@prefix doap: <http://usefulinc.com/ns/doap#> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix lv2: <http://lv2plug.in/ns/lv2core#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix state: <http://lv2plug.in/ns/ext/state#> .

<http://lv2plug.in/ns/ext/state>
	a doap:Project ;
	doap:created "2010-11-09" ;
	doap:name "LV2 State" ;
	doap:shortdesc "An interface for LV2 plugins to save and restore state." ;
	doap:license <http://opensource.org/licenses/isc> ;
	doap:developer <http://lv2plug.in/ns/meta#paniq> ,
		<http://drobilla.net/drobilla#me> ;
	doap:maintainer <http://drobilla.net/drobilla#me> ;
	lv2:documentation """

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 <q>state</q> 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 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](atom.html) extension. Use of these
types is recommended. Hosts MUST implement at least
[atom:String](atom.html#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 state:mapPath for details. Plugins MUST use the type
[atom:Path](atom.html#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

    :::c

    /* 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

    :::c
    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](patch.html) extension defines a generic set of
messages which can be used to access or manipulate properties, and the [LV2
Atom](atom.html) extension defines a port type and data container capable of
transmitting those messages.

"""^^lv2:Markdown .

state:interface
	lv2: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 URI LV2_STATE__interface.

The plugin data file should describe this like so:

    :::turtle
    @prefix state: <http://lv2plug.in/ns/ext/state#> .

    <plugin>
        a lv2:Plugin ;
        lv2:extensionData state:interface .

"""^^lv2:Markdown .

state:State
	lv2:documentation """

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).

"""^^lv2:Markdown .

state:loadDefaultState
	lv2:documentation """

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().

"""^^lv2:Markdown .

state:state
	lv2:documentation """

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

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

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

"""^^lv2:Markdown .

state:mapPath
	lv2:documentation """

This feature maps absolute paths to/from <q>abstract paths</q> 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:

    :::c
    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:

    :::c
    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);
    }

"""^^lv2:Markdown .

state:makePath
	lv2: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__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, 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:

    :::c
    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;
    }

"""^^lv2:Markdown .

state:threadSafeRestore
	lv2:documentation """

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](worker.html#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.

"""^^lv2:Markdown .

state:freePath
	lv2:documentation """

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

"""^^lv2:Markdown .

state:StateChanged
	lv2:documentation """

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](atom.html#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 explicitly makes
a change which it knows is likely to have that effect, such as changing a
parameter.

"""^^lv2:Markdown .

