feat(后端架构): 完善后端扩展与字体资源解耦，修复元数据保存和缩略图底边

ofdgo_backend.go

@@ -20,6 +20,7 @@ import (
 	"image"
 	"io"
 	"reflect"
+	"sync"
 )
 
 // ErrBackendUnavailable 表示未配置所需后端，不会自动切换其他实现
@@ -105,53 +106,83 @@ type EPSBackend interface {
 // RenderBackends 按职责组合后端，nil表示明确禁用，不隐式回退
 // 实例及其Renderer需串行使用，第三方实现无需引用任何内置绘图库
 type RenderBackends struct {
-	Fonts    FontBackend
-	Geometry GeometryBackend
-	Compiler PageCompiler
-	Raster   RasterBackend
-	SVG      SVGBackend
-	PDF      PDFBackend
-	EPS      EPSBackend
+	Resources FontResourceBackend
+	Fonts     FontBackend
+	Geometry  GeometryBackend
+	Compiler  PageCompiler
+	Raster    RasterBackend
+	SVG       SVGBackend
+	PDF       PDFBackend
+	EPS       EPSBackend
 }
 
 // BackendInfo 描述各项能力实际使用的后端，空标识表示未配置
 type BackendInfo struct {
-	Fonts    string `json:"fonts"`
-	Geometry string `json:"geometry"`
-	Compiler string `json:"compiler"`
-	Raster   string `json:"raster"`
-	SVG      string `json:"svg"`
-	PDF      string `json:"pdf"`
-	EPS      string `json:"eps"`
+	Resources string `json:"resources"`
+	Fonts     string `json:"fonts"`
+	Geometry  string `json:"geometry"`
+	Compiler  string `json:"compiler"`
+	Raster    string `json:"raster"`
+	SVG       string `json:"svg"`
+	PDF       string `json:"pdf"`
+	EPS       string `json:"eps"`
 }
 
-// NewRenderBackends 创建内置后端组合，各项能力通过Info报告实际提供者
-// 入参: name 内置后端标识
+// NewRenderBackends 创建已注册后端组合，各项能力通过Info报告实际提供者
+// 入参: name 已注册后端标识
 // 返回: RenderBackends 后端组合, error 未知后端错误
 func NewRenderBackends(name string) (RenderBackends, error) {
-	if name != "canvas" {
+	backendRegistry.RLock()
+	factory := backendRegistry.factories[name]
+	backendRegistry.RUnlock()
+	if factory == nil {
 		return RenderBackends{}, fmt.Errorf("unknown render backend %q", name)
 	}
-	return defaultRenderBackends(), nil
+	return factory(), nil
 }
 
-// RenderBackendInfos 列出内置组合及其实际能力，供原生程序和WASM共用
+// RenderBackendInfos 列出已注册组合及其实际能力，供原生程序和WASM共用
 // 返回: map[string]BackendInfo 后端能力
 func RenderBackendInfos() map[string]BackendInfo {
-	return map[string]BackendInfo{"canvas": defaultRenderBackends().Info()}
+	backendRegistry.RLock()
+	factories := make(map[string]func() RenderBackends, len(backendRegistry.factories))
+	for name, factory := range backendRegistry.factories {
+		factories[name] = factory
+	}
+	backendRegistry.RUnlock()
+	result := make(map[string]BackendInfo, len(factories))
+	for name, factory := range factories {
+		result[name] = factory().Info()
+	}
+	return result
 }
 
-// defaultRenderBackends 创建独立的默认配置
-// 返回: RenderBackends 默认后端组合
-func defaultRenderBackends() RenderBackends {
-	backend := CanvasBackend{}
-	return RenderBackends{Fonts: backend, Geometry: backend, Compiler: backend, Raster: backend, SVG: backend, PDF: backend, EPS: backend}
+// backendRegistry 保存后端工厂，不共享有状态的后端实例
+var backendRegistry = struct {
+	sync.RWMutex
+	factories map[string]func() RenderBackends
+}{factories: make(map[string]func() RenderBackends)}
+
+// RegisterRenderBackend 注册后端组合，工厂应返回独立实例，重复标识不会覆盖已有实现
+// 入参: name 后端标识, factory 后端工厂
+// 返回: error 无效或重复注册错误
+func RegisterRenderBackend(name string, factory func() RenderBackends) error {
+	if name == "" || factory == nil {
+		return fmt.Errorf("backend name and factory are required")
+	}
+	backendRegistry.Lock()
+	defer backendRegistry.Unlock()
+	if _, exists := backendRegistry.factories[name]; exists {
+		return fmt.Errorf("render backend %q already registered", name)
+	}
+	backendRegistry.factories[name] = factory
+	return nil
 }
 
 // Info 返回配置中各能力的提供者，不依赖WebUI推断
 // 返回: BackendInfo 后端能力
 func (b RenderBackends) Info() BackendInfo {
-	return BackendInfo{Fonts: backendName(b.Fonts), Geometry: backendName(b.Geometry), Compiler: backendName(b.Compiler), Raster: backendName(b.Raster), SVG: backendName(b.SVG), PDF: backendName(b.PDF), EPS: backendName(b.EPS)}
+	return BackendInfo{Resources: backendName(b.Resources), Fonts: backendName(b.Fonts), Geometry: backendName(b.Geometry), Compiler: backendName(b.Compiler), Raster: backendName(b.Raster), SVG: backendName(b.SVG), PDF: backendName(b.PDF), EPS: backendName(b.EPS)}
 }
 
 // backendName 获取已配置的后端标识