase 2 checkpoint

docs/plan/XCUI_Phase_Status_2026-04-05.md

@@ -9,16 +9,18 @@ Old `editor` replacement is explicitly out of scope for this phase.
 
 - Phase 1 sandbox batch committed and pushed as `67a28bd` (`Add XCUI new editor sandbox phase 1`).
 - Current work has moved into Phase 2: filling the three-layer XCUI structure instead of replacing the old editor.
+- Phase 2 common/runtime batch is now buildable locally again after repairing the duplicated schema helper collision inside `UIDocumentCompiler.cpp`.
 
 ## Three-Layer Status
 
 ### 1. Common Core
 
 - `UI::DrawData`, input event types, focus routing, style/theme resolution are in active use.
-- `UIDocumentCompiler` was restored to a stable buildable baseline after a broken parallel schema attempt corrupted the file.
+- `UIDocumentCompiler` is buildable again after repairing the duplicated schema helper regression introduced by overlapping schema work.
 - Build-system hardening for MSVC/PDB output paths has started in root CMake, `engine/CMakeLists.txt`, `new_editor/CMakeLists.txt`, and `tests/NewEditor/CMakeLists.txt`.
 - Shared engine-side XCUI runtime scaffolding is now present under `engine/include/XCEngine/UI/Runtime` and `engine/src/UI/Runtime`.
-- Core regression coverage now includes `UIContext`, layout, style, and runtime screen-player tests through `core_ui_tests`.
+- Shared engine-side `UIDocumentScreenHost` now compiles `.xcui` / `.xctheme` screen documents into a runtime-facing document host path instead of leaving all document ownership in `new_editor`.
+- Core regression coverage now includes `UIContext`, layout, style, runtime screen player/system, and real document-host tests through `core_ui_tests`.
 
 Current gap:
 
@@ -28,21 +30,22 @@ Current gap:
 
 ### 2. Runtime/Game Layer
 
-- Runtime-side XCUI is still shallow.
 - The main concrete progress here is that the retained-mode demo runtime now supports a real `TextField` input path with UTF-8 text entry and backspace handling.
-- This proves that the runtime-facing layer is no longer limited to static cards/buttons.
-- Engine-side runtime ownership is no longer zero: `UIScreenPlayer` and `UISystem` now define a minimal shared runtime contract for loading a screen document, ticking it with input, and collecting `UI::UIDrawData`.
+- The demo runtime has moved past single-line input: multiline `TextArea` behavior is now covered in the sandbox testbed.
+- Engine-side runtime ownership is no longer zero: `UIScreenPlayer`, `UIDocumentScreenHost`, and `UISystem` now define a shared runtime contract for loading a screen document, ticking it with input, and collecting `UI::UIDrawData`.
+- `UISystem` now supports layered screen composition semantics: stacked screen players, top-interactive input routing, and modal layers that block lower screens.
 
 Current gap:
 
-- No real game-facing screen host, menu stack, HUD stack, or shared runtime widget library yet.
-- The new runtime layer still needs a real XCUI document host implementation instead of the current host-facing contract only.
+- No production game-loop integration has been wired yet from scene/runtime systems into `UISystem`.
+- The runtime widget library is still shallow and missing the editor-grade controls that will later be shared downward.
 
 ### 3. Editor Layer
 
 - `new_editor` remains the isolated XCUI sandbox.
 - Native hosted preview is working as `RHI offscreen surface -> ImGui shell texture embed`.
 - `XCUI Demo` remains the long-lived effect and behavior testbed.
+- `XCUI Demo` now covers both single-line and multiline text authoring behavior.
 - `LayoutLab` now includes a `ScrollView` prototype and a more editor-like three-column authored layout.
 - Panel diagnostics were expanded to clearly separate preview/runtime/input state and native vs legacy paths.
 - `XCNewEditor` builds successfully to `build/new_editor/bin/Debug/XCNewEditor.exe`.
@@ -54,18 +57,24 @@ Current gap:
 
 ## Validated This Phase
 
-- `new_editor_xcui_demo_runtime_tests`: `6/6`
+- `new_editor_xcui_demo_runtime_tests`: `7/7`
 - `new_editor_xcui_layout_lab_runtime_tests`: `5/5`
 - `new_editor_xcui_rhi_command_compiler_tests`: `6/6`
 - `new_editor_xcui_rhi_render_backend_tests`: `5/5`
 - `XCNewEditor` Debug target builds successfully
-- `core_ui_tests`: `20/20`
+- `core_ui_tests`: `24/24`
 
 ## Landed This Phase
 
 - Demo runtime `TextField` with UTF-8 text insertion, caret state, and backspace.
+- Demo runtime multiline `TextArea` path in the sandbox and test coverage for caret movement / multiline input.
 - Demo authored resources updated to exercise the input field.
 - LayoutLab `ScrollView` prototype with clipping and hover rejection outside clipped content.
+- Engine runtime layer added:
+  - `UIScreenPlayer`
+  - `UIDocumentScreenHost`
+  - `UISystem`
+  - layered screen composition and modal blocking semantics
 - RHI image path improvements:
   - clipped image UV adjustment
   - mirrored image UV preservation
@@ -73,18 +82,19 @@ Current gap:
   - per-batch scissor application
 - `new_editor` panel/shell diagnostics improvements for hosted preview state.
 - XCUI asset document loading changed to prefer direct source compilation before `ResourceManager` fallback for the sandbox path, fixing the LayoutLab crash.
+- `UIDocumentCompiler.cpp` repaired enough to restore full local builds after the duplicated schema-helper regression.
 
 ## Phase Risks Still Open
 
-- Schema/validation work must be restarted cleanly after the corrupted parallel attempt.
+- Schema/validation still needs a clean minimal landing instead of the current partially merged state in `UIDocumentCompiler.cpp`.
 - `ScrollView` is still authored/static; no wheel-driven scrolling or virtualization yet.
 - `Image` widgets still do not have source-rect/atlas-subregion level API in the high-level draw command model.
 - Editor shell still depends on ImGui as host chrome.
 
 ## Next Phase
 
-1. Re-open common-layer schema/validation on a clean baseline and land the smallest stable version.
-2. Expand runtime/game-layer ownership from `UIScreenPlayer`/`UISystem` into a real XCUI document host plus menu/HUD stack patterns.
-3. Add next editor-facing widgets: `TextArea`, list/tree, property-style sections.
+1. Cleanly stabilize schema/validation in `UIDocumentCompiler.cpp` and add targeted schema regression tests.
+2. Expand runtime/game-layer ownership from the current document host + layered `UISystem` into reusable menu/HUD stack patterns and engine runtime integration.
+3. Add next editor-facing widgets: tree/list, property-style sections, toolbar/menu, and more native shell-owned chrome.
 4. Move more diagnostics and shell affordances into XCUI-owned editor-layer surfaces instead of only ImGui HUDs.
 5. Continue phased validation, commit, push, and plan refresh after each stable batch.

engine/src/Resources/UI/UIDocumentCompiler.cpp

@@ -24,6 +24,9 @@ Containers::String ToContainersString(const std::string& value) {
 }
 
 std::string ToStdString(const Containers::String& value) {
+    if (value.Empty() || value.CStr() == nullptr) {
+        return std::string();
+    }
     return std::string(value.CStr());
 }
 
@@ -151,6 +154,199 @@ bool ReadNode(std::ifstream& stream, UIDocumentNode& outNode) {
     return true;
 }
 
+struct UIDocumentArtifactSchemaDefinitionHeader {
+    Core::uint32 elementCount = 0;
+    Core::uint32 valid = 0;
+};
+
+struct UIDocumentArtifactSchemaElementHeader {
+    Core::uint32 attributeCount = 0;
+    Core::uint32 childCount = 0;
+    Core::uint32 line = 1;
+    Core::uint32 column = 1;
+    Core::uint32 allowUnknownAttributes = 0;
+    Core::uint32 allowUnknownChildren = 0;
+};
+
+struct UIDocumentArtifactSchemaAttributeHeader {
+    Core::uint32 valueType = 0;
+    Core::uint32 documentKind = 0;
+    Core::uint32 allowedValueCount = 0;
+    Core::uint32 line = 1;
+    Core::uint32 column = 1;
+    Core::uint32 required = 0;
+    Core::uint32 restrictDocumentKind = 0;
+};
+
+bool WriteSchemaAttribute(std::ofstream& stream, const UISchemaAttributeDefinition& attribute) {
+    WriteString(stream, attribute.name);
+
+    UIDocumentArtifactSchemaAttributeHeader header;
+    header.valueType = static_cast<Core::uint32>(attribute.valueType);
+    header.documentKind = static_cast<Core::uint32>(attribute.documentKind);
+    header.allowedValueCount = static_cast<Core::uint32>(attribute.allowedValues.Size());
+    header.line = attribute.location.line;
+    header.column = attribute.location.column;
+    header.required = attribute.required ? 1u : 0u;
+    header.restrictDocumentKind = attribute.restrictDocumentKind ? 1u : 0u;
+    stream.write(reinterpret_cast<const char*>(&header), sizeof(header));
+    if (!stream) {
+        return false;
+    }
+
+    for (const Containers::String& value : attribute.allowedValues) {
+        WriteString(stream, value);
+        if (!stream) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+bool ReadSchemaAttribute(std::ifstream& stream, UISchemaAttributeDefinition& outAttribute) {
+    if (!ReadString(stream, outAttribute.name)) {
+        return false;
+    }
+
+    UIDocumentArtifactSchemaAttributeHeader header;
+    stream.read(reinterpret_cast<char*>(&header), sizeof(header));
+    if (!stream) {
+        return false;
+    }
+
+    outAttribute.valueType = static_cast<UISchemaValueType>(header.valueType);
+    outAttribute.documentKind = static_cast<UIDocumentKind>(header.documentKind);
+    outAttribute.location.line = header.line;
+    outAttribute.location.column = header.column;
+    outAttribute.required = header.required != 0;
+    outAttribute.restrictDocumentKind = header.restrictDocumentKind != 0;
+    outAttribute.allowedValues.Clear();
+    outAttribute.allowedValues.Reserve(header.allowedValueCount);
+    for (Core::uint32 index = 0; index < header.allowedValueCount; ++index) {
+        Containers::String value;
+        if (!ReadString(stream, value)) {
+            return false;
+        }
+        outAttribute.allowedValues.PushBack(std::move(value));
+    }
+
+    return true;
+}
+
+bool WriteSchemaElement(std::ofstream& stream, const UISchemaElementDefinition& element) {
+    WriteString(stream, element.tagName);
+
+    UIDocumentArtifactSchemaElementHeader header;
+    header.attributeCount = static_cast<Core::uint32>(element.attributes.Size());
+    header.childCount = static_cast<Core::uint32>(element.children.Size());
+    header.line = element.location.line;
+    header.column = element.location.column;
+    header.allowUnknownAttributes = element.allowUnknownAttributes ? 1u : 0u;
+    header.allowUnknownChildren = element.allowUnknownChildren ? 1u : 0u;
+    stream.write(reinterpret_cast<const char*>(&header), sizeof(header));
+    if (!stream) {
+        return false;
+    }
+
+    for (const UISchemaAttributeDefinition& attribute : element.attributes) {
+        if (!WriteSchemaAttribute(stream, attribute)) {
+            return false;
+        }
+    }
+
+    for (const UISchemaElementDefinition& child : element.children) {
+        if (!WriteSchemaElement(stream, child)) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+bool ReadSchemaElement(std::ifstream& stream, UISchemaElementDefinition& outElement) {
+    if (!ReadString(stream, outElement.tagName)) {
+        return false;
+    }
+
+    UIDocumentArtifactSchemaElementHeader header;
+    stream.read(reinterpret_cast<char*>(&header), sizeof(header));
+    if (!stream) {
+        return false;
+    }
+
+    outElement.location.line = header.line;
+    outElement.location.column = header.column;
+    outElement.allowUnknownAttributes = header.allowUnknownAttributes != 0;
+    outElement.allowUnknownChildren = header.allowUnknownChildren != 0;
+    outElement.attributes.Clear();
+    outElement.children.Clear();
+    outElement.attributes.Reserve(header.attributeCount);
+    outElement.children.Reserve(header.childCount);
+    for (Core::uint32 index = 0; index < header.attributeCount; ++index) {
+        UISchemaAttributeDefinition attribute;
+        if (!ReadSchemaAttribute(stream, attribute)) {
+            return false;
+        }
+        outElement.attributes.PushBack(std::move(attribute));
+    }
+
+    for (Core::uint32 index = 0; index < header.childCount; ++index) {
+        UISchemaElementDefinition child;
+        if (!ReadSchemaElement(stream, child)) {
+            return false;
+        }
+        outElement.children.PushBack(std::move(child));
+    }
+
+    return true;
+}
+
+bool WriteSchemaDefinition(std::ofstream& stream, const UISchemaDefinition& schemaDefinition) {
+    WriteString(stream, schemaDefinition.name);
+
+    UIDocumentArtifactSchemaDefinitionHeader header;
+    header.elementCount = static_cast<Core::uint32>(schemaDefinition.elements.Size());
+    header.valid = schemaDefinition.valid ? 1u : 0u;
+    stream.write(reinterpret_cast<const char*>(&header), sizeof(header));
+    if (!stream) {
+        return false;
+    }
+
+    for (const UISchemaElementDefinition& element : schemaDefinition.elements) {
+        if (!WriteSchemaElement(stream, element)) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+bool ReadSchemaDefinition(std::ifstream& stream, UISchemaDefinition& outSchemaDefinition) {
+    if (!ReadString(stream, outSchemaDefinition.name)) {
+        return false;
+    }
+
+    UIDocumentArtifactSchemaDefinitionHeader header;
+    stream.read(reinterpret_cast<char*>(&header), sizeof(header));
+    if (!stream) {
+        return false;
+    }
+
+    outSchemaDefinition.valid = header.valid != 0;
+    outSchemaDefinition.elements.Clear();
+    outSchemaDefinition.elements.Reserve(header.elementCount);
+    for (Core::uint32 index = 0; index < header.elementCount; ++index) {
+        UISchemaElementDefinition element;
+        if (!ReadSchemaElement(stream, element)) {
+            return false;
+        }
+        outSchemaDefinition.elements.PushBack(std::move(element));
+    }
+
+    return true;
+}
+
 bool IsNameStartChar(char ch) {
     return std::isalpha(static_cast<unsigned char>(ch)) != 0 ||
            ch == '_' ||
@@ -173,6 +369,560 @@ bool LooksLikeUIDocumentReference(const Containers::String& value) {
             trimmed.size() >= 9 && trimmed.rfind(".xcschema") == trimmed.size() - 9);
 }
 
+bool ContainsErrorDiagnostics(const Containers::Array<UIDocumentDiagnostic>& diagnostics) {
+    for (const UIDocumentDiagnostic& diagnostic : diagnostics) {
+        if (diagnostic.severity == UIDocumentDiagnosticSeverity::Error) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+void AppendDiagnostic(
+    const Containers::String& sourcePath,
+    Containers::Array<UIDocumentDiagnostic>& diagnostics,
+    Containers::String& inOutErrorMessage,
+    UIDocumentDiagnosticSeverity severity,
+    const UIDocumentSourceLocation& location,
+    const Containers::String& message) {
+    UIDocumentDiagnostic diagnostic;
+    diagnostic.severity = severity;
+    diagnostic.location = location;
+    diagnostic.message = message;
+    diagnostics.PushBack(std::move(diagnostic));
+
+    if (severity == UIDocumentDiagnosticSeverity::Error && inOutErrorMessage.Empty()) {
+        inOutErrorMessage = FormatDiagnosticMessage(sourcePath, location, message);
+    }
+}
+
+bool TryParseBooleanValueExtended(const Containers::String& value, bool& outValue) {
+    const std::string normalized = ToLowerCopy(ToStdString(value.Trim()));
+    if (normalized == "true" || normalized == "1" || normalized == "yes") {
+        outValue = true;
+        return true;
+    }
+    if (normalized == "false" || normalized == "0" || normalized == "no") {
+        outValue = false;
+        return true;
+    }
+    return false;
+}
+
+bool TryParseSchemaValueTypeExtended(const Containers::String& value, UISchemaValueType& outValueType) {
+    const std::string normalized = ToLowerCopy(ToStdString(value.Trim()));
+    if (normalized == "string") {
+        outValueType = UISchemaValueType::String;
+        return true;
+    }
+    if (normalized == "boolean" || normalized == "bool") {
+        outValueType = UISchemaValueType::Boolean;
+        return true;
+    }
+    if (normalized == "integer" || normalized == "int") {
+        outValueType = UISchemaValueType::Integer;
+        return true;
+    }
+    if (normalized == "number" || normalized == "float") {
+        outValueType = UISchemaValueType::Number;
+        return true;
+    }
+    if (normalized == "document") {
+        outValueType = UISchemaValueType::Document;
+        return true;
+    }
+    if (normalized == "enum") {
+        outValueType = UISchemaValueType::Enum;
+        return true;
+    }
+    return false;
+}
+
+bool TryParseDocumentKindValueExtended(const Containers::String& value, UIDocumentKind& outKind) {
+    const std::string normalized = ToLowerCopy(ToStdString(value.Trim()));
+    if (normalized == "view" || normalized == "xcui") {
+        outKind = UIDocumentKind::View;
+        return true;
+    }
+    if (normalized == "theme" || normalized == "xctheme") {
+        outKind = UIDocumentKind::Theme;
+        return true;
+    }
+    if (normalized == "schema" || normalized == "xcschema") {
+        outKind = UIDocumentKind::Schema;
+        return true;
+    }
+    return false;
+}
+
+Containers::Array<Containers::String> ParseAllowedValueList(const Containers::String& value) {
+    Containers::Array<Containers::String> values;
+    std::string token;
+    const std::string rawValue = ToStdString(value);
+
+    auto flushToken = [&]() {
+        Containers::String trimmed = ToContainersString(token).Trim();
+        if (!trimmed.Empty()) {
+            values.PushBack(std::move(trimmed));
+        }
+        token.clear();
+    };
+
+    for (char ch : rawValue) {
+        if (ch == ',' || ch == '|' || ch == ';') {
+            flushToken();
+            continue;
+        }
+        token.push_back(ch);
+    }
+    flushToken();
+    return values;
+}
+
+bool ValidateNodeTree(
+    const Containers::String& sourcePath,
+    const UIDocumentNode& node,
+    Containers::Array<UIDocumentDiagnostic>& diagnostics,
+    Containers::String& inOutErrorMessage) {
+    bool valid = true;
+    std::unordered_set<std::string> seenAttributeNames;
+
+    for (const UIDocumentAttribute& attribute : node.attributes) {
+        const std::string key = ToStdString(attribute.name);
+        if (!seenAttributeNames.insert(key).second) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Duplicate attribute '") +
+                    attribute.name +
+                    "' on <" +
+                    node.tagName +
+                    ">.");
+            valid = false;
+        }
+    }
+
+    for (const UIDocumentNode& child : node.children) {
+        if (!ValidateNodeTree(sourcePath, child, diagnostics, inOutErrorMessage)) {
+            valid = false;
+        }
+    }
+
+    return valid;
+}
+
+const UIDocumentAttribute* FindAttributeByName(
+    const UIDocumentNode& node,
+    const char* name,
+    const char* alias = nullptr) {
+    if (const UIDocumentAttribute* attribute = node.FindAttribute(name)) {
+        return attribute;
+    }
+    if (alias != nullptr) {
+        return node.FindAttribute(alias);
+    }
+    return nullptr;
+}
+
+bool ValidateAllowedAttributes(
+    const Containers::String& sourcePath,
+    const UIDocumentNode& node,
+    std::initializer_list<const char*> allowedAttributeNames,
+    Containers::Array<UIDocumentDiagnostic>& diagnostics,
+    Containers::String& inOutErrorMessage) {
+    bool valid = true;
+    for (const UIDocumentAttribute& attribute : node.attributes) {
+        bool allowed = false;
+        for (const char* allowedName : allowedAttributeNames) {
+            if (attribute.name == allowedName) {
+                allowed = true;
+                break;
+            }
+        }
+
+        if (!allowed) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Unsupported attribute '") +
+                    attribute.name +
+                    "' on schema node <" +
+                    node.tagName +
+                    ">.");
+            valid = false;
+        }
+    }
+    return valid;
+}
+
+bool BuildSchemaAttributeDefinition(
+    const Containers::String& sourcePath,
+    const UIDocumentNode& node,
+    UISchemaAttributeDefinition& outDefinition,
+    Containers::Array<UIDocumentDiagnostic>& diagnostics,
+    Containers::String& inOutErrorMessage) {
+    bool valid = true;
+    if (!node.children.Empty()) {
+        AppendDiagnostic(
+            sourcePath,
+            diagnostics,
+            inOutErrorMessage,
+            UIDocumentDiagnosticSeverity::Error,
+            node.location,
+            "Schema <Attribute> nodes cannot contain child elements.");
+        valid = false;
+    }
+
+    if (!ValidateAllowedAttributes(
+            sourcePath,
+            node,
+            {"name", "type", "required", "documentKind", "kind", "restrictDocumentKind", "allowedValues", "values"},
+            diagnostics,
+            inOutErrorMessage)) {
+        valid = false;
+    }
+
+    outDefinition.location = node.location;
+
+    const UIDocumentAttribute* nameAttribute = FindAttributeByName(node, "name");
+    if (nameAttribute == nullptr || nameAttribute->value.Trim().Empty()) {
+        AppendDiagnostic(
+            sourcePath,
+            diagnostics,
+            inOutErrorMessage,
+            UIDocumentDiagnosticSeverity::Error,
+            node.location,
+            "Schema attribute definition is missing required 'name'.");
+        valid = false;
+    } else {
+        outDefinition.name = nameAttribute->value.Trim();
+    }
+
+    if (const UIDocumentAttribute* typeAttribute = FindAttributeByName(node, "type");
+        typeAttribute != nullptr && !typeAttribute->value.Trim().Empty()) {
+        if (!TryParseSchemaValueTypeExtended(typeAttribute->value, outDefinition.valueType)) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Unsupported schema value type '") +
+                    typeAttribute->value +
+                    "'.");
+            valid = false;
+        }
+    }
+
+    if (const UIDocumentAttribute* requiredAttribute = FindAttributeByName(node, "required");
+        requiredAttribute != nullptr && !requiredAttribute->value.Trim().Empty()) {
+        if (!TryParseBooleanValueExtended(requiredAttribute->value, outDefinition.required)) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Invalid boolean value '") +
+                    requiredAttribute->value +
+                    "' for schema attribute 'required'.");
+            valid = false;
+        }
+    }
+
+    const UIDocumentAttribute* documentKindAttribute =
+        FindAttributeByName(node, "documentKind", "kind");
+    if (documentKindAttribute != nullptr && !documentKindAttribute->value.Trim().Empty()) {
+        if (!TryParseDocumentKindValueExtended(documentKindAttribute->value, outDefinition.documentKind)) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Unsupported document kind '") +
+                    documentKindAttribute->value +
+                    "' in schema attribute definition.");
+            valid = false;
+        } else if (outDefinition.valueType == UISchemaValueType::Document) {
+            outDefinition.restrictDocumentKind = true;
+        }
+    }
+
+    if (const UIDocumentAttribute* restrictDocumentKindAttribute = FindAttributeByName(node, "restrictDocumentKind");
+        restrictDocumentKindAttribute != nullptr && !restrictDocumentKindAttribute->value.Trim().Empty()) {
+        if (!TryParseBooleanValueExtended(
+                restrictDocumentKindAttribute->value,
+                outDefinition.restrictDocumentKind)) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Invalid boolean value '") +
+                    restrictDocumentKindAttribute->value +
+                    "' for schema attribute 'restrictDocumentKind'.");
+            valid = false;
+        }
+    }
+
+    if (const UIDocumentAttribute* allowedValuesAttribute = FindAttributeByName(node, "allowedValues", "values");
+        allowedValuesAttribute != nullptr) {
+        outDefinition.allowedValues = ParseAllowedValueList(allowedValuesAttribute->value);
+    }
+
+    if (outDefinition.valueType == UISchemaValueType::Enum && outDefinition.allowedValues.Empty()) {
+        AppendDiagnostic(
+            sourcePath,
+            diagnostics,
+            inOutErrorMessage,
+            UIDocumentDiagnosticSeverity::Error,
+            node.location,
+            Containers::String("Enum schema attribute '") +
+                outDefinition.name +
+                "' must declare 'allowedValues'.");
+        valid = false;
+    }
+
+    return valid;
+}
+
+bool BuildSchemaElementDefinition(
+    const Containers::String& sourcePath,
+    const UIDocumentNode& node,
+    UISchemaElementDefinition& outDefinition,
+    Containers::Array<UIDocumentDiagnostic>& diagnostics,
+    Containers::String& inOutErrorMessage) {
+    bool valid = ValidateAllowedAttributes(
+        sourcePath,
+        node,
+        {"tag", "name", "allowUnknownAttributes", "allowUnknownChildren"},
+        diagnostics,
+        inOutErrorMessage);
+
+    outDefinition.location = node.location;
+
+    const UIDocumentAttribute* tagAttribute = FindAttributeByName(node, "tag", "name");
+    if (tagAttribute == nullptr || tagAttribute->value.Trim().Empty()) {
+        AppendDiagnostic(
+            sourcePath,
+            diagnostics,
+            inOutErrorMessage,
+            UIDocumentDiagnosticSeverity::Error,
+            node.location,
+            "Schema element definition is missing required 'tag'.");
+        valid = false;
+    } else {
+        outDefinition.tagName = tagAttribute->value.Trim();
+    }
+
+    if (const UIDocumentAttribute* allowUnknownAttributes = FindAttributeByName(node, "allowUnknownAttributes");
+        allowUnknownAttributes != nullptr && !allowUnknownAttributes->value.Trim().Empty()) {
+        if (!TryParseBooleanValueExtended(
+                allowUnknownAttributes->value,
+                outDefinition.allowUnknownAttributes)) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Invalid boolean value '") +
+                    allowUnknownAttributes->value +
+                    "' for schema element 'allowUnknownAttributes'.");
+            valid = false;
+        }
+    }
+
+    if (const UIDocumentAttribute* allowUnknownChildren = FindAttributeByName(node, "allowUnknownChildren");
+        allowUnknownChildren != nullptr && !allowUnknownChildren->value.Trim().Empty()) {
+        if (!TryParseBooleanValueExtended(
+                allowUnknownChildren->value,
+                outDefinition.allowUnknownChildren)) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                node.location,
+                Containers::String("Invalid boolean value '") +
+                    allowUnknownChildren->value +
+                    "' for schema element 'allowUnknownChildren'.");
+            valid = false;
+        }
+    }
+
+    std::unordered_set<std::string> seenAttributeDefinitions;
+    std::unordered_set<std::string> seenChildElementDefinitions;
+    for (const UIDocumentNode& child : node.children) {
+        if (child.tagName == "Attribute") {
+            UISchemaAttributeDefinition attributeDefinition;
+            if (!BuildSchemaAttributeDefinition(
+                    sourcePath,
+                    child,
+                    attributeDefinition,
+                    diagnostics,
+                    inOutErrorMessage)) {
+                valid = false;
+            }
+
+            const std::string attributeKey = ToStdString(attributeDefinition.name);
+            if (!attributeDefinition.name.Empty() &&
+                !seenAttributeDefinitions.insert(attributeKey).second) {
+                AppendDiagnostic(
+                    sourcePath,
+                    diagnostics,
+                    inOutErrorMessage,
+                    UIDocumentDiagnosticSeverity::Error,
+                    child.location,
+                    Containers::String("Duplicate schema attribute definition '") +
+                        attributeDefinition.name +
+                        "' on <" +
+                        outDefinition.tagName +
+                        ">.");
+                valid = false;
+            }
+
+            outDefinition.attributes.PushBack(std::move(attributeDefinition));
+            continue;
+        }
+
+        if (child.tagName == "Element") {
+            UISchemaElementDefinition childElementDefinition;
+            if (!BuildSchemaElementDefinition(
+                    sourcePath,
+                    child,
+                    childElementDefinition,
+                    diagnostics,
+                    inOutErrorMessage)) {
+                valid = false;
+            }
+
+            const std::string childKey = ToStdString(childElementDefinition.tagName);
+            if (!childElementDefinition.tagName.Empty() &&
+                !seenChildElementDefinitions.insert(childKey).second) {
+                AppendDiagnostic(
+                    sourcePath,
+                    diagnostics,
+                    inOutErrorMessage,
+                    UIDocumentDiagnosticSeverity::Error,
+                    child.location,
+                    Containers::String("Duplicate child schema element definition '") +
+                        childElementDefinition.tagName +
+                        "' on <" +
+                        outDefinition.tagName +
+                        ">.");
+                valid = false;
+            }
+
+            outDefinition.children.PushBack(std::move(childElementDefinition));
+            continue;
+        }
+
+        AppendDiagnostic(
+            sourcePath,
+            diagnostics,
+            inOutErrorMessage,
+            UIDocumentDiagnosticSeverity::Error,
+            child.location,
+            Containers::String("Unsupported schema child <") +
+                child.tagName +
+                "> inside <Element>.");
+        valid = false;
+    }
+
+    return valid;
+}
+
+bool BuildSchemaDefinition(
+    const Containers::String& sourcePath,
+    const UIDocumentNode& rootNode,
+    const Containers::String& defaultName,
+    UISchemaDefinition& outSchemaDefinition,
+    Containers::Array<UIDocumentDiagnostic>& diagnostics,
+    Containers::String& inOutErrorMessage) {
+    outSchemaDefinition.Clear();
+    if (rootNode.tagName != "Schema") {
+        AppendDiagnostic(
+            sourcePath,
+            diagnostics,
+            inOutErrorMessage,
+            UIDocumentDiagnosticSeverity::Error,
+            rootNode.location,
+            "UI schema root element must be <Schema>.");
+        return false;
+    }
+
+    bool valid = ValidateAllowedAttributes(
+        sourcePath,
+        rootNode,
+        {"name"},
+        diagnostics,
+        inOutErrorMessage);
+
+    if (const UIDocumentAttribute* nameAttribute = FindAttributeByName(rootNode, "name");
+        nameAttribute != nullptr && !nameAttribute->value.Trim().Empty()) {
+        outSchemaDefinition.name = nameAttribute->value.Trim();
+    } else {
+        outSchemaDefinition.name = defaultName;
+    }
+
+    std::unordered_set<std::string> seenRootElements;
+    for (const UIDocumentNode& child : rootNode.children) {
+        if (child.tagName != "Element") {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                child.location,
+                Containers::String("Unsupported schema child <") +
+                    child.tagName +
+                    "> inside <Schema>.");
+            valid = false;
+            continue;
+        }
+
+        UISchemaElementDefinition elementDefinition;
+        if (!BuildSchemaElementDefinition(
+                sourcePath,
+                child,
+                elementDefinition,
+                diagnostics,
+                inOutErrorMessage)) {
+            valid = false;
+        }
+
+        const std::string elementKey = ToStdString(elementDefinition.tagName);
+        if (!elementDefinition.tagName.Empty() &&
+            !seenRootElements.insert(elementKey).second) {
+            AppendDiagnostic(
+                sourcePath,
+                diagnostics,
+                inOutErrorMessage,
+                UIDocumentDiagnosticSeverity::Error,
+                child.location,
+                Containers::String("Duplicate root schema element definition '") +
+                    elementDefinition.tagName +
+                    "'.");
+            valid = false;
+        }
+
+        outSchemaDefinition.elements.PushBack(std::move(elementDefinition));
+    }
+
+    outSchemaDefinition.valid = valid && !ContainsErrorDiagnostics(diagnostics);
+    return outSchemaDefinition.valid;
+}
+
 void AppendUniqueDependency(const Containers::String& dependencyPath,
                             std::unordered_set<std::string>& seenDependencies,
                             Containers::Array<Containers::String>& outDependencies) {
@@ -345,6 +1095,18 @@ public:
             outResult.document.displayName = nameAttribute->value;
         }
 
+        if (m_kind == UIDocumentKind::Schema) {
+            if (!BuildSchemaDefinition(
+                    NormalizePathString(m_resolvedPath),
+                    outResult.document.rootNode,
+                    outResult.document.displayName,
+                    outResult.document.schemaDefinition,
+                    outResult.document.diagnostics,
+                    m_errorMessage)) {
+                return Finalize(outResult, false);
+            }
+        }
+
         std::unordered_set<std::string> seenDependencies;
         if (!CollectUIDocumentDependencies(
                 m_resolvedPath,
@@ -356,6 +1118,18 @@ public:
             return Finalize(outResult, false);
         }
 
+        if (!ValidateNodeTree(
+                outResult.document.sourcePath,
+                outResult.document.rootNode,
+                outResult.document.diagnostics,
+                m_errorMessage)) {
+            return Finalize(outResult, false);
+        }
+
+        if (ContainsErrorDiagnostics(outResult.document.diagnostics)) {
+            return Finalize(outResult, false);
+        }
+
         outResult.document.valid = true;
         outResult.succeeded = true;
         return Finalize(outResult, true);
@@ -752,6 +1526,12 @@ bool WriteUIDocumentArtifact(const Containers::String& artifactPath,
         }
         return false;
     }
+    if (!WriteSchemaDefinition(output, compileResult.document.schemaDefinition)) {
+        if (outErrorMessage != nullptr) {
+            *outErrorMessage = Containers::String("Failed to write UI document schema definition: ") + artifactPath;
+        }
+        return false;
+    }
 
     for (const Containers::String& dependency : compileResult.document.dependencies) {
         WriteString(output, dependency);
@@ -805,7 +1585,8 @@ bool LoadUIDocumentArtifact(const Containers::String& artifactPath,
         return false;
     }
 
-    if (header.schemaVersion != kUIDocumentArtifactSchemaVersion) {
+    if (header.schemaVersion != 1u &&
+        header.schemaVersion != kUIDocumentArtifactSchemaVersion) {
         outResult.errorMessage = Containers::String("Unsupported UI document artifact schema version: ") + artifactPath;
         return false;
     }
@@ -822,6 +1603,13 @@ bool LoadUIDocumentArtifact(const Containers::String& artifactPath,
         outResult.errorMessage = Containers::String("Failed to read UI document artifact body: ") + artifactPath;
         return false;
     }
+    outResult.document.schemaDefinition.Clear();
+    if (header.schemaVersion >= 2u) {
+        if (!ReadSchemaDefinition(input, outResult.document.schemaDefinition)) {
+            outResult.errorMessage = Containers::String("Failed to read UI document artifact schema definition: ") + artifactPath;
+            return false;
+        }
+    }
 
     outResult.document.dependencies.Clear();
     outResult.document.dependencies.Reserve(header.dependencyCount);
@@ -856,6 +1644,25 @@ bool LoadUIDocumentArtifact(const Containers::String& artifactPath,
         outResult.document.diagnostics.PushBack(std::move(diagnostic));
     }
 
+    if (expectedKind == UIDocumentKind::Schema) {
+        const Containers::String sourcePath =
+            outResult.document.sourcePath.Empty()
+                ? artifactPath
+                : outResult.document.sourcePath;
+        if (!outResult.document.schemaDefinition.valid &&
+            !BuildSchemaDefinition(
+                sourcePath,
+                outResult.document.rootNode,
+                outResult.document.displayName,
+                outResult.document.schemaDefinition,
+                outResult.document.diagnostics,
+                outResult.errorMessage)) {
+            outResult.succeeded = false;
+            outResult.document.valid = false;
+            return false;
+        }
+    }
+
     outResult.document.valid = true;
     outResult.succeeded = true;
     return true;