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 simple key:value dictionary, where keys are URIDs (URIs mapped to integers) and values are type-tagged blobs of any type. A single key:value pair is called a property. 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 property. The host is free to implement property storage and retrieval in any way. This state model is simple yet has many benefits: URID keys provide both fast performance and RDF compatibility. Fully extensible, no limitations on keys or value types. Easy to serialise in many formats (e.g. any RDF syntax, plain text, JSON, XML, key:value databases, SQL, s-expressions, etc.). Elegantly described in Turtle, which is useful for describing presets or default state in LV2 data files (the predicate state:instanceState is provided for this purpose). Does not impose any file formats, data structures, or file system requirements. Suitable for portable persistent state as well as fast in-memory snapshots. Easily stored in a typical map or dictionary data structure. Keys may be defined by extensions, making state meaningful between implementations and enabling dynamic state control. This extension defines a conceptual state model and a mechanism for saving and restoring it, but no interface for manipulating it dynamically. However, any such mechanism SHOULD work with the same properties used in this extension to avoid complicating the concept of plugin state. For example, an extension to the example plugin below could be to support a message like set(eg:greeting, "Bonjour"), which could be sent by the host, UIs, or other plugins (via the host) to dynamically control the plugin's state. Accordingly, plugins SHOULD use meaningful and well-defined keys wherever possible. Plugin Code Example #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, LV2_STATE_INTERFACE_URI)) { return &state_iface; } } Host Code Example 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; } Referring to Existing Files This extension deliberately avoids imposing any file formats or file system dependencies on implementations. However, some plugins need to refer to files in their state. This is done by storing the file's path as a property just like any other value. Plugins MUST use the type state:Path for all paths in their state. This allows hosts to know about all such files, which is necessary for making session export and archival possible (among other things). Hosts are free to map paths in any way, and plugins MUST NOT assume the restored path will be identical to the saved path. Creating New Files or Directories New implementations and basic plugins are strongly encouraged to simply store all state as properties using this API. However, some plugins have an existing file or directory formats for state. Plugins MAY create new files or directories by using the state:newPath feature, if it is provided by the host.
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 .
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 this extension.
This feature allows plugins to store paths inside files stored in its state (if, for example, a plugin saves to an existing file format which contains paths). To support this feature a host must pass an LV2_Feature with URI LV2_STATE_MAP_PATH_URI and data pointed to an LV2_State_Map_Path to the plugin's LV2_State_Interface methods. The plugin MUST use the provided functions to map all paths stored in any files it creates (using state:makePath) and stores in the state. This is necessary to enable host to handle file system references correctly, e.g. for session export or archival. For example, a plugin may write a path to a state file like so: void write_path(LV2_State_Map_Path* map_path, FILE* myfile, const char* path) { char* abstract_path = map_path->abstract_path(map_path->handle, path); fprintf(myfile, "%s", abstract_path); free(abstract_path); } Then, later reload the path like so: char* read_path(LV2_State_Map_Path* map_path, FILE* myfile) { /* Obviously this is not production quality code! */ char abstract_path[1024]; fscanf(myfile, "%s", abstract_path); return map_path->absolute_path(map_path->handle, abstract_path); }
Support for creating new files and directories
This feature allows plugins to create new files or directories. To support this feature the host passes an LV2_Feature with URI LV2_STATE_MAKE_PATH_URI and data pointed to an LV2_State_Make_Path to the plugin. The host may provide this feature during state saving only or at any time, by passing the feature to LV2_State_Interface::save() or LV2_Descriptor::instantiate(), respectively. For example, a plugin may create a file in a subdirectory like so: void save_myfile(LV2_State_Make_Path* make_path) { char* path = make_path->path(make_path->handle, "foo/bar/myfile.txt"); FILE* myfile = fopen(path, 'w'); fprintf(myfile, "Hello"); fclose(myfile); }
A path to a file or directory. The format of a state:Path is a C string escaped or otherwise restricted in a system-specific manner. This URI (LV2_STATE_PATH_URI), mapped to an integer, MUST be used as the type parameter for any files passed to the LV2_State_Store_Function; and will likewise be returned by the corresponding call to the LV2_State_Retrieve_Function. When storing and retrieving a path, the plugin MUST NOT assume the same path will be restored. However, the restored path will refer to a file with equivalent contents to the original.