iOS之使用QLPreviewController打开文件，处理txt文件出现乱码的情况

主要代码：

- (id <QLPreviewItem>)previewController:(QLPreviewController *)controller previewItemAtIndex:(NSInteger)index {
    //处理txt格式内容显示有乱码的情况
    NSData *fileData = [NSData dataWithContentsOfFile:self.files];
    //判断是UNICODE编码
    NSString *isUNICODE = [[NSString alloc] initWithData:fileData encoding:NSUTF8StringEncoding];
    //还是ANSI编码（-2147483623，-2147482591，-2147482062，-2147481296）encoding 任选一个就可以了
    NSString *isANSI = [[NSString alloc] initWithData:fileData encoding:-2147483623];
    if (isUNICODE) {
        NSString *retStr = [[NSString alloc]initWithCString:[isUNICODE UTF8String] encoding:NSUTF8StringEncoding];
        NSData *data = [retStr dataUsingEncoding:NSUTF16StringEncoding];
        [data writeToFile:self.files atomically:YES];
    }
    else if(isANSI){
        NSData *data = [isANSI dataUsingEncoding:NSUTF16StringEncoding];
        [data writeToFile:self.files atomically:YES];
    }
    
    return [NSURL fileURLWithPath:self.files];
}