is there some sane solution other than codegen?

# android-architecture

[ursus]

is there some sane solution other than codegen?

[joelpedraza]

Maybe I’m misunderstanding the question.
In DiffUtil.Callback
areItemsTheSame checks identity equality
areContentsTheSame checks visual equality

The adapter is only notified of changes if the return values of both are true

[joelpedraza]

areContentsTheSame is _only called for elements in which areItemsTheSame has returned true

[joelpedraza]

@ursus

[joelpedraza]

When `areItemsTheSame`returns `true `for two items and areContentsTheSame returns false for them, DiffUtil calls this method to get a payload about the change.

[joelpedraza]

Which makes sense, you only need a diff payload when the items have the same identity, but different contents

[ursus]

yes, im talking about the getChangePayload

[ursus]

problem is that you have to have a whitelist of fields in the payload function

[ursus]

however, this needs to be a complete list, you cannot forget to add here newly added model field

[ursus]

because this way that field value wont be updated in the view

[ursus]

= codgen is needed @joelpedraza

[joelpedraza]

The payload is any arbitrary object. I’m not sure what you mean by field whitelist

[ursus]

@Nullable
    @Override
    public Object getChangePayload(int oldItemPosition, int newItemPosition) {
        Contact newContact = newList.get(newItemPosition);
        Contact oldContact = oldList.get(oldItemPosition);

        Bundle diff = new Bundle();
        if(!newContact.getName().equals(oldContact.getName())){
            diff.putString("name", newContact.getName());
        }
        if(!newContact.getPhonenumber().equals (oldContact.getPhonenumber())){
            diff.putString("phone", newContact.getPhonenumber());
        }
        if (diff.size()==0){
            return null;
        }
        return diff;
    }

[ursus]

this is like very common what people do, I copy pasted it from some tutorial

[ursus]

the problem with it that it has the white list of checking for name & phone number change

[ursus]

so when you add a Image url to the model next time in future, and you forget to add it here; and Name and Image change for some object, your whitelist will only catch the Name change and your Image wont be refreshed

[joelpedraza]

Ah now I see what you mean. It does seem like a good use case for codegen.

If you want a hand written solution, consider some sort of double-dispatch, like a visitor pattern. Updating the model means adding a new visit method, then it’s a compile error unless you add an implementation for the new method to all your visitors

[joelpedraza]

WARNING: OPINION

Using a bundle is weird. What’s the purpose of even having the Contact type that stores all the data, only to stuff it into the bundle.

[joelpedraza]

I’d prefer an EnumSet, then you can pattern match with a when expression and non-exhastive matching will be a compiler error

That would come at the development cost of declaring a new enum for each view holder. This dosent seem much higher than declaring string constants though.

Also the runtime (bytecode) cost of adding new Class

[joelpedraza]

@ursus

[ursus]

@joelpedraza im not sure how would you enfore the visitor, one still has to add it

[joelpedraza]

You cant really enforce that the visitor has a method for each field.

If you put the declaration of the visitor as a nested class of the data type, and comments to update it when adding a field: you’ll probably be OK with just convention

[ursus]

Yea, cached reflection will probably be a better solution