An interface for LV2 plugins to save and restore state.
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/ext/atom#" LV2_Handle my_instantiate(...) { MyPlugin* plugin = ...; plugin->uris.atom_String = map_uri(NS_ATOM "String"); plugin->uris.eg_greeting = map_uri(NS_EG "greeting"); plugin->state.greeting = strdup("Hello"); return plugin; } void my_save(LV2_Handle instance, LV2_State_Store_Function store, void* handle, uint32_t flags, const LV2_Feature *const * features) { MyPlugin* plugin = (MyPlugin*)instance; const char* greeting = plugin->state.greeting; store(handle, plugin->uris.eg_greeting, greeting, strlen(greeting) + 1, plugin->uris.atom_String, LV2_STATE_IS_POD | LV2_STATE_IS_PORTABLE); } void my_restore(LV2_Handle instance, LV2_State_Retrieve_Function retrieve, void* handle, uint32_t flags, const LV2_Feature *const * features) { MyPlugin* plugin = (MyPlugin*)instance; size_t size; uint32_t type; uint32_t flags; const char* greeting = retrieve(handle, plugin->uris.eg_greeting, &size, &type, &flags); if (greeting) { free(plugin->state->greeting); plugin->state->greeting = strdup(greeting); } else { plugin->state->greeting = strdup("Hello"); } } const void* my_extension_data(const char* uri) { static const LV2_State_Interface state_iface = { my_save, my_restore }; if (!strcmp(uri, "http://lv2plug.in/ns/ext/state#Interface")) { return &state_iface; } } Similarly, a typical use case in a host is: int store_callback(void* handle, uint32_t key, const void* value, size_t size, uint32_t type, uint32_t flags) { if ((flags & LV2_STATE_IS_POD)) { /* We only care about POD since we're keeping state in memory only. If this was for disk or network storage/transmission, LV2_STATE_IS_PORTABLE would have to be checked as well. */ Map* state_map = (Map*)handle; state_map->insert(key, Value(copy(value), size, type, pod)); return 0; } else { return 1; /* Non-POD events are unsupported. */ } } Map get_plugin_state(LV2_Handle instance) { LV2_State* state = instance.extension_data("http://lv2plug.in/ns/ext/state"); Map state_map; /** Request a fast/native/POD save, since we're just copying in memory */ state.save(instance, store_callback, &state_map, LV2_STATE_IS_POD|LV2_STATE_IS_NATIVE); return state_map; }
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 URI "http://lv2plug.in/ns/ext/state#StateInterface". 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 .
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.
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.