iOS 判斷NSString值是否為空或null并轉換為空字元串

遇到了一個背景json誤将null作為字元串值導緻iOS用戶端崩潰閃退的問題，解決方法如下：套一層判斷，如果是各種情況的null則轉換為@""。

+ (NSString *)nullToString:(id)string {
    if ([string isEqual:@"NULL"] || [string isKindOfClass:[NSNull class]] || [string isEqual:[NSNull null]] || [string isEqual:NULL] || [[string class] isSubclassOfClass:[NSNull class]] || string == nil || string == NULL || [string isKindOfClass:[NSNull class]] || [[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length]==0 || [string isEqualToString:@"<null>"] || [string isEqualToString:@"(null)"]) {
        return @"";
    } else {
        return (NSString *)string;
    }
}