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diff --git a/lv2/state/state.ttl b/lv2/state/state.ttl new file mode 100644 index 0000000..c86fea7 --- /dev/null +++ b/lv2/state/state.ttl @@ -0,0 +1,382 @@ +@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, &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; + } +} +</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 & 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; +} +</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: <http://lv2plug.in/ns/ext/state#> . + +<plugin> + 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: <http://example.org/> . + +<plugin-instance> + 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> +""" . |