This changes how non-async module loading works. In general, module loading
is triggered by a v8 callback. We ask it to process a module (a <script type=
module>) and then for every module that it depends on, we get a callback. This
callback expects the nested v8.Module instance, so we need to load it then and
there (as opposed to dynamic imports, where we only have to return a promise).
Previously, we solved this by issuing a blocking HTTP get in each callback. The
HTTP loop was able to continuing downloading already-queued resources, but if
a module depended on 20 nested modules, we'd issue 20 blocking gets one after
the other.
Once a module is compiled, we can ask v8 for a list of its dependent module. We
can them immediately start to download all of those modules. We then evaluate
the original module, which will trigger our callback. At this point, we still
need to block and wait for the response, but we've already started the download
and it's much faster. Sure, for the first module, we might need to wait the same
amount of time, but for the other 19, chances are by the time the callback
executes, we already have it downloaded and ready.
Allows dynamic imports to be loading asynchronously. I know reddit isnt the
best example, since it doesn't fully load, but this reduced the load time from
~7.2s to ~4.8s.
The first time a `slotchange` event is registered, we setup a SlotChangeMonitor
on the page. This uses a global (ugh) MutationEvent to detect slot changes.
We could improve the perfomance of this by installing a MutationEvent per
custom element, but a global is obviously a lot easier.
Our MutationEvent currently fired _during_ the changes. This is problematic
(in general, but specifically for slotchange). You can image something like:
```
slot.addEventListener('slotchange', () => {
// do something with slot.assignedNodes()
});
```
But, if we dispatch the `slotchange` during the MutationEvent, assignedNodes
will return old nodes. So, our SlotChangeMonitor uses the page scheduler to
schedule dispatches on the next tick.
Refactors some of the module loading logic. Both normal modules import and
dynamic module import now share more of the same code - they both go through
the slightly modified `module` function.
Dynamic modules now check the cache first, before loading, and when cached,
resolve the correct promise. This can now happen regardless of the module
loading state.
Also tried to replace some page arenas with call arenas and added some basic
tests for both normal and dynamic module loading.
chromedp expects the nodeId starts to 1.
A start to 0 make it enter in infinite loop b/c it expects the Go's
default int, ie 0, to be nil from a map to stop the loop.
If the 0 index is set, it will loop...
exit_when_done is pretty much a sneaky way to get CDP knowledge into the page.
exit_when_done == true means "this isn't a CDP session".
extra_socket is another sneaky weay to get CDP knowledge into the page. When
we get an `extra_socket` message it means "Return control to the CDP server".
Therefore it should be impossible to get an `extra_socket` message (return to
CDP) when `exit_when_done == true` (this isn't a CDP session).
