This extension provides a mechanism for plugins to save and restore state across instances, allowing hosts to save configuration/state/data with a project or fully clone a plugin instance (including internal state). Unlike ports, this extension allows plugins to save private state data. The motivating ideal behind this extension is for the state of a plugin instance to be entirely described by port values (as with all LV2 plugins) and a key/value dictionary as defined by this extension. This mechanism is simple, yet sufficiently powerful to describe the state of very advanced plugins. The "state" described by this extension is conceptually a single key/value dictionary. Keys are URIs, and values are typed-tagged blobs of any type. The plugin provides a save and restore method for saving and restoring state. To initiate a save or restore, the host calls these methods, passing a callback to be used for saving or restoring a single key/value pair. In this way, the actual mechanism of saving and restoring state is completely abstract from the plugin's perspective. Because the state is a simple dictionary, hosts and plugins can work with state easily (virtually all programming languages have an appropriate dictionary type available). Additionally, this format is simple and terse to serialise 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 presets (among other things). Note that these are all simply possibilities enabled by this simple data model. This extension defines only a few function prototypes and does not impose any requirement to use a particular syntax, data structure, library, or other implementation detail. Hosts are free to work with plugin state in whatever way is most appropriate for that host. This extension makes it possible for plugins to save private data, but state is not necessarily private, e.g. a plugin could have a public interface via ports for manipulating internal state, which would be saved using this extension. Plugins are strongly encouraged to represent all state change as modifications of such key/value variables, to minimize implementation burden and enable the many benefits of having a universal model for describing plugin state. The use of URI keys prevents conflict and allows unrelated plugins to meaningfully describe state changes. Future extensions will describe a dynamic mechanism for manipulating plugin state, as well as define various keys likely to be useful to a wide range of plugins. In pseudo code, a typical use case in a plugin is: static const char* const KEY_GREETING = "http://example.org/greeting"; void my_save(LV2_Handle instance, LV2_Persist_Store_Function store, void* callback_data) { MyPlugin* plugin = (MyPlugin*)instance; const char* greeting = plugin->state->greeting; store(callback_data, KEY_GREETING, greeting, strlen(greeting) + 1, lv2_uri_map("http://lv2plug.in/ns/ext/atom#String")); } void my_restore(LV2_Handle instance, LV2_Persist_Retrieve_Function retrieve, void* callback_data) { MyPlugin* plugin = (MyPlugin*)instance; size_t size; uint32_t type; const char* greeting = retrieve(callback_data, KEY_GREETING, &size, &type); if (greeting) plugin->state->greeting = greeting; else plugin->state->greeting = "Hello"; } Similarly, a typical use case in a host is: void store_callback(void* callback_data, const char* key, const void* value, size_t size, uint32_t type) { Map* state_map = (Map*)callback_data; state_map->insert(key, Value(value, size, type)); } Map get_plugin_state(LV2_Handle instance) { LV2_Persist* persist = instance.extension_data("http://lv2plug.in/ns/ext/persist"); Map state_map; persist.save(instance, store_callback, &state_map); return state_map; }
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 persist:InstanceState as the subject (see persist: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 Persist extension.
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> persist:instanceState [ eg:somekey "some value" ; eg:someotherkey "some other value" ; eg:favouritenumber 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 Persist extension.