test/goodsignal.luau

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+--!nocheck
+-- wrapped task library for use in pure luau
+local task = { spawn = function(thread, ...)
+    local ok, err = coroutine.resume(thread, ...)
+    if not ok then error(err, 3) end
+end }
+
+export type Type = RBXScriptSignal & { Fire: (Type, ...any)-> () }
+
+----------------------------------------------------------------------------------------------------
+--               Batched Yield-Safe Signal Implementation                     --
+-- This is a Signal class which has effectively identical behavior to a       --
+-- normal RBXScriptSignal, with the only difference being a couple extra      --
+-- stack frames at the bottom of the stack trace when an error is thrown.     --
+-- This implementation caches runner coroutines, so the ability to yield in   --
+-- the signal handlers comes at minimal extra cost over a naive signal        --
+-- implementation that either always or never spawns a thread.                --
+--                                                                            --
+-- API:                                                                       --
+--   local Signal = require(THIS MODULE)                                      --
+--   local sig = Signal.new()                                                 --
+--   local connection = sig:Connect(function(arg1, arg2, ...) ... end)        --
+--   sig:Fire(arg1, arg2, ...)                                                --
+--   connection:Disconnect()                                                  --
+--   sig:DisconnectAll()                                                      --
+--   local arg1, arg2, ... = sig:Wait()                                       --
+--                                                                            --
+-- Licence:                                                                   --
+--   Licenced under the MIT licence.                                          --
+--                                                                            --
+-- Authors:                                                                   --
+--   stravant - July 31st, 2021 - Created the file.                           --
+----------------------------------------------------------------------------------------------------
+
+-- The currently idle thread to run the next handler on
+local freeRunnerThread = nil
+
+-- Function which acquires the currently idle handler runner thread, runs the
+-- function fn on it, and then releases the thread, returning it to being the
+-- currently idle one.
+-- If there was a currently idle runner thread already, that's okay, that old
+-- one will just get thrown and eventually GCed.
+local function acquireRunnerThreadAndCallEventHandler(fn, ...)
+	local acquiredRunnerThread = freeRunnerThread
+	freeRunnerThread = nil
+	fn(...)
+	-- The handler finished running, this runner thread is free again.
+	freeRunnerThread = acquiredRunnerThread
+end
+
+-- Coroutine runner that we create coroutines of. The coroutine can be 
+-- repeatedly resumed with functions to run followed by the argument to run
+-- them with.
+local function runEventHandlerInFreeThread()
+	-- Note: We cannot use the initial set of arguments passed to
+	-- runEventHandlerInFreeThread for a call to the handler, because those
+	-- arguments would stay on the stack for the duration of the thread's
+	-- existence, temporarily leaking references. Without access to raw bytecode
+	-- there's no way for us to clear the "..." references from the stack.
+	while true do
+		acquireRunnerThreadAndCallEventHandler(coroutine.yield())
+	end
+end
+
+-- Connection class
+local Connection = {}
+Connection.__index = Connection
+
+function Connection.new(signal, fn)
+	return setmetatable({
+		_connected = true,
+		_signal = signal,
+		_fn = fn,
+		_next = false,
+	}, Connection)
+end
+
+function Connection:Disconnect()
+	self._connected = false
+
+	-- Unhook the node, but DON'T clear it. That way any fire calls that are
+	-- currently sitting on this node will be able to iterate forwards off of
+	-- it, but any subsequent fire calls will not hit it, and it will be GCed
+	-- when no more fire calls are sitting on it.
+	if self._signal._handlerListHead == self then
+		self._signal._handlerListHead = self._next
+	else
+		local prev = self._signal._handlerListHead
+		while prev and prev._next ~= self do
+			prev = prev._next
+		end
+		if prev then
+			prev._next = self._next
+		end
+	end
+end
+
+-- Make Connection strict
+setmetatable(Connection, {
+	__index = function(tb, key)
+		error(("Attempt to get Connection::%s (not a valid member)"):format(tostring(key)), 2)
+	end,
+	__newindex = function(tb, key, value)
+		error(("Attempt to set Connection::%s (not a valid member)"):format(tostring(key)), 2)
+	end
+})
+
+-- Signal class
+local Signal = {}
+Signal.__index = Signal
+
+Signal.__type = "RBXScriptSignal"
+
+function Signal.new(): Type
+	return setmetatable({
+		_handlerListHead = false,
+	}, Signal) :: any
+end
+
+function Signal:Connect(fn)
+    if type(fn) ~= "function" then error(`attempt to connect non function (got { type(fn) })`, 2) end
+	local connection = Connection.new(self, fn)
+	if self._handlerListHead then
+		connection._next = self._handlerListHead
+		self._handlerListHead = connection
+	else
+		self._handlerListHead = connection
+	end
+	return connection
+end
+
+-- Disconnect all handlers. Since we use a linked list it suffices to clear the
+-- reference to the head handler.
+function Signal:DisconnectAll()
+	self._handlerListHead = false
+end
+
+-- Signal:Fire(...) implemented by running the handler functions on the
+-- coRunnerThread, and any time the resulting thread yielded without returning
+-- to us, that means that it yielded to the Roblox scheduler and has been taken
+-- over by Roblox scheduling, meaning we have to make a new coroutine runner.
+function Signal:Fire(...)
+	local item = self._handlerListHead
+	while item do
+		if item._connected then
+			if not freeRunnerThread then
+				freeRunnerThread = coroutine.create(runEventHandlerInFreeThread)
+				-- Get the freeRunnerThread to the first yield
+				coroutine.resume(freeRunnerThread)
+			end
+			task.spawn(freeRunnerThread, item._fn, ...)
+		end
+		item = item._next
+	end
+end
+
+-- Implement Signal:Wait() in terms of a temporary connection using
+-- a Signal:Connect() which disconnects itself.
+function Signal:Wait()
+	local waitingCoroutine = coroutine.running()
+	local cn;
+	cn = self:Connect(function(...)
+		cn:Disconnect()
+		task.spawn(waitingCoroutine, ...)
+	end)
+	return coroutine.yield()
+end
+
+-- Implement Signal:Once() in terms of a connection which disconnects
+-- itself before running the handler.
+function Signal:Once(fn)
+	local cn;
+	cn = self:Connect(function(...)
+		if cn._connected then
+			cn:Disconnect()
+		end
+		fn(...)
+	end)
+	return cn
+end
+
+return Signal