HarmonyOS鸿蒙Next开发者技术支持-PDF预览组件优化方案

1.1 问题说明

问题场景

在鸿蒙应用开发中，PDF文件预览功能存在以下问题：

1. 兼容性问题：不同格式的PDF文件在部分设备上显示异常
2. 性能瓶颈：大体积PDF文件加载缓慢，内存占用过高
3. 代码复用性差：各项目自行实现，代码重复且维护困难

1.2 解决方案

方案一：基于PDF.js的优化方案

// PDF预览组件核心代码
 import { PDFDocument } from '@ohos/pdf-parser';

@Component
 export struct PdfPreviewComponent {
   @State currentPage: number = 1;
   @State totalPages: number = 0;
   @State pageImages: Array<ImageBitmap> = [];
   private pdfDoc: PDFDocument | null = null;
   
   // 配置参数
   @Prop options: PdfOptions = {
     enableCache: true,
     maxCacheSize: 100, // MB
     renderQuality: 'high',
     enableAnnotations: true
   };

   // 初始化PDF文档
   async aboutToAppear() {
     await this.loadPdfDocument();
   }

   // 加载PDF文档
   async loadPdfDocument(filePath: string) {
     try {
       this.pdfDoc = await PDFDocument.load(filePath);
       this.totalPages = this.pdfDoc.getPageCount();
       await this.renderPage(1); // 渲染第一页
       this.preloadAdjacentPages(); // 预加载相邻页面
     } catch (error) {
       console.error('PDF加载失败:', error);
     }
   }

   // 渲染单页PDF
   async renderPage(pageNum: number) {
     if (!this.pdfDoc || pageNum < 1 || pageNum > this.totalPages) {
       return;
     }
     
     const page = await this.pdfDoc.getPage(pageNum);
     const viewport = page.getViewport({ scale: 2.0 });
     
     // 创建Canvas渲染
     const canvas = new OffscreenCanvas(viewport.width, viewport.height);
     const context = canvas.getContext('2d');
     
     const renderContext = {
       canvasContext: context,
       viewport: viewport
     };
     
     await page.render(renderContext).promise;
     
     // 转换为ImageBitmap
     const imageBitmap = await createImageBitmap(canvas);
     
     // 更新状态（带缓存检查）
     if (!this.options.enableCache || 
         !this.pageImages[pageNum - 1]) {
       this.pageImages[pageNum - 1] = imageBitmap;
     }
     
     this.currentPage = pageNum;
   }

   // 预加载相邻页面
   async preloadAdjacentPages() {
     const pagesToPreload = [this.currentPage + 1, this.currentPage + 2];
     
     for (const pageNum of pagesToPreload) {
       if (pageNum <= this.totalPages && !this.pageImages[pageNum - 1]) {
         setTimeout(() => this.renderPage(pageNum), 100);
       }
     }
   }

   // 构建UI
   build() {
     Column() {
       // 工具栏
       ToolbarComponent({
         currentPage: this.currentPage,
         totalPages: this.totalPages,
         onPageChange: (page) => this.onPageChange(page),
         onSearch: (text) => this.searchText(text)
       })

       // 页面显示区域
       Scroll() {
         Column() {
           if (this.pageImages[this.currentPage - 1]) {
             Image(this.pageImages[this.currentPage - 1])
               .width('100%')
               .height('auto')
               .objectFit(ImageFit.Contain)
           } else {
             LoadingComponent()
           }
         }
         .padding(10)
       }
       .height('80%')

       // 页面缩略图导航
       if (this.options.showThumbnails) {
         ThumbnailBarComponent({
           pageImages: this.pageImages,
           currentPage: this.currentPage,
           onThumbnailClick: (page) => this.jumpToPage(page)
         })
       }
     }
   }

   // 页面跳转
   private onPageChange(pageNum: number) {
     if (pageNum >= 1 && pageNum <= this.totalPages) {
       this.renderPage(pageNum);
     }
   }

   // 文本搜索
   private async searchText(text: string) {
     if (!this.pdfDoc) return;
     
     const searchResults = [];
     for (let i = 1; i <= this.totalPages; i++) {
       const page = await this.pdfDoc.getPage(i);
       const textContent = await page.getTextContent();
       
       textContent.items.forEach((item, index) => {
         if (item.str.includes(text)) {
           searchResults.push({
             page: i,
             index: index,
             text: item.str
           });
         }
       });
     }
     
     return searchResults;
   }
 }

方案二：缓存管理策略

// PDF缓存管理器
 export class PdfCacheManager {
   private static instance: PdfCacheManager;
   private cache: Map<string, CacheEntry> = new Map();
   private maxSize: number = 100 * 1024 * 1024; // 100MB
   private currentSize: number = 0;

   static getInstance(): PdfCacheManager {
     if (!PdfCacheManager.instance) {
       PdfCacheManager.instance = new PdfCacheManager();
     }
     return PdfCacheManager.instance;
   }

   // 缓存PDF页面
   async cachePage(pdfId: string, pageNum: number, data: ImageBitmap): Promise<void> {
     const cacheKey = `${pdfId}_${pageNum}`;
     const size = await this.calculateSize(data);
     
     // 检查缓存空间
     if (this.currentSize + size > this.maxSize) {
       this.clearOldCache();
     }
     
     this.cache.set(cacheKey, {
       data: data,
       timestamp: Date.now(),
       size: size
     });
     
     this.currentSize += size;
   }

   // 获取缓存页面
   getPage(pdfId: string, pageNum: number): ImageBitmap | null {
     const cacheKey = `${pdfId}_${pageNum}`;
     const entry = this.cache.get(cacheKey);
     
     if (entry) {
       entry.timestamp = Date.now(); // 更新访问时间
       return entry.data;
     }
     
     return null;
   }

   // 清理旧缓存
   private clearOldCache(): void {
     const entries = Array.from(this.cache.entries())
       .sort((a, b) => a[1].timestamp - b[1].timestamp);
     
     let clearedSize = 0;
     const targetSize = this.maxSize * 0.3; // 清理30%的空间
     
     for (const [key, entry] of entries) {
       if (clearedSize >= targetSize) break;
       
       this.cache.delete(key);
       clearedSize += entry.size;
       this.currentSize -= entry.size;
     }
   }

   private async calculateSize(image: ImageBitmap): Promise<number> {
     // 估算图片大小
     return image.width * image.height * 4; // RGBA 4字节
   }
 }

方案三：PDF配置类

// 配置接口
 export interface PdfOptions {
   // 显示配置
   defaultScale?: number;
   maxScale?: number;
   minScale?: number;
   
   // 功能配置
   enableSearch?: boolean;
   enableAnnotations?: boolean;
   enableBookmarks?: boolean;
   enablePrint?: boolean;
   
   // 性能配置
   enableCache?: boolean;
   maxCacheSize?: number;
   preloadPages?: number;
   renderQuality?: 'low' | 'medium' | 'high';
   
   // UI配置
   showToolbar?: boolean;
   showThumbnails?: boolean;
   showPageNumbers?: boolean;
   theme?: 'light' | 'dark';
 }

// 默认配置
 export const DefaultPdfOptions: PdfOptions = {
   defaultScale: 1.0,
   maxScale: 3.0,
   minScale: 0.5,
   
   enableSearch: true,
   enableAnnotations: false,
   enableBookmarks: true,
   enablePrint: true,
   
   enableCache: true,
   maxCacheSize: 100, // MB
   preloadPages: 2,
   renderQuality: 'medium',
   
   showToolbar: true,
   showThumbnails: true,
   showPageNumbers: true,
   theme: 'light'
 };

方案四：集成使用示例

// 主页面使用示例
 @Entry
 @Component
 struct MainPage {
   @State pdfPath: string = '/data/storage/el1/base/files/sample.pdf';
   
   build() {
     Column() {
       // 使用PDF预览组件
       PdfPreviewComponent({
         filePath: this.pdfPath,
         options: {
           enableSearch: true,
           enableBookmarks: true,
           showThumbnails: true,
           enableCache: true,
           maxCacheSize: 150,
           theme: 'dark'
         }
       })
       
       // 操作按钮
       Row() {
         Button('打开PDF')
           .onClick(() => this.openFilePicker())
         
         Button('搜索文本')
           .onClick(() => this.showSearchDialog())
         
         Button('添加书签')
           .onClick(() => this.addBookmark())
       }
       .padding(10)
       .justifyContent(FlexAlign.SpaceAround)
     }
   }
   
   private async openFilePicker() {
     // 文件选择逻辑
     const filePicker = new FilePicker();
     const result = await filePicker.pick({
       type: ['pdf'],
       multiple: false
     });
     
     if (result && result.length > 0) {
       this.pdfPath = result[0].path;
     }
   }
 }

1.3 结果展示

可复用资产

1. 核心组件
   • PdfPreviewComponent：主预览组件
   • PdfCacheManager：缓存管理器
   • PdfSearchService：搜索服务
2. 工具类库
   • pdf-utils.ets：工具函数集合
   • pdf-types.d.ts：类型定义
   • pdf-constants.ets：常量定义
3. 配置文件
   • pdf-config.json：默认配置
   • theme-config.json：主题配置