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authorDavid Robillard <d@drobilla.net>2020-03-22 16:36:44 +0100
committerDavid Robillard <d@drobilla.net>2020-04-10 19:46:04 +0200
commit430284545345539c9ffb31df889debac1d3888b5 (patch)
treedc9bb1f32f0d6fe34a7339221389048e199f14a5 /lv2/state/state.meta.ttl
parentc4514483da1ab4f49148f9c4fe4ff5b559323217 (diff)
downloadlv2-430284545345539c9ffb31df889debac1d3888b5.tar.xz
Move documentation to metadata files and convert it to Markdown
Diffstat (limited to 'lv2/state/state.meta.ttl')
-rw-r--r--lv2/state/state.meta.ttl381
1 files changed, 380 insertions, 1 deletions
diff --git a/lv2/state/state.meta.ttl b/lv2/state/state.meta.ttl
index 784946d..ac3f2d6 100644
--- a/lv2/state/state.meta.ttl
+++ b/lv2/state/state.meta.ttl
@@ -1,7 +1,9 @@
@prefix dcs: <http://ontologi.es/doap-changeset#> .
@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 ;
@@ -73,5 +75,382 @@
rdfs:label "Initial release."
]
]
- ] .
+ ] ;
+ 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 (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](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: &lt;http://lv2plug.in/ns/ext/state#&gt; .
+
+ &lt;plugin&gt;
+ 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: &lt;http://example.org/&gt; .
+
+ &lt;plugin-instance&gt;
+ 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:Changed
+ 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 explicity makes
+a change which it knows is likely to have that effect, such as changing a
+parameter.
+
+"""^^lv2:Markdown .