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Diffstat (limited to 'ext/persist.lv2/persist.ttl')
| -rw-r--r-- | ext/persist.lv2/persist.ttl | 123 |
1 files changed, 91 insertions, 32 deletions
diff --git a/ext/persist.lv2/persist.ttl b/ext/persist.lv2/persist.ttl index 1cbbdb7..490e2c1 100644 --- a/ext/persist.lv2/persist.ttl +++ b/ext/persist.lv2/persist.ttl @@ -47,38 +47,97 @@ This extension provides a mechanism for plugins to save and restore state across instances, allowing hosts to save configuration/state/data with a project or fully clone a plugin instance (including internal state). -The motivating idea behind this extension is that a plugin instance's state -is entirely represented by port values and a single key/value dictionary. -This makes state well-defined and easily manageable by hosts. Keys are URIs, -avoiding conflicts and allowing the same dictionary to be used to store plugin -state in any context. Values are typed tagged (by URI mapped integers), -but otherwise are simple binary blobs. - -This extension defines plugin instance state and provides a mechanism -for saving/restoring it, but otherwise makes no restrictions on how a -plugin works with state. For example, other extensions may define dynamic -ways to control plugin state at runtime. The idea is that <em>all</em> -plugin state can be represented with a single (conceptual) dictionary. -This state representation is tried-and-true, universal, and works well with -many existing technologies. Accordingly, plugins/extensions that deal with -instance state in any way SHOULD represent it in a way compatible with this -extension, i.e. URI keys with URI-typed values. Similarly, plugins SHOULD NOT -use any other mechanism to store/restore state; this <strong>will</strong> -cause serious problems, don't do it! Note that you can store values of any -format whatsoever, so if you have an existing state representation to use -(e.g. XML), simply store it as a single value under some key. - -Files may be persisted using this extension in conjunction with the -<a href="http://lv2plug.in/ns/ext/files">LV2 Files</a> extension. - -Instance state as defined by this extension is RDF compatible, allowing for -simple and seamless integration with existing technology (LV2 or otherwise). -An obvious benefit of this is that plugin state can be elegantly described -in Turtle files; the persist:instanceState predicate is provided for this -purpose. RDF compatibility is also convenient since LV2 hosts are likely -to already have mechanisms for working with RDF-style data. Note, however, -that hosts may store state in any way, and are not required to use any -specific technology or file format to support this extension. +Unlike ports, this extension allows plugins to save private state data. +The motivating ideal behind this extension is for the state of a plugin +instance to be entirely described by port values (as with all LV2 plugins) and +a key/value dictionary as defined by this extension. This mechanism is simple, +yet sufficiently powerful to describe the state of very advanced plugins. + +The "state" described by this extension is conceptually a single key/value +dictionary. Keys are URIs, and values are typed-tagged blobs of any type. +The plugin provides a save and restore method for saving and restoring state. +To initiate a save or restore, the host calls these methods, passing a callback +to be used for saving or restoring a single key/value pair. In this way, the +actual mechanism of saving and restoring state is completely abstract from the +plugin's perspective. + +Because the state is a simple dictionary, hosts and plugins can work with state +easily (virtually all programming languages have an appropriate dictionary +type available). Additionally, this format is simple and terse to serialise +in many formats (e.g. any RDF syntax, JSON, XML, key/value databases such as +BDB, etc.). In particular, state can be elegantly described in a plugin's +Turtle description, which is useful for presets (among other things). +Note that these are all simply possibilities enabled by this simple data +model. This extension defines only a few function prototypes and does not +impose any requirement to use a particular syntax, data structure, library, +or other implementation detail. Hosts are free to work with plugin state +in whatever way is most appropriate for that host. + +This extension makes it possible for plugins to save private data, but state is +not necessarily private, e.g. a plugin could have a public interface via ports +for manipulating internal state, which would be saved using this extension. +Plugins are strongly encouraged to represent all state change as modifications +of such key/value variables, to minimize implementation burden and enable +the many benefits of having a universal model for describing plugin state. +The use of URI keys prevents conflict and allows unrelated plugins to +meaningfully describe state changes. Future extensions will describe a +dynamic mechanism for manipulating plugin state, as well as define various +keys likely to be useful to a wide range of plugins. + +In pseudo code, a typical use case in a plugin is: +<pre> +static const char* const KEY_GREETING = "http://example.org/greeting"; + +void my_save(LV2_Handle instance, + LV2_Persist_Store_Function store, + void* callback_data) +{ + MyPlugin* plugin = (MyPlugin*)instance; + const char* greeting = plugin->state->greeting; + + store(callback_data, KEY_GREETING, + greeting, strlen(greeting) + 1, + lv2_uri_map("http://lv2plug.in/ns/ext/atom#String")); +} + +void my_restore(LV2_Handle instance, + LV2_Persist_Retrieve_Function retrieve, + void* callback_data) +{ + MyPlugin* plugin = (MyPlugin*)instance; + + size_t size; + uint32_t type; + const char* greeting = retrieve(callback_data, KEY_GREETING, &size, &type); + + if (greeting) + plugin->state->greeting = greeting; + else + plugin->state->greeting = "Hello"; + +} +</pre> + +Similarly, a typical use case in a host is: +<pre> +void store_callback(void* callback_data, + const char* key, + const void* value, + size_t size, + uint32_t type) +{ + Map* state_map = (Map*)callback_data; + state_map->insert(key, Value(value, size, type)); +} + +Map get_plugin_state(LV2_Handle instance) +{ + LV2_Persist* persist = instance.extension_data("http://lv2plug.in/ns/ext/persist"); + Map state_map; + persist.save(instance, store_callback, &state_map); + return state_map; +} +</pre> """ . persist:InstanceState |