scripts/lib: add a new settings manager lua library

This intends to reduce the amount of code needed to maintain
tables of settings values across different scripts

src/scripts/lib/settings-manager.lua

@@ -0,0 +1,124 @@
+-- WirePlumber
+--
+-- Copyright © 2022 Collabora Ltd.
+--
+-- SPDX-License-Identifier: MIT
+--
+-- Settings manager helper
+--
+-- This sets up a lua table that will automatically populate itself with
+-- values from the settings, falling back to the default values specified.
+-- Any changes in the settings will automatically change the values of this
+-- table.
+--
+-- Usage:
+--
+-- local settings_manager = require ("settings-manager")
+-- local defaults = {
+--   ["foo"] = true,
+--   ["foo-bar"] = 3,
+-- }
+-- local settings = settings_manager.new ("example.prefix.", defaults)
+--
+-- The above "settings" table should now look like this internally:
+-- {
+--   foo = true, -- or the value specified in the Settings
+--   foo_bar = 3, -- or the value specified in the Settings
+-- }
+--
+-- Additionally, a "subscribe" method is present in the "settings" table, which
+-- allows subscribing to changes of the values:
+--
+-- settings:subscribe ("foo-bar", function (new_value)
+--   Log.info ("foo-bar value changed to " .. new_value)
+-- end)
+--
+
+local settings_manager = {}
+local private_api = {}
+
+function private_api.subscribe (self, key, closure)
+  if not self.subscribers[key] then
+    self.subscribers[key] = {}
+  end
+  table.insert (self.subscribers, closure)
+end
+
+function private_api.call_subscribers (self, key, new_value)
+  if self.subscribers[key] then
+    for i, closure in ipairs (self.subscribers[key]) do
+      closure (new_value)
+    end
+  end
+end
+
+function private_api.update_value (self, key, json_value, default)
+  local new_value = json_value and json_value:parse () or default
+
+  -- only accept values that have the same type as the default value
+  if type (new_value) ~= type (default) then
+    new_value = default
+  end
+
+  -- store only if the new value is not equal to the default
+  self.values[key] = (new_value ~= default) and new_value or nil
+
+  return new_value
+end
+
+function settings_manager.new (_prefix, _defaults)
+  -- private storage table
+  local private = {
+    prefix = _prefix,
+    prefix_len = string.len (_prefix),
+    defaults = _defaults,
+    values = {},
+    subscribers = {},
+  }
+  setmetatable (private, { __index = private_api })
+
+  -- initialize with the values found in Settings
+  for key, default in pairs (private.defaults) do
+    local json_value = Settings.get (private.prefix .. key)
+    private:update_value (key, json_value, default)
+  end
+
+  -- subscribe for changes in Settings
+  Settings.subscribe (private.prefix .. "*", function (_, setting, json_value)
+    local key = string.sub (setting, private.prefix_len, -1)
+    local default = private.defaults[key]
+
+    -- unknown key, ignore it
+    if default == nil then
+      return
+    end
+
+    local new_value = private:update_value (key, json_value, default)
+    private:call_subscribers (key, new_value)
+  end)
+
+  -- return an empty table with a metatable that will resolve
+  -- keys to the values stored in `private` or their default values
+  return setmetatable ({}, {
+    __private = private,
+    __index = function (self, key)
+      if key == "subscribe" then
+        -- special case, "subscribe" is a method
+        return function (self, key, closure)
+          local private = getmetatable (self) ["__private"]
+          private:subscribe (key, closure)
+        end
+      else
+        local private = getmetatable (self) ["__private"]
+        key = string.gsub (key, "_", "-") -- foo_bar_baz -> foo-bar-baz
+        local value = private.values[key]
+        return (value ~= nil) and value or private.defaults[key]
+      end
+    end,
+    __newindex = function(_, key, _)
+      error ('Not allowed to modify configuration value')
+    end
+  })
+end
+
+return settings_manager