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+@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/atom.html">LV2 Atom</a>
+extension. Use of these types is recommended. Hosts MUST implement at least
+<a href="../atom/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/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/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/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/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: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/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>
+""" .