sfizz/plugins/lv2/lv2/state/state.ttl

@prefix lv2:   <http://lv2plug.in/ns/lv2core#> .
@prefix rdf:   <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@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 lv2:Specification ;
	rdfs:seeAlso <state.h> ,
		<lv2-state.doap.ttl> ;
	lv2:documentation """
<p>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
<em>completely</em> described by port values (as with all LV2 plugins) and a
simple dictionary.</p>

<p>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 <q>URID</q>, a URI mapped to an integer.  A single
key:value pair is called a <q>property</q>.</p>

<p>This state model is simple yet has many benefits:</p>
<ul>
  <li>Both fast and extensible thanks to URID keys.</li>
  <li>No limitations on possible value types.</li>
  <li>Easy to serialise in almost any format.</li>
  <li>Easy to store in a typical <q>map</q> or <q>dictionary</q> data
  structure.</li>
  <li>Elegantly described in Turtle, so state can be described in LV2 data
  files (including presets).</li>
  <li>Does not impose any file formats, data structures, or file system
  requirements.</li>
  <li>Suitable for portable persistent state as well as fast in-memory
  snapshots.</li>
  <li>Keys <em>may</em> be well-defined and used meaningfully across several
  implementations.</li>
  <li>State <em>may</em> be dynamic, but plugins are not required to have a
  dynamic dictionary data structure available.</li>
</ul>

<p>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.</p>

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

<h3>Referring to Files</h3>

<p>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 <a
href="atom.html#Path">atom:Path</a> for all paths in their state.</p>

<p>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.</p>

<h3>Plugin Code Example</h3>

<pre class="c-code">

/* 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, &amp;size, &amp;type, &amp;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 &amp;state_iface;
    }
}
</pre>

<h3>Host Code Example</h3>

<pre class="c-code">
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 &amp; 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, &amp;state_map,
               LV2_STATE_IS_POD|LV2_STATE_IS_NATIVE);
    return state_map;
}
</pre>

<h3>Extensions to this Specification</h3>

<p>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 <em>model</em> defined here.  That is, <strong>do not complicate the
state model</strong>.  Implementations can assume the following:</p>

<ul>
<li>The current port values and state dictionary completely describe a plugin
instance, at least well enough that saving and restoring will yield an
<q>identical</q> instance from the user's perspective.</li>
<li>Hosts are not expected to save and/or restore any other attributes of a
plugin instance.</li>
</ul>

<h3>The <q>Property Principle</q></h3>

<p>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 <q>for free</q>, since there is no need to define
commands <em>and</em> a set of properties for storing their effects.  In
particular, this is a good way for UIs to achieve more advanced control of
plugins.</p>

<p>This <q>property principle</q> is summed up in the phrase:
<q>Don't stop; set playing to false</q>.</p>

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

state:interface
	a lv2:ExtensionData ;
	lv2:documentation """
<p>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.</p>

<p>The plugin data file should describe this like so:</p>
<pre class="turtle-code">
@prefix state: &lt;http://lv2plug.in/ns/ext/state#&gt; .

&lt;plugin&gt;
    a lv2:Plugin ;
    lv2:extensionData state:interface .
</pre>
""" .

state:State
	a rdfs:Class ;
	rdfs:label "State" ;
	lv2:documentation """
<p>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).</p>
""" .

state:loadDefaultState
	a lv2:Feature ;
	lv2:documentation """
<p>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.</p>

<p>To support this feature, the host MUST <q>restore</q> the default state
after instantiating the plugin but before calling run().</p>
""" .

state:state
	a rdf:Property ;
	rdfs:label "state" ;
	rdfs:range state:State ;
	lv2:documentation """
<p>The state of this instance.  This property may be used anywhere a state
needs to be described, for example:</p>

<pre class="turtle-code">
@prefix eg: &lt;http://example.org/&gt; .

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

state:mapPath
	a lv2:Feature ;
	rdfs:label "map file paths" ;
	lv2:documentation """
<p>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.</p>

<p>The plugin MUST map <em>all</em> 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.</p>

<p>For example, a plugin may write a path to a state file like so:</p>

<pre class="c-code">
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);
}
</pre>

<p>Then, later reload the path like so:</p>

<pre class="c-code">
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);
}
</pre>
""" .

state:makePath
	a lv2:Feature ;
	rdfs:label "create new file paths" ;
	lv2:documentation """
<p>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.</p>

<p>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 <em>one</em> such namespace, even if the feature is passed to
both LV2_Descriptor::instantiate() <em>and</em> LV2_State_Interface::save().
Beyond this, the plugin MUST NOT make any assumptions about the returned
paths.</p>

<p>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.</p>

<p>For example, a plugin may create a file in a subdirectory like so:</p>

<pre class="c-code">
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;
}
</pre>
""" .

state:threadSafeRestore
	a lv2:Feature ;
	rdfs:label "thread-safe restore" ;
	lv2:documentation """
<p>If a plugin supports this feature, its LV2_State_Interface::restore method
is thread-safe and may be called concurrently with audio class functions.</p>

<p>To support this feature, the host MUST pass a <a
href="worker.html#schedule">work:schedule</a> 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.</p>

<p>The host is not required to block run() while restore() and work() load the
state, so this feature allows state to be restored without dropouts.</p>
""" .

state:freePath
	a lv2:Feature ;
	rdfs:label "free a file path" ;
	lv2:documentation """
<p>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).</p>
""" .

state:Changed
	a rdfs:Class ;
	rdfs:label "State changed" ;
	lv2:documentation """
<p>A notification that the internal state of the plugin has been changed in a
way that the host can not otherwise know about.</p>

<p>This is a one-way notification, intended to be used as the type of an <a
href="atom.html#Object">Object</a> sent from plugins when
necessary.</p>

<p>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.</p>
""" .