David Robillard
type: Person

state
type: Specification
name: LV2 State
shortdesc: An interface for LV2 plugins to save and restore state.
license: isc
release:
  revision: 0.5
  created: 2012-01-29
  blame: David Robillard
developer: Leonard Ritter
maintainer: David Robillard
documentation: This extension provides a simple mechanism for plugins to save and restore state across instances, allowing hosts to save and restore a plugin instance's state at any time. The goal is for an instance's state to be completely described by port values (as with all LV2 plugins) and a 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. To implement state, the plugin provides a state:Interface to the host. 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:state 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 and used by several plugins, making state meaningful enabling dynamic state control. Implementations or further extensions which work with plugin state (including dynamic plugin control) SHOULD work entirely within this model. That is, do not complicate the state model. All information required to express an instance's state at any given time can, and should, be expressed within this model. Plugin Code Example #define NS_EG "http://example.org/" #define NS_ATOM …

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
documentation: 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:State as the subject (see state:state 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.

state
type: Property
range: Plugin Instance State
documentation: Predicate to relate a plugin instance to its State. 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:state [ 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.

Support for storing paths in files
type: Feature
documentation: This feature maps absolute paths to/from abstract paths which are stored in state. 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 map all paths stored in its state (including in any files in its state). This is necessary to enable host to handle file system references correctly, e.g. for distribution 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
type: Feature
documentation: This feature allows plugins to create new files and/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 make this feature available only during save by passing it to LV2_State_Interface::save(), or available any time by passing it to LV2_Descriptor::instantiate(). If passed to LV2_State_Interface::save(), the feature MUST NOT be used beyond the scope of that call. The plugin is guaranteed a hierarchial namespace unique to that plugin instance, and may expect the returned path to have the requested path as a suffix. There is one such namespace, even if the feature is passed to both LV2_Descriptor::instantiate() and LV2_State_Interface::save(). Beyond this, the plugin MUST NOT make any assumptions about the returned paths. Like any other paths, the plugin MUST map these paths using state:mapPath before storing them in state. The plugin MUST NOT assume these paths will be available across a save/restore otherwise, i.e. only mapped paths saved to state are persistent, any other created paths are temporary. For example, a plugin may create a file in a subdirectory like so: char* 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); return path; }

Path
type: Class
documentation: A path to a file or directory. The format of a state:Path is a C string, possibly 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.