Description

This extension defines a simple mechanism which 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 (as with all LV2 plugins) 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:

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.

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 (e.g. 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 state: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 state: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.
           For disk or network use, LV2_STATE_IS_PORTABLE must also be checked.
        */
        Map* state_map = (Map*)handle;
        state_map->insert(key, Value(copy(value), size, type));
        return 0;
    } else {
        return 1; /* Non-POD events are unsupported. */
    }
}

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

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 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
state
Instances
interface, loadDefaultState, makePath, mapPath

Reference

Class state:State

State

A state dictionary. 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).

In range ofstate:state

Property state:state

state

The state of this 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
    ] .
Rangestate:State

Instance state:interface

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 .
Typelv2:ExtensionData

Instance state:loadDefaultState

This feature indicates that the plugin has default state listed with the state: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().

Typelv2:Feature

Instance state:makePath

create new file paths

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 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, "I am some data");
    fclose(myfile);
    return path;
}
Typelv2:Feature

Instance state:mapPath

map 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 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);
}
Typelv2:Feature

History

Version 2.2 (2014-08-08)
  • Add patch:sequenceNumber for associating replies with requests.
Version 2.2 (2014-01-04)
  • Add missing include string.h to logger.h for memset.
Version 1.6 (2013-05-26)
  • Fix crash in forge.h when pushing atoms to a full buffer.
Version 1.4 (2013-01-27)
  • Fix lv2_atom_sequence_end().
  • Remove atom:stringType in favour of owl:onDatatype so generic tools can understand and validate atom literals.
  • Improve atom documentation.
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 1.10 (2013-01-13)
  • Fix incorrect return type in lv2_event_get().
Version 2.0 (2013-01-10)
  • Make patch:Set a compact message for setting one property.
  • Add patch:readable and patch:writable for describing available properties.
Version 2.0 (2013-01-08)
  • Add logger convenience API.
Version 1.2 (2012-12-21)
  • Fix typo in bufsz:sequenceSize label.
Version 1.2 (2012-10-14)
  • Use consistent label style.
Version 1.0 (2012-04-17)
  • Initial release.
Version 2.2 (2011-11-21)
  • Update packaging.
  • Improve documentation.
Version 1.4 (2011-11-21)
  • Improve documentation.
  • Update packaging.
Version 1.4 (2011-11-21)
  • Improve documentation.
  • Update packaging.
Version 1.4 (2011-11-21)
  • Update packaging.
Version 1.4 (2011-11-21)
  • Improve documentation.
  • Update packaging.
Version 1.2 (2011-05-26)
  • Switch to ISC license.
  • Add build system for installation.
Version 1.2 (2011-05-26)
  • Switch to ISC license.
  • Add build system for installation.
Version 1.2 (2011-05-26)
  • Use lv2:Specification to be discovered as an extension.
  • Add build system (for installation).
  • Convert documentation to HTML and use lv2:documentation.
Version 1.2 (2011-05-26)
  • Switch to ISC license.
  • Add build system for installation.
Version 1.0 (2010-11-24)
  • Initial release.
Version 2.0 (2010-10-04)
  • Initial release.
Version 1.0 (2010-10-04)
  • Initial release.
Version 1.0 (2010-10-04)
  • Initial release.
Version 1.0 (2010-10-04)
  • Initial release.