NSString 常用分类

#pragma mark 清空字符串中的空白字符
- (NSString *)trimString
{
    return [self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
}

#pragma mark 是否空字符串
- (BOOL)isEmptyString
{
    return (self == nil || self.length == 0);
}

#pragma mark 返回沙盒中的文件路径
- (NSString *)documentsPath
{
    NSString *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
    return [path stringByAppendingString:self];
}

#pragma mark 写入系统偏好
- (void)saveToNSDefaultsWithKey:(NSString *)key
{
    [[NSUserDefaults standardUserDefaults] setObject:self forKey:key];
    [[NSUserDefaults standardUserDefaults] synchronize];
}