[Webkit-unassigned] [Bug 67458] Generate a WebKitCSSMatrix constructor of V8 using the IDL 'Constructor' extended attribute

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=67458





--- Comment #5 from Kentaro Hara <haraken at google.com>  2011-09-02 02:27:35 PST ---
Adam and Antonm: The motivation for the change is as follows. 

As you can see in V8WebKitCSSMatrixConstructor.cpp, constructorCallback() expects that...

- If the first argument is a DOMString, the corresponding WTF::String is passed to the first argument of WebKitCSSMatrix::create().
- If the first argument is omitted, a null WTF::String (i.e. String()) is passed to the first argument of WebKitCSSMatrix::create().

I first thought that describing 'Constructor(in [Optional=CallWithDefaultValue] DOMString cssValue)' in IDL will generate the code that behaves as above, but it does not. In fact, the generated code behaves...

- If the first argument is a DOMString, the corresponding WTF::String is passed to the first argument of WebKitCSSMatrix::create().
- If the first argument is omitted, a string "undefined" is passed to the first argument of WebKitCSSMatrix::create(). This is because when we execute 'V8Parameter cssValue(args[0])' in the situation where args[0] is omitted, args[0] becomes v8::Undefined(), which will make cssValue.toString() "undefined".

Thus, we need a way to express a null string in V8Parameter (note: V8Parameter is something like an abstraction class for String and AtomicString). The change in V8Binding.h is just doing this. 

The same scenario happens in other constructors, such as V8SharedWorker::constructorCallback().

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