Add stackTrace getter

CHANGELOG.md

@@ -7,6 +7,7 @@
 * Added the static lifecycle method `getDerivedStateFromProps` ([#57](https://github.com/Roblox/roact/pull/57))
 * Allow canceling render by returning nil from setState callback ([#64](https://github.com/Roblox/roact/pull/64))
 * 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))
 
 ## 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))

docs/api-reference.md

@@ -241,6 +241,13 @@ Right now, components are re-rendered any time a parent component updates, or wh
 
 `PureComponent` implements `shouldUpdate` to only trigger a re-render any time the props are different based on shallow equality. In a future Roact update, *all* components may implement this check by default.
 
+### getElementTraceback
+```
+getElementTraceback() -> string | nil
+```
+
+`getElementTraceback` gets the stack trace that the component was created in. This allows you to report error messages accurately.
+
 ## Lifecycle Methods
 In addition to the base Component API, Roact exposes additional lifecycle methods that stateful components can hook into to be notified of various steps in the rendering process.
 

lib/Component.lua

@@ -239,6 +239,14 @@ function Component:setState(partialState)
 	self:_update(self.props, newState)
 end
 
+--[[
+	Returns the current stack trace for this component, or nil if the
+	elementTracing configuration flag is set to false.
+]]
+function Component:getElementTraceback()
+	return self._handle._element.source
+end
+
 --[[
 	Notifies the component that new props and state are available. This function
 	is invoked by the reconciler.

lib/Component.spec.lua

@@ -2,6 +2,7 @@ return function()
 	local Core = require(script.Parent.Core)
 	local Reconciler = require(script.Parent.Reconciler)
 	local Component = require(script.Parent.Component)
+	local GlobalConfig = require(script.Parent.GlobalConfig)
 
 	it("should be extendable", function()
 		local MyComponent = Component:extend("The Senate")
@@ -523,4 +524,51 @@ return function()
 			Reconciler.unmount(instance)
 		end)
 	end)
+
+	describe("getElementTraceback", function()
+		it("should return stack traces", function()
+			local stackTraceCallback = nil
+
+			GlobalConfig.set({
+				elementTracing = true
+			})
+
+			local TestComponent = Component:extend("TestComponent")
+
+			function TestComponent:init()
+				stackTraceCallback = function()
+					return self:getElementTraceback()
+				end
+			end
+
+			function TestComponent:render()
+				return Core.createElement("StringValue")
+			end
+
+			local handle = Reconciler.mount(Core.createElement(TestComponent))
+			expect(stackTraceCallback()).to.be.ok()
+			Reconciler.unmount(handle)
+			GlobalConfig.reset()
+		end)
+
+		it("should return nil when elementTracing is off", function()
+			local stackTraceCallback = nil
+
+			local TestComponent = Component:extend("TestComponent")
+
+			function TestComponent:init()
+				stackTraceCallback = function()
+					return self:getElementTraceback()
+				end
+			end
+
+			function TestComponent:render()
+				return Core.createElement("StringValue")
+			end
+
+			local handle = Reconciler.mount(Core.createElement(TestComponent))
+			expect(stackTraceCallback()).to.never.be.ok()
+			Reconciler.unmount(handle)
+		end)
+	end)
 end