//存储publickey和sessionID -- writeContent： nil － 仅取出数据， 其他 － 修改原内容并提取
+(NSString *)storeFile:(NSString *)fileName content:(NSString *)writeContent
{
    NSString *pathDocuments=[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
     
    NSString *createPath=[NSString stringWithFormat:@"%@/%@",pathDocuments,fileName];//用文件名补全路径
    NSError *ReadFileError;
    NSString *readContent ;
    NSData *data ;
    if ([[NSFileManager defaultManager] fileExistsAtPath:createPath])//判断文件是否已存在
    {
        if (nil == writeContent) {
            readContent = [NSString stringWithContentsOfFile:createPath encoding:NSUTF8StringEncoding error:&ReadFileError;];
        }else{
            data = [writeContent dataUsingEncoding:NSUTF8StringEncoding];//新文件的初始数据
            [[NSFileManager defaultManager] createFileAtPath:createPath contents:data attributes:nil];//创建文件 
            readContent = [NSString stringWithContentsOfFile:createPath encoding:NSUTF8StringEncoding error:&ReadFileError;];            
        }
    }
    else
    {
        if (nil == writeContent) {
            return nil;
        }else{
            data = [writeContent dataUsingEncoding:NSUTF8StringEncoding];//新文件的初始数据
            [[NSFileManager defaultManager] createFileAtPath:createPath contents:data attributes:nil];//创建文件
            readContent = [NSString stringWithContentsOfFile:createPath encoding:NSUTF8StringEncoding error:&ReadFileError;]; 
        }
    } 
    return readContent;
}



//objectc/5098