Object Interest

The Interest object allows you to declare interest in a specific object, or a set of objects, and filter them. This is used in the ObjectManager but also in other places where methods allow you to iterate over a set of objects or lookup a specific object.

Interest is a binding of WpObjectInterest, but it uses a Lua-idiomatic way of construction instead of mapping directly the C functions, for convenience.

Construction

The most common use for Interest is in the ObjectManager, where you have to use the following construction method to create it.

Interest(decl)

Parameters:

decl (table) – an interest declaration

Returns:

the interest

Return type:

Interest (WpObjectInterest)

The interest consists of a GType and an array of constraints. It is constructed with a table that contains all the constraints, plus the GType in the type field.

local om = ObjectManager {
  Interest {
    type = "node",
    Constraint { "node.name", "matches", "alsa*", type = "pw-global" },
    Constraint { "media.class", "equals", "Audio/Sink", type = "pw-global" },
  }
}

In the above example, the interest will match all objects of type WpNode that contain the following 2 global properties:

• “node.name”, with a value that begins with the string “alsa”

• “media.class”, with a value that equals exactly the string “Audio/Sink”

When an object method requires an Interest as an argument, you may as well directly pass the declaration table as argument instead of using the Interest() constructor function. For instance,

local fl_port = node:lookup_port {
  Constraint { "port.name", "equals", "playback_FL" }
}

In the above example, we lookup a port in a node. The Node.lookup_port() method takes an Interest as an argument, but we can pass the interest declaration table directly. Note also that such a method does not require declaring the type of the interest, as it has WpPort as a hardcoded default.

The type

The type of an interest must be a valid GType, written as a string without the “Wp” prefix and with the first letter optionally being lowercase. The type may match any wireplumber object or interface.

Examples:

type string	parsed as
node	WpNode
Node	WpNode
device	WpDevice
plugin	WpPlugin
siLink	WpSiLink
SiLink	WpSiLink
properties	WpProperties

The Constraint

The Constraint is constructed also with a table, like Interest itself. This table must have the following items in this very strict order:

• a subject string: a string that specifies the name of a property to match

• a verb string: a string that specifies the match operation

• an object, if the verb requires it

The verb may be any verb listed in WpConstraintVerb, using either the nickname of the enum or the character value of it.

Allowed verbs:

nickname	character	value
“equals”	“=”	WP_CONSTRAINT_VERB_EQUALS
“not-equals”	“!”	WP_CONSTRAINT_VERB_NOT_EQUALS
“in-list”	“c”	WP_CONSTRAINT_VERB_IN_LIST
“in-range”	“~”	WP_CONSTRAINT_VERB_IN_RANGE
“matches”	“#”	WP_CONSTRAINT_VERB_MATCHES
“is-present”	“+”	WP_CONSTRAINT_VERB_IS_PRESENT
“is-absent”	“-”	WP_CONSTRAINT_VERB_IS_ABSENT

The values that are expected for each verb are primarily documented in wp_object_interest_add_constraint(). In Lua, though, native types are expected instead of GVariant and then they are converted according to the rules documented in the GObject Integration page.

Examples of constraints:

Constraint { "node.id", "equals", "42" }
Constraint { "node.id", "equals", 42 }
Constraint { "port.physical", "=", true }

Constraint { "audio.channel", "not-equals", "FL" }

Constraint { "node.name", "matches", "v4l2_input*" }
Constraint { "format.dsp", "#", "*mono audio*" }

-- matches either "default" or "settings" as a possible value for "metadata.name"
Constraint { "metadata.name", "in-list", "default", "settings" }

-- matches any priority.session between 0 and 1000, inclusive
Constraint { "priority.session", "in-range", 0, 1000 }

-- matches when the object has a "media.role" property
Constraint { "media.role", "is-present" }

-- matches when "media.role" is not present in the properties list
Constraint { "media.role", "is-absent" }

Constraint types

PipeWire objects have multiple properties lists, therefore constraints also need to have a way to specify on which property list they apply. The constraint type may be any of the types listed in WpConstraintType and can be specified with an additional field in the Constraint construction table, called type.

For instance:

Constraint { "node.id", "equals", "42", type = "pw-global" }
Constraint { "api.alsa.card.name", "matches", "*Intel*", type = "pw" }
Constraint { "bound-id", "=", 42, type = "gobject" }

Valid types are:

type	value
pw-global	WP_CONSTRAINT_TYPE_PW_GLOBAL_PROPERTY
pw	WP_CONSTRAINT_TYPE_PW_PROPERTY
gobject	WP_CONSTRAINT_TYPE_G_PROPERTY

Methods

Interest.matches(self, obj)

Binds wp_object_interest_matches()

Checks if a specific object matches the interest. The object may be a GObject or a table convertible to WpProperties, if the interest is for properties.

This is rarely useful to use directly on objects, but it may be useful to check if a specific table contains key-value pairs that match specific constraints, using “properties” as the interest type and passing a table as the object.

Parameters:

• self – the interest

• obj (GObject or table) – an object to check for a match

Returns:

whether the object matches the interest

Return type:

boolean