Implement MessageEvent.initMessageEvent

[pshaughn]

This is expected by WPT html/webappapis/scripting/events/messageevent-constructor.https.html and has IDL in https://html.spec.whatwg.org/multipage/comms.html#the-messageevent-interface

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄

[jdm]

Code: components/script/dom/webidls/MessageEvent.webidl, components/script/dom/messageevent.rs
Test: ./mach test-wpt tests/wpt/web-platform-tests/html/webappapis/scripting/events/messageevent-constructor.https.html

[kunalmohan]

I would like to try and solve this one.
@highfive: assign me

[highfive]

Hey @kunalmohan! Thanks for your interest in working on this issue. It's now assigned to you!

[kunalmohan]

I am having trouble initializing variables which are DOMString type. How should I get past it?

[jdm]

@kunalmohan Are you using DOMString::new() for an empty string and DOMString::from(...) to create a DOMString from an existing Rust string value?

[kunalmohan]

I have defined the function as

fn InitMessageEvent(
        &self,
        _cx: JSContext,
        type_: DOMString,
        bubbles: bool,
        cancelable: bool,
        data: HandleValue,
        origin: DOMString,
        lastEventId: DOMString,
        source: Option<WindowProxyOrMessagePortOrServiceWorker>,
        ports: Vec<DomRoot<MessagePort>>
    )

I am not taking origin and lastEventId as string because then I get the following error:

method `InitMessageEvent` has an incompatible type for trait

[jdm]

What is the trouble you are having with initialization, in that case?

[kunalmohan]

Actually it's not just the DOMString.
First, I cannot initialize a key like self.[key]=value because self is not mutable reference. But initializing it using *self.[key]=value gives the error doesn't have size known at compile time for lastEventId, ports and origin.
Second, I cannot figure out why the statements *self.origin=origin (and same for lastEventId) are giving the error

expected `str`, found struct `dom::bindings::str::DOMString`

for the values obtained as parameters.

[jdm]

Ah. You will need to put the members that need to be reinitialized inside DOMRefCell or Cell values. These allow mutating the values inside of methods that take &self instead of &mut self, like all of our DOM APIs. As for the str vs. DOMString error, that also surprises me. See if it goes away when you use the appropriate DOMRefCell/Cell wrappers?

[kunalmohan]

Okay, I'll use the DOMRefCell/Cell wrappers. Would there be a need to edit other functions for the MessageEvent struct after implementing the wrappers?

[jdm]

Any method that uses the members that are now wrapped in cells will need to be adjusted in order to compile, yes.

[kunalmohan]

That was quite helpful. Thanks :)
I'll get back to you about the str vs DOMString error.