GObject Integration

The Lua engine that powers WirePlumber’s scripts provides direct integration with GObject. Most of the objects that you will deal with in the lua scripts are wrapping GObjects. In order to work with the scripts, you will first need to have a basic understanding of GObject’s basic concepts, such as signals and properties.

Properties

All GObjects have the ability to have properties. In C we normally use g_object_get to retrieve them and g_object_set to set them.

In WirePlumber’s lua engine, these properties are exposed as object members of the Lua object.

For example:

-- read the "bound-id" GObject property from the proxy
local proxy = function_that_returns_a_wp_proxy()
local proxy_id = proxy["bound-id"]
print("Bound ID: " .. proxy_id)

Writable properties can also be set in a similar fashion:

-- set the "scale" GObject property to the enum value "cubic"
local mixer = ...
mixer["scale"] = "cubic"

Signals

GObjects also have a generic mechanism to deliver events to external callbacks. These events are called signals. To connect to a signal and handle it, you may use the connect method:

GObject.connect(self, detailed_signal, callback)

Connects the signal to a callback. When the signal is emitted by the underlying object, the callback will be executed.

The signature of the callback is expected to match the signature of the signal, with the first parameter being the object itself.

Example:

-- connects the "bound" signal from WpProxy to a callback
local proxy = function_that_returns_a_wp_proxy()
proxy:connect("bound", function(p, id)
  print("Proxy " .. tostring(p) .. " bound to " .. tostring(id))
end)

In this example, the p variable in the callback is the proxy object, while id is the first parameter of the “bound” signal, as documented in WpProxy

Parameters:

• detailed_signal – the signal name to listen to (of the form “signal-name::detail”)

• callback – a lua function that will be called when the signal is emitted

Signals may also be used as a way to have dynamic methods on objects. These signals are meant to be called by external code and not handled. These signals are called action signals. You may call an action signal using the call method:

GObject.call(self, action_signal, ...)

Calls an action signal on this object.

Example:

Core.require_api("default-nodes", "mixer", function(...)
  local default_nodes, mixer = ...

  -- "get-default-node" and "get-volume" are action signals of the
  -- "default-nodes-api" and "mixer-api" plugins respectively
  local id = default_nodes:call("get-default-node", "Audio/Sink")
  local volume = mixer:call("get-volume", id)

  -- the return value of "get-volume" is a GVariant(a{sv}),
  -- which gets translated to a Lua table
  Debug.dump_table(volume)
end)

Parameters:

• action_signal – the signal name to call

• ... – a list of arguments that will be passed to the signal

Returns:

the return value of the action signal, if any

Type conversions

When working with GObject properties and signals, variables need to be converted from C types to Lua types and vice versa. The following tables list the type conversions that happen automatically:

C to Lua

Conversion from C to lua is based on the C type.

C	Lua
gchar, guchar, gint, guint	integer
glong, gulong, gint64, guint64	integer
gfloat, gdouble	number
gboolean	boolean
gchar *	string
gpointer	lightuserdata
WpProperties *	table (keys: string, values: string)
enum	string containing the nickname (short name) of the enum, or integer if the enum is not registered with GType
flags	integer (as in C)
GVariant *	a native type, see below
other GObject, GInterface	userdata holding reference to the object
other GBoxed	userdata holding reference to the object

Lua to C

Conversion from Lua to C is based on the expected type in C.

Expecting	Lua
gchar, guchar, gint, guint,	convertible to integer
glong, gulong, gint64, guint64	convertible to integer
gfloat, gdouble	convertible to number
gboolean	convertible to boolean
gchar *	convertible to string
gpointer	must be lightuserdata
WpProperties *	must be table (keys: string, values: convertible to string)
enum	must be string holding the nickname of the enum, or convertible to integer
flags	convertible to integer
GVariant *	see below
other GObject, GInterface	must be userdata holding a compatible GObject type
other GBoxed	must be userdata holding the same GBoxed type

GVariant to Lua

GVariant	Lua
NULL or G_VARIANT_TYPE_UNIT	nil
G_VARIANT_TYPE_INT16	integer
G_VARIANT_TYPE_INT32	integer
G_VARIANT_TYPE_INT64	integer
G_VARIANT_TYPE_UINT16	integer
G_VARIANT_TYPE_UINT32	integer
G_VARIANT_TYPE_UINT64	integer
G_VARIANT_TYPE_DOUBLE	number
G_VARIANT_TYPE_BOOLEAN	boolean
G_VARIANT_TYPE_STRING	string
G_VARIANT_TYPE_VARIANT	converted recursively
G_VARIANT_TYPE_DICTIONARY	table (keys & values converted recursively)
G_VARIANT_TYPE_ARRAY	table (children converted recursively)

Lua to GVariant

Conversion from Lua to GVariant is based on the lua type and is quite limited.

There is no way to recover an array, for instance, because there is no way in Lua to tell if a table contains an array or a dictionary. All Lua tables are converted to dictionaries and integer keys are converted to strings.

Lua	GVariant
nil	G_VARIANT_TYPE_UNIT
boolean	G_VARIANT_TYPE_BOOLEAN
integer	G_VARIANT_TYPE_INT64
number	G_VARIANT_TYPE_DOUBLE
string	G_VARIANT_TYPE_STRING
table	G_VARIANT_TYPE_VARDICT (a{sv})

Closures

When a C function is expecting a GClosure, in Lua it is possible to pass a Lua function directly. The function is then wrapped into a custom GClosure.

When this GClosure is invalidated, the reference to the Lua function is dropped. Similarly, when the lua engine is stopped, all the GClosures that were created by this engine are invalidated.

Reference counting

GObject references in Lua always hold a reference to the underlying GObject. When moving this reference around to other variables in Lua, the underlying GObject reference is shared, but Lua reference counts the wrapper “userdata” object.

-- creating a new FooObject instance; obj holds the GObject reference
local obj = FooObject()

-- GObject reference is dropped and FooObject is finalized
obj = nil

-- creating a new FooObject instance; obj holds the GObject reference
local obj = FooObject()

function store_global(o)
  -- o is now stored in the global 'obj_global' variable
  -- the GObject ref count is still 1
  obj_global = o
end

-- obj userdata reference is passed to o, the GObject ref count is still 1
store_global(obj)

-- userdata reference dropped from obj, the GObject is still alive
obj = nil

-- userdata reference dropped from obj_global,
-- the GObject ref is dropped and FooObject is finalized
obj_global = nil

Note

When assigning a variable to nil, Lua may not immediately drop the reference of the underlying object. This is because Lua uses a garbage collector and goes through all the unreferenced objects to cleanup when the garbage collector runs.

When a GObject that is already referenced in Lua re-appears somewhere else through calling some API or because of a callback from C, a new reference is added on the GObject.

-- ObjectManager is created in Lua, om holds 1 ref
local om = ObjectManager(...)
om:connect("objects-changed", function (om)
  -- om in this scope is a local function argument that was created
  -- by the signal's closure marshaller and holds a second reference
  -- to the ObjectManager

  do_some_stuff()

  -- this second reference is dropped when the function goes out of scope
end)

Danger

Because Lua variables hold strong references to GObjects, it is dangerous to create closures that reference such variables, because these closures may create reference loops and leak objects

local om = ObjectManager(...)

om:connect("objects-changed", function (obj_mgr)
  -- using 'om' here instead of the local 'obj_mgr'
  -- creates a dangerous reference from the closure to 'om'
  for obj in om:iterate() do
     do_stuff(obj)
  end
end)

-- local userdata reference dropped, but the GClosure that was generated
-- from the above function is still holding a reference and keeps
-- the ObjectManager alive; the GClosure is referenced by the ObjectManager
-- because of the signal connection, so the ObjectManager is leaked
om = nil