state
type: Specification
name: LV2 State
shortdesc: An interface for LV2 plugins to save and restore state.
license: isc-license
release:
  revision: 0.2
  created: 2011-11-14
developer: Leonard Ritter
maintainer: David Robillard
documentation: This extension provides a mechanism for plugins to save and restore state across instances, allowing hosts to save, restore, clone, or take a snapshot of a plugin instance's state at any point in time. The intention is for a plugin instance's state to be completely described by port values (as with all LV2 plugins) and a simple dictionary. The state described by this extension is conceptually a single key/value dictionary, where keys are URIDs and values are type-tagged blobs of any type. The plugin provides an LV2_State_Interface for working with this state. To save or restore, the host calls LV2_State_Interface::save() or LV2_State_Interface::restore(), passing a callback to be used for handling a single key/value pair. The host is free to implement saving and restoring in any way; the actual mechanism is completely abstract from the plugin's perspective. Because state is a simple dictionary, hosts and plugins can work with it easily from many languages and protocols. Keys are URIDs for performance reasons as well as RDF compatibility, which makes it simple to serialise state in many formats (e.g. any RDF syntax, JSON, XML, key/value databases such as BDB, etc.). In particular, state can be elegantly described in a plugin's Turtle description, which is useful for e.g. presets or default state. Specific keys may be described in Turtle on the fly or in extensions, allowing plugins to use common well-defined keys. This extension defines a conceptual model of state and a mechanism for saving and restoring it, but no interface for manipulating it dynamically. While no such mechanism is defined here, dynamic control of plugins SHOULD be achieved by generic manipulations of the same conceptual state dictionary used by this extension (e.g. plugin->set(key, value)). Accordingly, plugins SHOULD use meaningful and well-defined keys wherever possible. In pseudo code, a typical use case in a plugin is: #define NS_EG "http://example.org/" #define NS_ATOM "http://lv2plug.in/ns/e…

Interface
type: Class
subclass of: ExtensionData
documentation: A structure (LV2_State_Interface) which contains functions to be called by the host to save and restore state. In order to support this extension, the plugin must return a valid LV2_State_Interface from LV2_Descriptor::extension_data() when it is called with LV2_STATE_INTERFACE_URI. The plugin data file should describe this like so: @prefix state: <http://lv2plug.in/ns/ext/state#> . <plugin> a lv2:Plugin ; lv2:extensionData state:Interface .

Plugin Instance State
type: Class
comment: This class is used to express a plugin instance's state in RDF. The key/value properties of the instance form the predicate/object (respectively) of triples with a state:InstanceState as the subject (see state:instanceState for an example). This may be used wherever it is useful to express a plugin instance's state in RDF (e.g. for serialisation, storing in a model, or transmitting over a network). Note that this class is provided because it may be useful for hosts, plugins, or extensions that work with instance state, but its use is not required to support the LV2 State extension.

instanceState
type: Property
range: Plugin Instance State
documentation: Predicate to relate a plugin instance to an InstanceState. This may be used wherever the state of a particular plugin instance needs to be represented. Note that the domain of this property is unspecified, since LV2 does not define any RDF class for plugin instance. This predicate may be used wherever it makes sense to do so, e.g.: @prefix eg: <http://example.org/> . <plugininstance> state:instanceState [ eg:somekey "some value" ; eg:someotherkey "some other value" ; eg:favourite-number 2 ] . Note that this property is provided because it may be useful for hosts, plugins, or extensions that work with instance state, but its use is not required to support the LV2 State extension.