Allow View containers to specify custom measure functions

[obipawan]

This PR allows ViewManagers that are containers (extending ViewGroupManager) to specify it's custom CSSNode.MeasureFunction.
ViewManagers that don't need to specify this can extend SimpleViewGroupManager instead

[facebook-github-bot]

By analyzing the blame information on this pull request, we identified @mkonicek, @kmagiera and @dmmiller to be potential reviewers.

[mkonicek]

Hey @obipawan, what's the motivation for this?

[obipawan]

Hey @mkonicek
This change is basically to allow custom Views that are also containers(ViewGroup) to other views to be able to specify a measure function if they wanted to. With the current ViewGroupManager, it's not possible to specify a custom LayoutShadowNode.
It also brings this in parity with iOS where one can create any native view from RCTViewManager and then specify a custom RCTShadowView.

[mkonicek]

OK thanks for the explanation! @kmagiera, @foghina are you OK with this?

[astreet]

Why can you not provide a custom LayoutShadowNode by extending ViewGroupManager?

[kmagiera]

I don't see this change being necessary to support the use case you're describing. If you want your view group manager to use custom class as a shadow node you can simply override createShadowNodeInstance and getShadowNodeClass (note that both methods are not marked as final in ViewGroupManager). TBH I'd rather avoid adding yet another base class to the namespace

[obipawan]

Hey @kmagiera It's possible to override createShadowNodeInstance, but as for getShadowNodeClass you'd have to make the return type to an unchecked Class type for it to work. Though this is yet another base class, it would make it safer to have the return type checked IMO.

[kmagiera]

@obipawan Are you sure that is the case here? The return type on getShadowNodeClass in ViewGroupManager is Class<? extends LayoutShadowNode> (not Class<LayoutShadowNode>) which should allow you to return class object of any class that extends LayoutShadowNode.

[obipawan]

@kmagiera Apologies! you're absolutely right. I missed that this was changed for 0.18 and missed this even while submitting this PR. My bad!