Implement dispatching generic runnables to script event loops

[jdm]

For events that are dispatched to the script event loop from the script crate it would be cleaner to avoid adding implementation knowledge to script_task.rs - events like the XHR ones belong entirely to the XHR code. If we had runnables with closures, the script task could just invoke the closure without having to know anything about its contents. Something like RunnableMsg(Box<Runnable>) where Runnable is a trait with a method to invoke the closure would make a lot of sense; any state the closure required could be stored in whatever type is implementing the Runnable trait for that particular event.

[thiagopnts]

You mean to replace the current ScriptMsg approach? I'll start to work on this.

[jdm]

I'm pretty sure we'll need to retain at least some of the messages in there, but I would also be happy to be proved wrong. I was imaging RunnableMsg being another variant of ScriptMsg.

[thiagopnts]

Ok, I'll start to implement this approach with the XHR events you mentioned, then I'll ping you to check if I'm on the right track.

[thiagopnts]

what I did so far: https://github.com/thiagopnts/servo/blob/b014a7417cc9e8abf06528653cafa89ab2fbbd9b/components/script/dom/xmlhttprequest.rs#L83-L95

https://github.com/thiagopnts/servo/blob/b014a7417cc9e8abf06528653cafa89ab2fbbd9b/components/script/script_task.rs#L123

The main problem is processing the progress on the handler, I end up getting a:

dom/xmlhttprequest.rs:92:42: 92:46 error: cannot move out of dereference of `&`-pointer
dom/xmlhttprequest.rs:92             xhr.process_partial_response(self.progress);

Any thoughts on how to workaround this? I would also appreciate any resources on how to deal with boxed traits on channels.

[jdm]

I would try making the handler method take self by value instead of reference.

[thiagopnts]

This makes the object not safe:

dom/xmlhttprequest.rs:232:43: 232:92 error: cannot convert to a trait object because trait `script_task::Runnable` is not object-safe [E0038]
dom/xmlhttprequest.rs:232                     chan.send(RunnableMsg(box XHRProgressWrapper{addr: addr, progress: msg}));
                                                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: cannot call a method (`handler`) with a by-value receiver through a trait object
error: aborting due to previous error

[jdm]

:<

Maybe &mut self? If not, then just clone the needed values for now.

[jdm]

rust-lang/rust#19878 (comment) might be related (and/or suggest alternatives).

[thiagopnts]

@gankro's post is also a good resource on the subject. The &mut self approach didn't work too, so I made a PR cloning the values.