Objective-C NSFileHandle 檔案讀寫完整指南

NSFileHandle vs NSFileManager

• NSFileManager：檔案/目錄操作 (建立、刪除、移動、複製、屬性查詢)
• NSFileHandle：檔案內容讀寫 (開啟、讀取、寫入、定位、截斷、同步、關閉)

核心 API 速查

實戰範例

1. 附加資料到檔案末尾

@autoreleasepool {
    NSString *path = [NSHomeDirectory() stringByAppendingPathComponent:@"Desktop/testfile"];
    NSFileHandle *fh = [NSFileHandle fileHandleForUpdatingAtPath:path];
    [fh seekToEndOfFile];
    NSString *str = @"測試附加資料";
    [fh writeData:[str dataUsingEncoding:NSUTF8StringEncoding]];
    [fh closeFile];
}

2. 從檔案中間讀取

NSString *path = [NSHomeDirectory() stringByAppendingPathComponent:@"Desktop/testfile"];
NSFileHandle *fh = [NSFileHandle fileHandleForReadingAtPath:path];
NSUInteger len = [fh availableData].length;
[fh seekToFileOffset:len / 2]; // 移至中間
NSData *data = [fh readDataToEndOfFile];
NSString *str = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSLog(@"%@", str);
[fh closeFile];

3. 複製檔案內容

NSString *srcPath = [NSHomeDirectory() stringByAppendingPathComponent:@"Desktop/testfile"];
NSString *dstPath = [NSHomeDirectory() stringByAppendingPathComponent:@"Desktop/outfile"];

[[NSFileManager defaultManager] createFileAtPath:dstPath contents:nil attributes:nil];

NSFileHandle *inFH = [NSFileHandle fileHandleForReadingAtPath:srcPath];
NSFileHandle *outFH = [NSFileHandle fileHandleForWritingAtPath:dstPath];

[outFH writeData:[inFH availableData]]; // 一次性讀寫 (小檔案適用)

[inFH closeFile];
[outFH closeFile];

大檔案分塊複製 (記憶體安全)

const size_t CHUNK = 1024 * 1024; // 1MB
NSFileHandle *in = [NSFileHandle fileHandleForReadingAtPath:srcPath];
NSFileHandle *out = [NSFileHandle fileHandleForWritingAtPath:dstPath];

while (1) {
    NSData *chunk = [in readDataOfLength:CHUNK];
    if (chunk.length == 0) break;
    [out writeData:chunk];
}
[in closeFile];
[out closeFile];

NSData 常用轉換

• [str dataUsingEncoding:NSUTF8StringEncoding] → NSData
• [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding] → NSString
• [data bytes] → const void* 原始指標
• [data length] → 位元組長度

錯誤處理最佳實踐

NSFileHandle *fh = [NSFileHandle fileHandleForReadingAtPath:path];
if (!fh) {
    NSLog(@"無法開啟檔案: %@", path);
    return;
}
@try {
    NSData *data = [fh readDataToEndOfFile];
    // 處理 data
}
@catch (NSException *e) {
    NSLog(@"讀取異常: %@", e.reason);
}
@finally {
    [fh closeFile];
}

進階：非同步檔案 I/O (iOS/macOS)

使用 NSFileHandleReadCompletionNotification、NSFileHandleWriteCompletionNotification 搭配 readInBackgroundAndNotify/writeData:completionHandler: 實現非阻塞讀寫。