Refactor the DebugHandler

[jtpio]

As mentioned in #357, the DebugHandler is getting more complicated as it now handles session context changes (for notebooks and consoles), as well as session connection changes for files:

debugger/src/handler.ts

Lines 164 to 310 in e442a89

	private async _update(
	widget: DebuggerHandler.SessionWidget[DebuggerHandler.SessionType],
	connection: Session.ISessionConnection
	): Promise<void> {
	if (!this._service.model) {
	return;
	}
	const hasFocus = () => {
	return this._shell.currentWidget && this._shell.currentWidget === widget;
	};
	const updateAttribute = () => {
	if (!this._handlers[widget.id]) {
	widget.node.removeAttribute('data-jp-debugger');
	return;
	}
	widget.node.setAttribute('data-jp-debugger', 'true');
	};
	const createHandler = async () => {
	if (this._handlers[widget.id]) {
	return;
	}
	switch (this._type) {
	case 'notebook':
	this._handlers[widget.id] = new NotebookHandler({
	debuggerService: this._service,
	widget: widget as NotebookPanel
	});
	break;
	case 'console':
	this._handlers[widget.id] = new ConsoleHandler({
	debuggerService: this._service,
	widget: widget as ConsolePanel
	});
	break;
	case 'file':
	this._handlers[widget.id] = new FileHandler({
	debuggerService: this._service,
	widget: widget as DocumentWidget<FileEditor>
	});
	break;
	default:
	throw Error(`No handler for the type ${this._type}`);
	}
	updateAttribute();
	};
	const removeHandlers = () => {
	const handler = this._handlers[widget.id];
	if (!handler) {
	return;
	}
	handler.dispose();
	delete this._handlers[widget.id];
	delete this._kernelChangedHandlers[widget.id];
	delete this._statusChangedHandlers[widget.id];
	delete this._contextKernelChangedHandlers[widget.id];
	// clear the model if the handler being removed corresponds
	// to the current active debug session
	if (this._service.session?.connection?.path === connection?.path) {
	const model = this._service.model as DebuggerModel;
	model.clear();
	}
	updateAttribute();
	};
	const addToolbarButton = () => {
	const button = this._buttons[widget.id];
	if (button) {
	return;
	}
	const newButton = updateToolbar(widget, toggleDebugging);
	this._buttons[widget.id] = newButton;
	};
	const removeToolbarButton = () => {
	const button = this._buttons[widget.id];
	if (!button) {
	return;
	}
	button.parent = null;
	button.dispose();
	delete this._buttons[widget.id];
	};
	const toggleDebugging = async () => {
	// bail if the widget doesn't have focus
	if (!hasFocus()) {
	return;
	}
	if (
	this._service.isStarted &&
	this._previousConnection.id === connection.id
	) {
	this._service.session.connection = connection;
	await this._service.stop();
	removeHandlers();
	} else {
	this._service.session.connection = connection;
	this._previousConnection = connection;
	await this._service.restoreState(true);
	await createHandler();
	}
	};
	const debuggingEnabled = await this._service.isAvailable(connection);
	if (!debuggingEnabled) {
	removeHandlers();
	removeToolbarButton();
	return;
	}
	// update the active debug session
	if (!this._service.session) {
	this._service.session = new DebugSession({ connection });
	} else {
	this._previousConnection = this._service.session.connection.kernel
	? this._service.session.connection
	: null;
	this._service.session.connection = connection;
	}
	await this._service.restoreState(false);
	addToolbarButton();
	// check the state of the debug session
	if (!this._service.isStarted) {
	removeHandlers();
	this._service.session.connection = this._previousConnection ?? connection;
	await this._service.restoreState(false);
	return;
	}
	// if the debugger is started but there is no handler, create a new one
	await createHandler();
	this._previousConnection = connection;
	// listen to the disposed signals
	widget.disposed.connect(removeHandlers);
	this._service.model.disposed.connect(removeHandlers);
	}

[jtpio]

Closing as this should eventually be addressed in https://github.com/jupyterlab/jupyterlab directly.