pulseaudio/doc/messaging_api.txt

Message API reference

The message API allows any object within pulseaudio to register a message
handler. A message handler is a function that can be called by clients using
PA_COMMAND_SEND_OBJECT_MESSAGE. A message consists at least of an object path
and a message command, both specified as strings. Additional parameters can
be specified using a single string, but are not mandatory. The message handler
returns an error number as defined in def.h and also returns a string in
the "response" variable. If the string is not empty it consists of elements.
Curly braces are used to separate elements. Each element can itself contain
further elements. For example consider a message that returns multiple elements
which each contain an integer and an array of float. A response string would
look like that:
{{Integer} {{1st float} {2nd float} ...}}{...}
Any characters that are not enclosed in curly braces are ignored (all characters
between { and {, between } and } and between } and {). The same syntax is used
to specify message parameters. The reference further down lists available messages,
their parameters and return values. If a return value is enclosed in {}, this
means that multiple elements of the same type may be returned.

For string parameters that contain curly braces or backslashes, those characters
must be escaped by adding a "\" before them.

Reference:

Object path: /core
Message: list-handlers
Parameters: None
Return value: {{{Handler name} {Description}} ...}