Objective-C创建PDF映射文件求助：解决大PDF加载缓慢问题

我明白你遇到的痛点——大体积PDF（比如5.5MB）用UIDocumentInteractionController加载时耗时太久，而内存映射（mapped file）正是解决这个问题的关键。它能让系统按需加载文件内容，不用一次性把整个文件塞进内存，下面我给你一步步讲具体实现：

核心原理 #

内存映射是把磁盘文件直接映射到应用的虚拟内存空间，读取文件时系统会自动按需加载对应区块，避免了传统IO操作一次性读取整个大文件的开销，自然能加快UIDocumentInteractionController的加载速度。

Swift 实现步骤 #

1. 创建内存映射的PDF副本 #

我们需要把原始PDF文件通过内存映射的方式复制一份，这样后续加载时就能利用内存映射的优势：

import Foundation

func createMappedPDF(from originalPDFURL: URL, to destinationURL: URL) throws {
    // 确保目标文件夹存在
    try FileManager.default.createDirectory(
        at: destinationURL.deletingLastPathComponent(),
        withIntermediateDirectories: true
    )
    
    // 获取原始文件大小
    let fileAttrs = try FileManager.default.attributesOfItem(atPath: originalPDFURL.path)
    guard let fileSize = fileAttrs[.size] as? UInt64 else {
        throw NSError(
            domain: "PDFMappingError",
            code: 1,
            userInfo: [NSLocalizedDescriptionKey: "无法获取原始PDF文件大小"]
        )
    }
    
    // 创建空的目标文件并设置对应大小
    FileManager.default.createFile(atPath: destinationURL.path, contents: nil)
    let writeHandle = try FileHandle(forWritingTo: destinationURL)
    try writeHandle.truncate(atOffset: Int64(fileSize))
    writeHandle.closeFile()
    
    // 打开源文件和目标文件，获取内存映射
    let readHandle = try FileHandle(forReadingFrom: originalPDFURL)
    let mappedWriteHandle = try FileHandle(forWritingTo: destinationURL)
    
    guard let sourceMapping = readHandle.mappedFileContent,
          let destMapping = mappedWriteHandle.mappedFileContent else {
        throw NSError(
            domain: "PDFMappingError",
            code: 2,
            userInfo: [NSLocalizedDescriptionKey: "无法创建文件内存映射"]
        )
    }
    
    // 直接复制内存映射内容
    destMapping.copyMemory(from: sourceMapping)
    
    // 关闭文件句柄
    readHandle.closeFile()
    mappedWriteHandle.closeFile()
}

2. 用映射后的PDF展示预览 #

拿到映射后的文件URL后，直接传给UIDocumentInteractionController即可：

import UIKit

func presentPDFPreview(with mappedPDFURL: URL, from viewController: UIViewController) {
    let docController = UIDocumentInteractionController(url: mappedPDFURL)
    docController.delegate = viewController
    docController.presentPreview(animated: true)
}

3. 额外注意事项 #

• 权限检查：确保目标目录（比如Documents文件夹）是可读写的，可以用FileManager.default.isWritableFile(atPath:)提前验证。
• 清理临时文件：如果映射后的PDF只是临时使用，记得在预览结束后删除它，避免占用磁盘空间：

  try? FileManager.default.removeItem(at: mappedPDFURL)
  

Objective-C 版本实现 #

如果你还在用Objective-C，对应的代码如下：

创建内存映射PDF #

#import <Foundation/Foundation.h>

NSError *createMappedPDFFromURL(NSURL *originalURL, NSURL *destinationURL) {
    // 创建目标目录
    [[NSFileManager defaultManager] createDirectoryAtURL:[destinationURL URLByDeletingLastPathComponent]
                           withIntermediateDirectories:YES
                                            attributes:nil
                                                 error:nil];
    
    // 获取原始文件大小
    NSDictionary *fileAttrs = [[NSFileManager defaultManager] attributesOfItemAtPath:originalURL.path error:nil];
    unsigned long long fileSize = [fileAttrs[NSFileSize] unsignedLongLongValue];
    
    // 创建空目标文件并设置大小
    [[NSFileManager defaultManager] createFileAtPath:destinationURL.path contents:nil attributes:nil];
    NSFileHandle *writeHandle = [NSFileHandle fileHandleForWritingToURL:destinationURL error:nil];
    [writeHandle truncateFileAtOffset:(off_t)fileSize];
    [writeHandle closeFile];
    
    // 打开文件并创建内存映射
    NSFileHandle *readHandle = [NSFileHandle fileHandleForReadingFromURL:originalURL error:nil];
    NSFileHandle *mappedWriteHandle = [NSFileHandle fileHandleForWritingToURL:destinationURL error:nil];
    
    NSData *sourceMapping = [readHandle mappedFileContent];
    NSData *destMapping = [mappedWriteHandle mappedFileContent];
    
    if (!sourceMapping || !destMapping) {
        return [NSError errorWithDomain:@"PDFMappingError"
                                   code:2
                               userInfo:@{NSLocalizedDescriptionKey:@"无法创建文件内存映射"}];
    }
    
    // 复制映射内容
    memcpy((void *)destMapping.bytes, sourceMapping.bytes, sourceMapping.length);
    
    // 关闭句柄
    [readHandle closeFile];
    [mappedWriteHandle closeFile];
    
    return nil;
}

展示PDF预览 #

#import <UIKit/UIKit.h>

void presentPDFPreviewWithMappedURL(NSURL *mappedURL, UIViewController *vc) {
    UIDocumentInteractionController *docController = [UIDocumentInteractionController interactionControllerWithURL:mappedURL];
    docController.delegate = vc;
    [docController presentPreviewAnimated:YES];
}

这样实现后，UIDocumentInteractionController加载大PDF时就不会因为一次性读取整个文件而卡顿了，内存映射会帮你按需加载所需的文件内容。

内容的提问来源于stack exchange，提问作者user979331