[ASViewController] Support optional node

[nguyenhuy]

This PR makes ASViewController's node optional. This allows developers to use ASViewController like a normal UIViewController. More importantly, it can be used as a base class for view controllers among which some use a node hierarchy and some don't.

Resolve #2760.

[hannahmbanana]

This is really awesome Huy! I've heard that ASViewController's initWithNode: pattern is the most confusing aspect of ASDK.

[maicki]

@nguyenhuy After looking over this PR it's a really nice improvement idea, but I'm not pretty sure if we should introduce having to test if the _node is nil in almost every method. Especially going forward we would need to be careful to always consider that.

What about having something like a ASViewControllerNullNode that would act like a proxy and this would be used as root node for the ASViewController and has a reference to the view controller if no node is given? In that null node we would just not do anything I guess in case we call method's like layoutThatFits: but if the view is accessed we actually would return the view from the view controller. I would have to lookup in the ASViewController implementation what we exactly would need, but I think that would be a bit cleaner solution.

[nguyenhuy]

@maicki Thank you for the feedbacks. I've came up with a simpler fix. In case users don't provide ASViewController with a node (by using -initWithNibName:bundle: or -initWithCoder:, I create a default node that simply loads an UIView. And so the node is non-optional as it has always been. This introduces a small overhead but helps reduce complexity of ASViewController. The overhead might even encourage people to opt-in to a node backed hierarchy and take full advantage of ASViewController.

I tested this approach with ASDKgram and didn't find any problem. Given the fact that we currently don't even support -initWithNibName:bundle: or -initWithCoder:, this change shouldn't cause any new regression on existing codebases and thus it should be safe.

[Adlai-Holler]

We should use simply [[ASDisplayNode alloc] init] here because using a view block results in a wrapper node, which has limited capacity compared to a node using _ASDisplayView.

[nguyenhuy]

Good catch! Thanks. Fixing.

[Adlai-Holler]

It looks like this has been updated by @maicki in the linked PR. Closing.