.. _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