state
type: Specification
name: LV2 State
shortdesc: An interface for LV2 plugins to save and restore state.
license: isc
release:
  revision: 0.4
  created: 2011-11-22
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 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 pl…

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: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 this extension.

Support for storing paths in files
type: Feature
documentation: 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
type: Feature
documentation: 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); }

Path
type: Class
documentation: 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.