Add a Context class (#110)

* Don't expose TestRunner nodes through TestSession

* Remove unused returns

* Add function comments.

* EOF newline

* Add Context object.

* Remove unused variables

* Add a comment about what a Context does

* Add missing test case

src/Context.lua

@@ -0,0 +1,26 @@
+--[[
+	The Context object implements a write-once key-value store. It also allows
+	for a new Context object to inherit the entries from an existing one.
+]]
+local Context = {}
+
+function Context.new(parent)
+	local meta = {}
+	local index = {}
+	meta.__index = index
+
+	if parent then
+		for key, value in pairs(getmetatable(parent).__index) do
+			index[key] = value
+		end
+	end
+
+	function meta.__newindex(_obj, key, value)
+		assert(index[key] == nil, string.format("Cannot reassign %s in context", tostring(key)))
+		index[key] = value
+	end
+
+	return setmetatable({}, meta)
+end
+
+return Context

tests/Context.lua

@@ -0,0 +1,56 @@
+local TestEZ = script.Parent.Parent.TestEZ
+local Context = require(TestEZ.Context)
+
+return {
+	["Context.new returns a new context"] = function()
+		assert(Context.new(), "Context.new() returned nil")
+	end,
+	["context.foo returns nil if it wasn't set"] = function()
+		local context = Context.new()
+		assert(context.foo == nil, string.format("Got %s, expected nil", tostring(context.foo)))
+	end,
+	["context.foo returns the value from setting context.foo"] = function()
+		local context = Context.new()
+		context.foo = "BAR"
+		assert(context.foo == "BAR", string.format("Got %s, expected BAR", tostring(context.foo)))
+	end,
+	["context.foo can't be set twice"] = function()
+		local context = Context.new()
+		context.foo = "foo"
+		local success, _ = pcall(function()
+			context.foo = "bar"
+		end)
+		assert(not success, "Expected second context.foo to error")
+	end,
+	["Context.new accepts a parent"] = function()
+		local parent = Context.new()
+		assert(Context.new(parent), "Context.new(parent) returned nil")
+	end,
+	["A child context can still read its parent values"] = function()
+		local parent = Context.new()
+		parent.foo = "BAR"
+		local child = Context.new(parent)
+		assert(child.foo == "BAR", string.format("Got %s, expected BAR", tostring(child.foo)))
+	end,
+	["A parent context can't read its child values"] = function()
+		local parent = Context.new()
+		local child = Context.new(parent)
+		child.foo = "BAR"
+		assert(parent.foo == nil, string.format("Got %s, expected nil", tostring(parent.foo)))
+	end,
+	["A child can't overwrite parent values"] = function()
+		local parent = Context.new()
+		parent.foo = "foo"
+		local child = Context.new(parent)
+		local success, _ = pcall(function()
+			child.foo = "bar"
+		end)
+		assert(not success, "Expected setting child.foo to error")
+	end,
+	["A child won't see changes to the parent"] = function()
+		local parent = Context.new()
+		local child = Context.new(parent)
+		parent.foo = "foo"
+		assert(child.foo == nil, string.format("Got %s, expected nil", tostring(parent.foo)))
+	end,
+}