Improve cell node relayout

[nguyenhuy]

As discussed in #792:

• Introduce ASCellNodeDelegate. Cell node automatically notifies the delegate after a relayout (caused by -setNeedsLayout). Confirming to ASCellNodeDelegate; ASTableView and ASCollectionView reload the calling cell upon notifications. These views automatically set themselves as delegate of every node.
• The result is that ASCellNode subclasses don't need to manually notify the containing view. Thus, -relayoutItemAtIndexPath and -relayoutRowAtIndexPath are removed (Add relayout item/row APIs to ASTableView and ASCollectionView #722, [ASLayoutSpec] ASNetworkImageNode with unknown height don't update ASCellNode's height #718).
• Kittens example is updated to reflect the change.

[levi]

What's the purpose of having a suggestedAnimation param when this is always automatic? How do you see this being extended?

[nguyenhuy]

It can be set to another value, by another object or in the subclass's initializer.

[Adlai-Holler]

@nguyenhuy Does this mean cells will flash when they resize? In client apps I specifically avoid calling reloadRowsAtIndexPaths in favor of beginUpdates() / endUpdates() because UITableView seems to flash when reloading rows with animation no matter what you pass.

[nguyenhuy]

@Adlai-Holler:

Does this mean cells will flash when they resize?

"Reload" destroys the existing node and loads a new one. "Relayout" (as in this PR) means the existing node is reused and only its layout is updated. This PR allows you to pass relayout animation (UITableViewRowAnimation) to a cell node, which will then be passed to ASTableView and the underlying UITableView (here). You can also animate the layout changes by wrapping -setNeedsLayout inside an animation block (like this).

In client apps I specifically avoid calling reloadRowsAtIndexPaths in favor of beginUpdates() / endUpdates() because UITableView seems to flash when reloading rows with animation no matter what you pass.

Yeah, that's because of the delete + insert behaviour. Though you can do some tricks to hide the flash like this, I would recommend avoiding reload methods, as discussed here.

[appleguy]

It's only safe to call from the background if the node isn't loaded, right? We should probably just remove that.

[nguyenhuy]

Done in #812.

[appleguy]

Thanks for the naming and formatting improvements. This is really a great change and reflects the development philosophy of ASDK, aiming for a smaller API surface that integrates functionality where it makes sense for the developer (e.g. ASCellNode itself bridging table and collection, layout being a concept that should extend to cells' height, etc)