打开文本文件前，你需要知道其编码。如果用QuickLook打开（UIDocumentInteractionController封装了QLPreviewController），需要先将其编码转为UTF16（即NSUnicodeStringEncoding）。下面的方法将url对应的本地文件转化为可以查看的编码（UTF16）））

text_file_encoding_conversion_to_UTF16.m

- (void)reEncodingTextFileAtUrl:(NSURL*)url{
    NSString * str = nil;
    NSStringEncoding fileEncoding = 0;
    NSError * __autoreleasing error;

    if (![url checkResourceIsReachableAndReturnError:&error]) { // 非必须
        NSLog(@"not reachable: %@", error.localizedDescription);
        return;
    }
    // 自动获取编码，但探测的编码范围公限于CFString.h头文件里提到的编码
    str = [NSString stringWithContentsOfURL:url usedEncoding:&fileEncoding error:&error];
    if (error == nil) {
        NSLog(@"txt file encoding: %@(%zd)", [NSString localizedNameOfStringEncoding:fileEncoding], fileEncoding);
    }
    if (fileEncoding == NSUnicodeStringEncoding) {
        return; // 编码正确，无需转换
    }

#if 0
    // 另一种探测编码的方法。理应与上述api等价，但由于与下述suggestedEncodings数组的排列顺序相关，事实上不等价。——故不推荐
    NSArray *suggestedEncodings = @[
                                    //@(NSUnicodeStringEncoding),
                                    @(NSUTF16StringEncoding),
                                    @(NSUTF8StringEncoding),
                                    @(NSUTF16BigEndianStringEncoding),
                                    @(NSUTF16LittleEndianStringEncoding),
                                    @(NSUTF32StringEncoding),
                                    @(NSUTF32BigEndianStringEncoding),
                                    @(NSUTF32LittleEndianStringEncoding),
                                    @(NSNEXTSTEPStringEncoding),
                                    @(NSJapaneseEUCStringEncoding),
                                    @(NSISOLatin1StringEncoding),
                                    @(NSSymbolStringEncoding),
                                    @(NSShiftJISStringEncoding),
                                    @(NSISOLatin2StringEncoding),
                                    @(NSWindowsCP1251StringEncoding),
                                    @(NSWindowsCP1252StringEncoding),
                                    @(NSWindowsCP1253StringEncoding),
                                    @(NSWindowsCP1254StringEncoding),
                                    @(NSWindowsCP1250StringEncoding),
                                    @(NSISO2022JPStringEncoding),
                                    @(NSMacOSRomanStringEncoding),
                                    @(NSNonLossyASCIIStringEncoding),
                                    @(NSASCIIStringEncoding),
                                    ];
    for (NSNumber *encodingNum in suggestedEncodings) {
        error = nil;
        str = [NSString stringWithContentsOfURL:url encoding:encodingNum.integerValue error:&error];
        if (error == nil && str != nil) {
            fileEncoding = encodingNum.integerValue;
            if (fileEncoding == NSUnicodeStringEncoding) {
                return;
            }
            break;
        }
    }
#endif

    if (error) {
        //都没有则猜测中文编码（暂未考虑下述编码的顺序问题）
        NSArray * chineseEncodings = @[
                                       @(kCFStringEncodingGB_18030_2000),
                                       @(kCFStringEncodingGB_2312_80),
                                       @(kCFStringEncodingGBK_95),
                                       @(kCFStringEncodingBig5),
                                       @(kCFStringEncodingBig5_E),
                                       @(kCFStringEncodingBig5_HKSCS_1999),
                                       @(kCFStringEncodingHZ_GB_2312),
                                       @(kCFStringEncodingISO_2022_CN),
                                       @(kCFStringEncodingISO_2022_CN_EXT),
                                       ];
        NSData *data = [NSData dataWithContentsOfURL:url];
        str = nil;
        for (NSNumber *encValue in chineseEncodings) {
            NSStringEncoding enc = CFStringConvertEncodingToNSStringEncoding(encValue.unsignedIntValue);
            str = [[NSString alloc] initWithData:data encoding:enc];
            if (str) {
                NSLog(@"txt file encoding: %@(%zd)", [NSString localizedNameOfStringEncoding:enc], enc);
                break;
            }
        }
    }
    // 重新以正确编码写入文件
    NSData *data = [str dataUsingEncoding:NSUTF16StringEncoding];
    [data writeToURL:url atomically:YES];
}