I'd be happy with simple solution like this:
in the very beginning of AddWatchAction.performAction add a lookup check in the current session and call provider (if any) method like:
AddWatchSessionProvider sp = DebuggerManager.getDebuggerManager().getCurrentEngine().lookup(null, AddWatchSessionProvider.class);
if (sp != null) {
   sp.perform();
} else {
...
}
//open watches view
...

O.K. If I got it right, the intent is to provide your own UI for submission of watches? Do you expect that AddWatchActionProvider should provide a UI component that would be placed into the New Watch dialog instead of the current single field?
What is the proposed AddWatchSessionProvider.perform() method supposed to do?

Do you need this only when a debugger session is active, or you would make some use of a MIME type-sensitive provider (e.g. use your provider if a C/C++ file is active)?

Oh, I've read it too fast... perform() would replace all dialog construction that AddWatchAction.performAction() does. Fine, that's simple enough to implement.
Do you need also to override getName()? And probably asynchronous() as well...

It is only required if there native session is active.
I think it would be easier if we provide whole UI component.
It does not look like we would need to override getName() and asynchronous(). At least for now.

Created attachment 94127 [details]
The Add Watch action taken from a provider

I've made the change really minimalistic. If we use existing ActionsProvider for this purpose it's enough to just add ACTION_ADD_WATCH constant into the API.

Please let me know if you're fine with this solution and I'd post it to apireviews.

Created attachment 94128 [details]
The testing Add Watch action demonstrating the usage.

I'm fine with one small request:
default performAction does "open watches view" stuff at the end, I'd prefer not to do that by myself if possible. Can you make it:
if (!performEngineAddWatchAction()) {
... default code ...
}
// open watches view
...

Thanks

Well, for that we would really need some specialized provider :-(
We should not open watches view if you cancel the Add Watch action. But when I simply run
manager.postAction(ActionsManager.ACTION_ADD_WATCH)
I'm not able to find out if the action was canceled later or not.

Or, we can implement a "detector": listen for watch add while the action is running. And when a watch is added, open the watches view automatically.

I'll write such a detector and will attach it here for review...

Created attachment 94197 [details]
Add Watch action now tracks created watches and opens watches window when a new watch is created.

Created attachment 94198 [details]
Tested also with a lazy action provider.

Created attachment 94201 [details]
I've renamed the action from ADD_WATCH to NEW_WATCH, added apichanges, it's hopefully ready for the review now...

Please review addition of ACTION_NEW_WATCH constant. Thanks.

look fine for me

Thanks for the review, I'll push the change tomorrow.

Implemented in changeset:   160994:92a2e5078f66
http://hg.netbeans.org/main/rev/92a2e5078f66

Integrated into 'main-golden', will be available in build *201002261426* on http://bits.netbeans.org/dev/nightly/ (upload may still be in progress)
Changeset: http://hg.netbeans.org/main/rev/92a2e5078f66
User: mentlicher@netbeans.org
Log: #180558 - Debuggers can provide session-specific New Watch dialog.

Thanks a lot! It works good.
Question is how to add new watch action to watch nodes popup menu?
NodeActionsProvider.getActions have to contain an instance of global newWatch action, how can I obtain it?

Well, debugger core adds the new watch action into the Watches window. It's the first one, therefore if you implement NodeActionsProviderFilter, you can get it.
Other approach can be to retrieve it from the actions registry:
NewWatchAction action = (Action) DataObject.find (FileUtil.getConfigFile ("Actions/Debug/org-netbeans-modules-debugger-ui-actions-AddWatchAction.instance")).getLookup().lookup(InstanceCookie.class).instanceCreate();

Thanks!