iOS 判断字符串是否为空

写一个字符串的扩展，实现判断字符串是否为空


- (BOOL) isBlankString {
    
    if ([self isEqualToString:@"(null)"]) {
        return YES;
    }
    
    if ([self isEqualToString:@"<null>"]) {
        return YES;
    }
    
    if (self == nil || self == NULL){
        return YES;
    }
    
    if ([self isKindOfClass:[NSNull class]]){
        return YES;
    }
    
    if ([[self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0){
        return YES;
    }
    
    return NO;
}