add signal module

data/scripts/types/signal.lua

@@ -0,0 +1,30 @@
+local util = require "util"
+
+local Signal = {}
+
+Signal.__index = Signal
+
+function Signal:connect(f)
+  util.list_insert_once(self._connections, f)
+end
+
+function Signal:disconnect(f)
+  util.list_remove_value(self._connections, f)
+end
+
+function Signal:emit(...)
+  -- don't care about order
+  for _, v in pairs(self._connections) do
+    v(...)
+  end
+end
+
+function Signal.new()
+  local s = {
+    _connections = {},
+  }
+  
+  return setmetatable(s, Signal)
+end
+
+return Signal