Change refs in one pass, making switching refs atomic (#98)

CHANGELOG.md

@@ -9,6 +9,7 @@
 * Added `defaultProps` value on stateful components to define values for props that aren't specified ([#79](https://github.com/Roblox/roact/pull/79))
 * Added `getElementTraceback` ([#81](https://github.com/Roblox/roact/issues/81), [#93](https://github.com/Roblox/roact/pull/93))
 * Added `createRef` ([#70](https://github.com/Roblox/roact/issues/70), [#92](https://github.com/Roblox/roact/pull/92))
+* Ref switching now occurs in one pass, which should fix edge cases where the result of a ref is `nil`, especially in property changed events ([#98](https://github.com/Roblox/roact/pull/98))
 
 ## 1.0.0 Prerelease 2 (March 22, 2018)
 * Removed `is*Element` methods, this is unlikely to affect anyone ([#50](https://github.com/Roblox/roact/pull/50))

lib/Reconciler.lua

@@ -331,11 +331,13 @@ function Reconciler._reconcileInternal(instanceHandle, newElement)
 
 		local oldRef = oldElement.props[Core.Ref]
 		local newRef = newElement.props[Core.Ref]
-		local refChanged = (oldRef ~= newRef)
 
-		-- Cancel the old ref before we make changes. Apply the new one after.
-		if refChanged and oldRef then
+		-- Change the ref in one pass before applying any changes.
+		-- Roact doesn't provide any guarantees with regards to the sequencing
+		-- between refs and other changes in the commit phase.
+		if newRef ~= oldRef then
 			applyRef(oldRef, nil)
+			applyRef(newRef, instanceHandle._rbx)
 		end
 
 		-- Update properties and children of the Roblox object.
@@ -344,11 +346,6 @@ function Reconciler._reconcileInternal(instanceHandle, newElement)
 
 		instanceHandle._element = newElement
 
-		-- Apply the new ref if there was a ref change.
-		if refChanged and newRef then
-			applyRef(newRef, instanceHandle._rbx)
-		end
-
 		return instanceHandle
 	elseif isFunctionalElement(newElement) then
 		instanceHandle._element = newElement