Remove children when updating a host node to an element with nil children

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Roact Changelog
 
+## Unreleased
+
+* Fixed an issue where updating a host element with children to an element with `nil` children caused the old children to not be unmounted. ([#210](https://github.com/Roblox/roact/pull/210))
+
 ## [1.0.0](https://github.com/Roblox/roact/releases/tag/v1.0.0)
 This release significantly reworks Roact internals to enable new features and optimizations.
 

src/RobloxRenderer.lua

@@ -269,7 +269,7 @@ function RobloxRenderer.updateHostNode(reconciler, virtualNode, newElement)
 	end
 
 	local children = newElement.props[Children]
-	if children ~= nil then
+	if children ~= nil or oldProps[Children] ~= nil then
 		reconciler.updateVirtualNodeWithChildren(virtualNode, virtualNode.hostObject, children)
 	end
 
@@ -280,4 +280,4 @@ function RobloxRenderer.updateHostNode(reconciler, virtualNode, newElement)
 	return virtualNode
 end
 
-return RobloxRenderer
+return RobloxRenderer

src/RobloxRenderer.spec.lua

@@ -419,6 +419,33 @@ return function()
 				expect(message:find("RobloxRenderer%.spec")).to.be.ok()
 			end)
 		end)
+
+		it("should delete instances when reconciling to nil children", function()
+			local parent = Instance.new("Folder")
+			local key = "Some Key"
+
+			local element = createElement("Frame", {
+				Size = UDim2.new(1, 0, 1, 0),
+			}, {
+				child = createElement("Frame"),
+			})
+
+			local node = reconciler.createVirtualNode(element, parent, key)
+
+			RobloxRenderer.mountHostNode(reconciler, node)
+
+			expect(#parent:GetChildren()).to.equal(1)
+
+			local instance = parent:GetChildren()[1]
+			expect(#instance:GetChildren()).to.equal(1)
+
+			local newElement = createElement("Frame", {
+				Size = UDim2.new(0.5, 0, 0.5, 0),
+			})
+
+			RobloxRenderer.updateHostNode(reconciler, node, newElement)
+			expect(#instance:GetChildren()).to.equal(0)
+		end)
 	end)
 
 	describe("unmountHostNode", function()