Swift中NSData第二次无法访问文件及邮件附件添加问题

Hey there, let's break down why you're running into trouble accessing that PDF file the second time around, and how to fix it.

First off, a couple of red flags in your current code that might be causing the issue:

• Manual string-based path concatenation (newDir+"/file.pdf") is error-prone—you might end up with invalid paths (like duplicate slashes) without realizing it.
• Using NSData(contentsOfFile:) gives you no visibility into why the read failed on the second attempt (it just returns nil silently).

Here's how to resolve this step by step:

1. Ditch String Paths—Use URLs Instead #

URLs are the safe, recommended way to handle file paths in iOS. They automatically handle path separators and avoid common string concatenation mistakes:

let filemgr = FileManager.default
let dirPaths = filemgr.urls(for: .documentDirectory, in: .userDomainMask)
let docsURL = dirPaths[0]

// Build folder and file URLs properly
let folderURL = docsURL.appendingPathComponent("folder")
let fileURL = folderURL.appendingPathComponent("file.pdf")

2. Use Swift's Data with Error Handling #

Instead of NSData, use Swift's native Data type with a do/catch block. This will tell you exactly why the file read fails (e.g., file missing, permission issues):

// Initialize your mail controller (create a fresh instance for each email attempt)
let mail = MFMailComposeViewController()

do {
    let fileData = try Data(contentsOf: fileURL)
    // Add the attachment with correct MIME type
    mail.addAttachmentData(fileData, mimeType: "application/pdf", fileName: "file.pdf")
} catch {
    // Debug the error to find the root cause
    print("Failed to read PDF file: \(error.localizedDescription)")
    
    // Add extra checks to diagnose further
    if !filemgr.fileExists(atPath: fileURL.path) {
        print("Error: File does NOT exist at \(fileURL.path)")
    }
    if !filemgr.isReadableFile(atPath: fileURL.path) {
        print("Error: File exists but isn't readable (permission issue?)")
    }
}

3. Check for Hidden File Changes #

If the first read works but the second doesn't, ask yourself:

• Is any other code deleting/moving the PDF file after the first access?
• Are you reusing the same MFMailComposeViewController instance instead of creating a new one for each email attempt? (It's best to create a fresh instance every time you need to send mail.)

4. Verify Folder Existence #

Make sure the "folder" directory actually exists before trying to access the file. If it's created dynamically, add a check:

if !filemgr.fileExists(atPath: folderURL.path) {
    do {
        try filemgr.createDirectory(at: folderURL, withIntermediateDirectories: true)
    } catch {
        print("Failed to create folder: \(error)")
    }
}

By switching to URLs and adding proper error handling, you'll either fix the issue immediately or get a clear error message that points to the real problem (like a missing file or permission error).

内容的提问来源于stack exchange，提问作者EatMyDust