Consider shutting off DOMAgent

[njx]

While investigating #2154, we discovered that DOMAgent can cause a couple of issues in Live Development when trying to load files. _onFinishedLoadingDOM() appears to try to load files via XHR rather than just getting them from the local disk, and in various edge cases this can fail, causing the agent to load incompletely or fail to load, even though none of our current features rely on the results of this load as far as I can tell.

From what I can tell, DOMAgent's main functionality is the experimental DOM highlighting that @jdiehl had written originally, but we're not using that functionality right now. However, simply commenting out the loading of the DOMAgent seems to cause other parts of Live Development to fail, so there are some other dependencies on it. We should consider shutting it off for now, or at least shutting off the parts that aren't being used.

[jdiehl]

Removing the DOMAgent will break the following other agents:

• EditAgent (in-browser html editing)
• GotoAgent (navigate from brackets to browser and back)
• HighlightAgent (highlight in brackets / in browser)

These agents rely on the rendered DOM tree being mapped to the source, which is what the DOMAgent provides.

The ScriptAgent will also break, but only because it also manages traces, which are unused at the moment. To remove the DOMAgent and keep the ScriptAgent working, you will have to:

Edit ScriptAgent:

• remove DOMAgent
• remove _insertTrace
• remove the functions _onGetDocument, _onChildNodeInserted, _onPaused
• replace the load and unload functions with:

    function load() {
        _urlToScript = {};
        _idToScript = {};
        _load = new $.Deferred();
        _load.resolve();
        Inspector.Debugger.enable();
        $(Inspector.Debugger)
            .on("scriptParsed.ScriptAgent", _onScriptParsed)
            .on("scriptFailedToParse.ScriptAgent", _onScriptFailedToParse);
        return _load;
    }

    function unload() {
        $(Inspector.Debugger).off(".ScriptAgent");
    }

Then, the DOMAgent can be disabled in LiveDevelopment.js

[njx]

Marking move to backlog. We should add a task to one of the upcoming HTML Live Development stories to audit the existing code and figure out what we want to keep/reuse/disable/remove.

[njx]

This is in sprint 22 as part of the RESEARCH: Live Development Robustness card. No need to add to the bug card. Should assign it once someone picks up the task.

[redmunds]

I took a closer look at DOMAgent. I'm not sure why, but disabling DOMAgent causes the Inspector.Page object to stop firing the loadEventFired event, so CSSAgent and RemoteAgent both fail to load. Page appears to load in browser, but status stops at "Initializing...", and highlighting breaks. I looked through Inspector.json, but I don't see any other load events.

I also tried disabling code in _onFinishedLoadingDOM() in DOMAgent.js (by making it "experimental"). This is the code that loads the page via XHR and builds the DOM. Everything still works after disabling this code, except that I discovered that one of the Live Dev unit tests uses the DOM nodes to verify an edit to document has been made.

@njx Do you think it's worth taking this change and removing 1 of the Live Dev unit tests?

[jdiehl]

To enable the loadEventFired you must enable Page events by issuing the following command as soon as the connection is established:

Inspector.Page.enable();

[redmunds]

Thanks, @jdiehl. That should be moved out of DOMAgent since those events are used by other agents.

Unfortunately, the Live Development "should reapply in-memory css changes after saving changes in html document" unit test relies on the DOM built by DOMAgent (via nodeAtLocation() method) to verify changes to html document were loaded in browser. This also cannot use LiveDevelopment.getLiveDocForPath() because HTML docs are still experimental. We could disable this test for now, otherwise we'll have to keep DOMAgent enabled.

[redmunds]

This was researched during Sprint 22. We don't want to take any action now, but want to keep this open, so clearing milestone.