swxmlhash conditional deserialization that may return nil

Question

I'm using SWXMLHASH Swift library to parse some 3rd party XML data. This data can sometimes be returned with values that are not valid for my application. When this happens I wish to ignore this data and return nil instead of Self in my deserialize function.

For example, I have some XML like this:

<Root>
    <ColourChanges>
        <ChangeCount>0</ChangeCount>
        <ChangeItems></ChangeItems>
    </ColourChanges>
</Root>

I want to be able to test the ChangeCount value, and if it equals 0 I want to be able to return nil in the deserialize function of my ColourChanges struct.

It appears that when I define my deserialize function as follows:

static func deserialize(_ node: XMLIndexer) throws -> ColourChanges?

I get a runtime error complaining the deserialize implementation doesn't exist.

I believe I could handle this by throwing from my struct's deserialize method when I encounter an unwanted value and handle that in the caller, however, it would be cleaner if I could just return nil.

Has anyone encountered a similar issue, or any suggestions on how this may be achieved?

Thanks for any help.

A pattern I've used is cases like this is the null object pattern - basically you return something like `ColourChanges.empty` where `empty` is a method that builds a `ColourChanges` instance with preset values to indicate that it is empty or null. Would that work for you? — David Mohundro, May 04 '17 at 23:23
@DavidMohundro - Thank you for responding to the question. Your suggestion might be problematic in this use case, there are nodes returned from the XML where it _may_ entirely be made up of nil values (i.e. the associated WSDL with the XML doesn't mandate values), so I wonder whether this will cause issues attempting to come up with an _empty_ version. Currently, to work around, I'm using the 'throwing approach' I described in my post. However, this may also potentially hide issues because I'm using it in conjunction with `try?` during the `myobject.value()` call. I will investigate your idea. — AlexM, May 06 '17 at 12:55
I've played with your suggestion a little, and the solution I've arrived at is to have a static function that returns ColourChanges?. I use this to wrap the call to value(). I can then test my object for validity and return nil if necessary. Please feel free to change your comment into an answer and I can mark it as correct. Thanks again for responding to the question. — AlexM, May 07 '17 at 14:44

score 0 · Accepted Answer · answered May 09 '17 at 14:02

Adding my suggestion from above:

A pattern I've used is cases like this is the null object pattern - basically you return something like ColourChanges.empty where empty is a method that builds a ColourChanges instance with preset values to indicate that it is empty or null. Would that work for you?

swxmlhash conditional deserialization that may return nil

1 Answers1