# LV2 State

An interface for LV2 plugins to save and restore state.

ID | http://lv2plug.in/ns/ext/state
Version | 2.8
Date | 2021-01-09
Discuss | devel@lists.lv2plug.in (subscribe)

- Index
- History
- API

This extension defines a simple mechanism that allows hosts to save and restore a plugin instance's state. The goal is for an instance's state to be completely described by port values and a simple dictionary.

The state defined here is conceptually a key:value dictionary, with URI keys and values of any type. For performance reasons the key and value type are actually a "URID", a URI mapped to an integer. A single key:value pair is called a "property".

This state model is simple yet has many benefits:

- Both fast and extensible thanks to URID keys.

- No limitations on possible value types.

- Easy to serialise in almost any format.

- Easy to store in a typical "map" or "dictionary" data structure.

- Elegantly described in Turtle, so state can be described in LV2 data files (including presets).

- Does not impose any file formats, data structures, or file system requirements.

- Suitable for portable persistent state as well as fast in-memory snapshots.

- Keys may be well-defined and used meaningfully across several implementations.

- State may be dynamic, but plugins are not required to have a dynamic dictionary data structure available.

To implement state, the plugin provides a 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.

Since value types are defined by URI, any type is possible. However, a set of standard types is defined by the LV2 Atom extension. Use of these types is recommended. Hosts MUST implement at least atom:String, which is simply a C string.

### Referring to Files

Plugins may need to refer to existing files (such as loaded samples) in their state. This is done by storing the file's path as a property just like any other value. However, there are some rules which MUST be followed when storing paths, see mapPath for details. Plugins MUST use the type atom:Path for all paths in their state.

Plugins are strongly encouraged to avoid creating files, instead storing all state as properties. However, occasionally the ability to create files is necessary. To make this possible, the host can provide the feature makePath which allocates paths for plugin-created files. Plugins MUST NOT create files in any other locations.

### Plugin Code Example

    /* Namespace for this plugin's keys.  This SHOULD be something that could be
       published as a document, even if that document does not exist right now.
    */
    #define NS_MY "http://example.org/myplugin/schema#"

    #define DEFAULT_GREETING "Hello"

    LV2_Handle
    my_instantiate(...)
    {
        MyPlugin* plugin = ...;
        plugin->uris.atom_String = map_uri(LV2_ATOM__String);
        plugin->uris.my_greeting = map_uri(NS_MY "greeting");
        plugin->state.greeting   = strdup(DEFAULT_GREETING);
        return plugin;
    }

    LV2_State_Status
    my_save(LV2_Handle                 instance,
            LV2_State_Store_Function   store,
            LV2_State_Handle           handle,
            uint32_t                   flags,
            const LV2_Feature *const * features)
    {
        MyPlugin*   plugin   = (MyPlugin*)instance;
        const char* greeting = plugin->state.greeting;

        store(handle,
              plugin->uris.my_greeting,
              greeting,
              strlen(greeting) + 1,  // Careful!  Need space for terminator
              plugin->uris.atom_String,
              LV2_STATE_IS_POD | LV2_STATE_IS_PORTABLE);

        return LV2_STATE_SUCCESS;
    }

    LV2_State_Status
    my_restore(LV2_Handle                  instance,
               LV2_State_Retrieve_Function retrieve,
               LV2_State_Handle            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.my_greeting, &size, &type, &flags);

        if (greeting) {
            free(plugin->state->greeting);
            plugin->state->greeting = strdup(greeting);
        } else {
            plugin->state->greeting = strdup(DEFAULT_GREETING);
        }

        return LV2_STATE_SUCCESS;
    }

    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)) {
            return &state_iface;
        }
    }

### Host Code Example

    LV2_State_Status
    store_callback(LV2_State_Handle 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.
            // Disk or network use would also require LV2_STATE_IS_PORTABLE.
            Map* state_map = (Map*)handle;
            state_map->insert(key, Value(copy(value), size, type));
            return LV2_STATE_SUCCESS;;
        } else {
            return LV2_STATE_ERR_BAD_FLAGS; // Non-POD events are unsupported
        }
    }

    Map
    get_plugin_state(LV2_Handle instance)
    {
        LV2_State* state = instance.extension_data(LV2_STATE__interface);

        // Request a fast/native/POD save, since we're just copying in memory
        Map state_map;
        state.save(instance, store_callback, &state_map,
                   LV2_STATE_IS_POD|LV2_STATE_IS_NATIVE);

        return state_map;
    }

### Extensions to this Specification

It is likely that other interfaces for working with plugin state will be developed as needed. This is encouraged, however everything SHOULD work within the state model defined here. That is, do not complicate the state model. Implementations can assume the following:

- The current port values and state dictionary completely describe a plugin instance, at least well enough that saving and restoring will yield an "identical" instance from the user's perspective.

- Hosts are not expected to save and/or restore any other attributes of a plugin instance.

### The "Property Principle"

The main benefit of this meaningful state model is that it can double as a plugin control/query mechanism. For plugins that require more advanced control than simple control ports, instead of defining a set of commands, define properties whose values can be set appropriately. This provides both a way to control and save that state "for free", since there is no need to define commands and a set of properties for storing their effects. In particular, this is a good way for UIs to achieve more advanced control of plugins.

This "property principle" is summed up in the phrase: "Don't stop; set playing to false".

This extension does not define a dynamic mechanism for state access and manipulation. The LV2 Patch extension defines a generic set of messages which can be used to access or manipulate properties, and the LV2 Atom extension defines a port type and data container capable of transmitting those messages.

## Index

Classes | Properties | Instances
--- | --- | ---
State StateChanged | state | freePath interface loadDefaultState makePath mapPath threadSafeRestore

## Classes

### State

Class

Label | State
In range of | state

LV2 plugin state.

This type should be used wherever instance state is described. The properties of a resource with this type correspond directly to the properties of the state dictionary (except the property that states it has this type).

### StateChanged

Class

Label | State Changed

A notification that the internal state of the plugin has changed.

A notification that the internal state of the plugin has been changed in a way that the host can not otherwise know about.

This is a one-way notification, intended to be used as the type of an Object sent from plugins when necessary.

Plugins SHOULD emit such an event whenever a change has occurred that would result in a different state being saved, but not when the host explicity makes a change which it knows is likely to have that effect, such as changing a parameter.

## Properties

### state

Property

Label | state
Range | State
Type | Object Property

The state of an LV2 plugin instance.

This property may be used anywhere a state needs to be described, for example:

    @prefix eg: <http://example.org/> .

    <plugin-instance>
        state:state [
            eg:somekey "some value" ;
            eg:someotherkey "some other value" ;
            eg:favourite-number 2
        ] .

## Instances

### freePath

Instance

Label | free path
Type | lv2:Feature

A feature for freeing paths allocated by the host.

This feature provides a function that can be used by plugins to free paths that were allocated by the host via other state features (mapPath and makePath).

### interface

Instance

Label | interface
Type | lv2:ExtensionData

A plugin interface for saving and restoring state.

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 LV2_STATE__interface.

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 .

### loadDefaultState

Instance

Label | load default state
Type | lv2:Feature

A feature indicating that the plugin has default state.

This feature indicates that the plugin has default state listed with the state property which should be loaded by the host before running the plugin. Requiring this feature allows plugins to implement a single state loading mechanism which works for initialisation as well as restoration, without having to hard-code default state.

To support this feature, the host MUST restore the default state after instantiating the plugin but before calling run().

### makePath

Instance

Label | make path
Type | lv2:Feature

A feature for creating new files and directories.

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__makePath 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 hierarchical 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 mapPath before storing them in state. The plugin MUST NOT assume these paths will be available across a save/restore otherwise, that is, 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, "I am some data");
        fclose(myfile);
        return path;
    }

### mapPath

Instance

Label | map path
Type | lv2:Feature

A feature for mapping between absolute and abstract file paths.

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__mapPath 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 those inside any files). This is necessary so that hosts can handle file system references correctly, for example to share common files, or bundle state 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);
    }

### threadSafeRestore

Instance

Label | thread-safe restore
Type | lv2:Feature

A feature indicating support for thread-safe state restoration.

If a plugin supports this feature, its LV2_State_Interface::restore method is thread-safe and may be called concurrently with audio class functions.

To support this feature, the host MUST pass a work:schedule feature to the restore method, which will be used to complete the state restoration. The usual mechanics of the worker apply: the host will call the plugin's work method, which emits a response which is later applied in the audio thread.

The host is not required to block audio processing while restore() and work() load the state, so this feature allows state to be restored without dropouts.

## History

Version 2.8 (2021-01-07)
  - Fix state:StateChanged URI in metadata and documentation.
Version 2.6 (2020-04-26)
  - Add state:freePath feature.
Version 2.4 (2019-02-03)
  - Add state:StateChanged for notification events.
Version 2.2 (2016-07-31)
  - Add LV2_STATE_ERR_NO_SPACE status flag.
  - Add state:threadSafeRestore feature for dropout-free state restoration.
Version 2.0 (2013-01-16)
  - Add state:loadDefaultState feature so plugins can have their default state loaded without hard-coding default state as a special case.
Version 1.2 (2012-10-14)
  - Use consistent label style.
Version 1.0 (2012-04-17)
  - Initial release.