From 430284545345539c9ffb31df889debac1d3888b5 Mon Sep 17 00:00:00 2001 From: David Robillard Date: Sun, 22 Mar 2020 16:36:44 +0100 Subject: Move documentation to metadata files and convert it to Markdown --- lv2/state/state.meta.ttl | 381 ++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 380 insertions(+), 1 deletion(-) (limited to 'lv2/state/state.meta.ttl') 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: . @prefix doap: . @prefix foaf: . +@prefix lv2: . @prefix rdfs: . +@prefix 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 state 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: <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 abstract paths 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 . -- cgit v1.2.1