feat(后端架构): 完善后端解耦

ofdgo_font_backend.go

@@ -0,0 +1,75 @@
+// Copyright 2025-2026 肖其顿 (XIAO QI DUN)
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ofdgo
+
+import "fmt"
+
+// FontMetrics 提供不依赖绘图库的字体度量，所有整数尺寸使用字体设计单位
+// VerticalMetrics依次返回非负的上升高度、下降高度和行间隙
+// Write返回独立的SFNT数据，调用方不得修改字体实例
+type FontMetrics interface {
+	UnitsPerEm() uint16
+	NumGlyphs() uint16
+	GlyphIndex(rune) uint16
+	GlyphAdvance(uint16) uint16
+	VerticalMetrics() (uint16, uint16, uint16)
+	Write() []byte
+}
+
+// ResolvedFont 保存字体来源和只读字体数据，不将外部字体写入文档
+type ResolvedFont struct {
+	Data   []byte
+	Source string
+	Face   *FontFace
+	Exact  bool
+}
+
+// FontBackend 统一字体来源与度量，exact禁止使用无关回退字体
+// ResolveFont不得递归调用Renderer.ResolveFont，OpenFont不得修改输入数据
+type FontBackend interface {
+	Backend
+	ResolveFont(renderer *Renderer, id string, exact bool) (ResolvedFont, error)
+	OpenFont(data []byte) (FontMetrics, error)
+}
+
+// resolvedFontKey 区分只读资源的字体标识与精确匹配要求
+type resolvedFontKey struct {
+	id         string
+	exact      bool
+	definition *Font
+}
+
+// resolvedFontResult 缓存字体解析结果及不可用原因
+type resolvedFontResult struct {
+	font ResolvedFont
+	err  error
+}
+
+// ResolveFont 使用配置的字体后端解析资源，保留精确匹配与回退的区别
+// 入参: id 字体资源标识, exact 是否要求精确匹配
+// 返回: ResolvedFont 字体与来源, error 解析错误
+func (r *Renderer) ResolveFont(id string, exact bool) (ResolvedFont, error) {
+	if r.backends.Fonts == nil {
+		return ResolvedFont{}, fmt.Errorf("fonts: %w", ErrBackendUnavailable)
+	}
+	key := resolvedFontKey{id: id, exact: exact, definition: r.Reader.fontCache[id]}
+	if cached, ok := r.resolvedFonts[key]; ok {
+		return cached.font, cached.err
+	}
+	resolved, err := r.backends.Fonts.ResolveFont(r, id, exact)
+	key.definition = r.Reader.fontCache[id]
+	r.resolvedFonts[key] = resolvedFontResult{font: resolved, err: err}
+	return resolved, err
+}