Bug 154656 – Improve support for nullable DOMString attributes / parameters

• Component: Bindings
• Status: RESOLVED FIXED

[Chris Dumez]

Improve support for nullable DOMString attributes / parameters and use them when possible to match specifications:
http://heycam.github.io/webidl/#idl-nullable-type

Once we support and use properly nullable DOMString attributes / parameters, we should be able to get rid of [TreatUndefined=NullString] WebKit IDL attribute.
Also, [TreatNullAs=NullString] should then only be used in cases where the specification has [TreatNullAs=EmptyString] (see Bug 154654).

* Currently, to emulate nullable DOMString parameters, WebKit uses:
void method([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString parameter)
or worse:
void method([TreatNullAs=NullString] DOMString parameter) // Wrong behavior for undefined.

When it should simply be:
void method(DOMString? parameter)

* And to emulate nullable DOMString attributes, WebKit uses:
[TreatReturnedNullStringAs=Null, TreatNullAs=NullString, TreatUndefinedAs=NullString] attribute DOMString myAttr;

or worse:
[TreatReturnedNullStringAs=Null, TreatNullAs=NullString] attribute DOMString myAttr; // Wrong behavior for undefined.

when it should simply be:
attribute DOMString? myAttr;

--
Note that the change should not be that hard because our bindings generator is already able to generate the right code for these nullable attributes / parameters, although via the use of WebKit-specific IDL attributes. Also, I believe I already added support for nullable attributes recently:
attribute DOMString? myAttr;
should already work and be equivalent to
[TreatReturnedNullStringAs=Null, TreatNullAs=NullString, TreatUndefinedAs=NullString] attribute DOMString myAttr;

I am not sure what's our support for nullable DOMString parameters at the moment but I seem to remember messing with that code as well a while back.

[Chris Dumez]

This was dealt with in other bugs recently. Nullable DOMString parameters, attributes and return values should not be supported.