.. _lua_gobject:
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:
.. code-block:: lua
-- 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:
.. code-block:: lua
-- 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:
.. function:: 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:**
.. code-block:: lua
-- 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 :c:struct:`WpProxy`
:param detailed_signal: the signal name to listen to
(of the form "signal-name::detail")
:param 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:
.. function:: GObject.call(self, action_signal, ...)
Calls an action signal on this object.
**Example:**
.. code-block:: lua
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)
:param action_signal: the signal name to call
:param ...: 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_gobject_lua_to_c:
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.
.. code-block:: lua
-- creating a new FooObject instance; obj holds the GObject reference
local obj = FooObject()
-- GObject reference is dropped and FooObject is finalized
obj = nil
.. code-block:: lua
-- 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.
.. code-block:: lua
-- 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
.. code-block:: lua
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
.. _GObject: https://developer.gnome.org/gobject/stable/
.. _properties: https://developer.gnome.org/gobject/stable/gobject-properties.html
.. _g_object_get: https://developer.gnome.org/gobject/stable/gobject-The-Base-Object-Type.html#g-object-get
.. _g_object_set: https://developer.gnome.org/gobject/stable/gobject-The-Base-Object-Type.html#g-object-set
.. _signals: https://developer.gnome.org/gobject/stable/signal.html