# LV2 UI

http://lv2plug.in/ns/extensions/ui
LV2 plugin UIs of any type.

Version | 2.20
Date | 2018-03-16
Discuss | devel@lists.lv2plug.in (subscribe)
Developer | Lars Luthman
Maintainer | David Robillard

- Index
- Reference
- History
- API

This extension is used to create User Interfaces (UIs) for LV2 plugins.

UIs are implemented as an LV2UI_Descriptor loaded via lv2ui_descriptor() in a shared library, and are distributed in bundles just like plugins. See the API reference for details on the C API.

UIs are associated with plugins in data:

    @prefix ui: <http://lv2plug.in/ns/extensions/ui#> .

    <http://my.plugin>   ui:ui     <http://my.pluginui> .
    <http://my.pluginui> a         ui:GtkUI ;
                         ui:binary <myui.so> .

where <http://my.plugin> is the URI of the plugin, <http://my.pluginui> is the URI of the plugin UI and <myui.so> is the relative URI to the shared object file.

While it is possible to have the plugin UI and the plugin in the same shared object file it is probably a good idea to keep them separate so that hosts that don't want UIs don't have to load the UI code. A UI MUST specify its class in the RDF data (ui:GtkUI in the above example). The class defines what type the UI is, e.g. what graphics toolkit it uses. Any type of UI class can be defined separately from this extension.

It is possible to have multiple UIs for the same plugin, or to have the UI for a plugin in a different bundle from the actual plugin - this way people other than the plugin author can write plugin UIs independently without editing the original plugin bundle.

Note that the process that loads the shared object file containing the UI code and the process that loads the shared object file containing the actual plugin implementation are not necessarily the same process (and not even necessarily on the same machine). This means that plugin and UI code MUST NOT use singletons and global variables and expect them to refer to the same objects in the UI and the actual plugin. The function callback interface defined in this header is the only method of communication between UIs and plugin instances (extensions may define more, though this is discouraged unless absolutely necessary since the significant benefits of network transparency and serialisability are lost).

UI functionality may be extended via features, much like plugins:

    <http://my.pluginui> lv2:requiredFeature <http://my.feature> .
    <http://my.pluginui> lv2:optionalFeature <http://my.feature> .

The rules for a UI with a required or optional feature are identical to those of lv2:Plugin instances: if a UI declares a feature as required, the host is NOT allowed to load it unless it supports that feature; and if it does support a feature, it MUST pass an appropriate LV2_Feature struct to the UI's instantiate() method. This extension defines several standard features for common functionality.

UIs written to this specification do not need to be thread-safe. All functions may only be called in the UI thread. There is only one UI thread (for toolkits, the one the UI main loop runs in). There is no requirement that a UI actually be a graphical widget.

Note that UIs are completely separate from plugins. From the plugin's perspective, control from a UI is indistinguishable from any other control, as it all occcurs via ports.

## Index

Classes | Properties | Instances | Files
--- | --- | --- | ---
PortNotification PortProtocol UI CocoaUI Gtk3UI GtkUI Qt4UI Qt5UI WindowsUI X11UI | binary notifyType plugin portIndex portNotification protocol ui updateRate windowTitle | fixedSize floatProtocol idleInterface makeSONameResident noUserResize parent peakProtocol portMap portSubscribe resize showInterface touch | lv2-ui.doap.ttl ui.h ui.ttl

## Reference

### Class ui:CocoaUI

Cocoa UI

A UI where the LV2_Widget is a pointer to a NSView, the basic view type for the Cocoa API (formerly OpenStep). This is the native UI type on Mac OS X.

Sub-class of | ui:UI

### Class ui:Gtk3UI

GTK3 UI

A UI where the LV2_Widget is a pointer to a Gtk+ 3.0 compatible GtkWidget, and the host guarantees that the Gtk+ library has been initialised and the Glib main loop is running before a UI of this type is instantiated.

Sub-class of | ui:UI

### Class ui:GtkUI

GTK2 UI

A UI where the LV2_Widget is a pointer to a Gtk+ 2.0 compatible GtkWidget, and the host guarantees that the Gtk+ library has been initialised and the Glib main loop is running before a UI of this type is instantiated.

Sub-class of | ui:UI

### Class ui:PortNotification

A port notification. This describes which ports the host must send notifications to the UI about. The port can be specific by index, using the ui:portIndex property, or symbol, using the lv2:symbol property. Since port indices are not guaranteed to be stable between different revisions (or even instantiations) of a plugin, symbol is recommended, and index may only be used by UIs shipped in the same bundle as the plugin.

A ui:PortNotification MUST have either a ui:portIndex or a lv2:symbol to indicate which port it refers to.

Restriction on ui:plugin |
owl:cardinality | 1
A PortNotification MUST have exactly one ui:plugin.
In domain of | ui:notifyType
| ui:protocol
| ui:plugin
| ui:portIndex
In range of | ui:portNotification

### Class ui:PortProtocol

Port Protocol

A PortProtocol defines a method to communicate port data between a UI and plugin.

Any PortProtocol MUST define:

Port Type | Which plugin port types the buffer type is valid for.
Feature Data | What data (if any) should be passed in the LV2_Feature.

Any PortProtocol for an output port MUST define:

Update Frequency | When the host should call port_event().
Update Format | The format of the data in the buffer passed to port_event().
Options | The format of the options passed to subscribe() and unsubscribe().

Any PortProtocol for an input port MUST define:

Write Format | The format of the data in the buffer passed to write_port().
Write Effect | What happens when the UI calls write_port().

For an example, see ui:floatProtocol or ui:peakProtocol.

PortProtocol is a subclass of lv2:Feature, so UIs use lv2:optionalFeature and lv2:requiredFeature to specify which PortProtocols they want to use.

Sub-class of | lv2:Feature
In range of | ui:protocol

### Class ui:Qt4UI

Qt4 UI

A UI where the LV2_Widget is a pointer to a Qt4 compatible QWidget, and the host guarantees that the Qt4 library has been initialised and the Qt4 main loop is running before a UI of this type is instantiated.

Sub-class of | ui:UI

### Class ui:Qt5UI

Qt5 UI

A UI where the LV2_Widget is a pointer to a Qt5 compatible QWidget, and the host guarantees that the Qt5 library has been initialised and the Qt5 main loop is running before a UI of this type is instantiated.

Sub-class of | ui:UI

### Class ui:UI

User Interface

A UI for an LV2 plugin

In domain of | ui:portNotification
In range of | ui:ui

### Class ui:WindowsUI

Windows UI

A UI where the LV2_Widget is a Windows HWND window ID. Note this is actually an unsigned 32-bit integer, i.e. the LV2_Widget is not a pointer to a HWND but should be interpreted as an HWND itself. This is the native UI type on Microsoft Windows.

Sub-class of | ui:UI

### Class ui:X11UI

X11 UI

A UI where the LV2_Widget is an X11 window ID. Note this is actually an integer, i.e. the LV2_Widget is not a pointer to an X11 window ID, but should be itself taken as an integer value. This is the native UI type on most POSIX systems.

Sub-class of | ui:UI

### Property ui:binary

binary
DEPRECATED

The shared library a UI resides in. This property is redundant, new UIs SHOULD use lv2:binary, however hosts MUST still support ui:binary at this time.

owl:deprecated | true
owl:sameAs | lv2:binary

### Property ui:notifyType

notify type

Indicates a particular type that the UI should be notified of. In the case of ports where several types of data can be present (e.g. event ports), this can be used to indicate that only a particular type of data should cause notification. This is useful where port traffic is very dense, but only a certain small number of events are actually of interest to the UI.

Domain | ui:PortNotification

### Property ui:plugin

plugin

The plugin a portNotification applies to.

OWL Type | Object Property
Domain | ui:PortNotification
Range | lv2:Plugin

### Property ui:portIndex

port index

The index of the port a portNotification applies to.

OWL Type | Datatype Property
Domain | ui:PortNotification
Range | xsd:decimal

### Property ui:portNotification

port notification

Indicates that a UI should receive notification (via LV2UI_Descriptor::port_event()) when a particular port's value changes.

For example:

    eg:ui
        a ui:GtkUI ;
        ui:portNotification [
            ui:plugin eg:plugin ;
            lv2:symbol "gain" ;
            ui:protocol ui:floatProtocol
        ] .

OWL Type | Object Property
Domain | ui:UI
Range | ui:PortNotification

### Property ui:protocol

protocol

The protocol to be used for this notification.

Domain | ui:PortNotification
Range | ui:PortProtocol

### Property ui:ui

user interface

Relates a plugin to a UI that applies to it.

Domain | lv2:Plugin
Range | ui:UI

### Property ui:updateRate

update rate

The target rate, in Hz, to send updates to the UI.

Range | xsd:float

### Property ui:windowTitle

window title

The title for the window shown by LV2UI_Show_Interface.

Range | xsd:string

### Instance ui:fixedSize

If a UI requires this feature it indicates the same thing as ui:noUserResize, and additionally it means that the UI will not resize the main widget on its own - it will always remain the same size (e.g. a pixmap based GUI). This feature may not make sense for all UI types. The data pointer for the LV2_Feature for this feature should always be set to NULL.

Type | lv2:Feature

### Instance ui:floatProtocol

floating point value

A protocol for transferring single floating point values. The rules for this protocol are:

Port Type | lv2:ControlPort
Feature Data | None.
Update Frequency | The host SHOULD call port_event() as soon as possible when the port value has changed, but there are no timing guarantees.
Update Format | A single `float`.
Options | None.
Write Format | A single `float`.
Write Effect | The host SHOULD set the port to the received value as soon as possible, but there is no guarantee that run() actually sees this value.

Type | ui:PortProtocol

### Instance ui:idleInterface

A feature that provides a callback for the host to call rapidly to drive the UI. To support this feature, the UI should list it as a lv2:optionalFeature or lv2:requiredFeature in its data, and also as lv2:extensionData. When the UI's extension_data() is called with this URI (LV2_UI__idleInterface), it should return a pointer to an LV2UI_Idle_Interface.

To indicate support, the host should pass a feature to instantiate() with this URI, with NULL for data.

Type | lv2:Feature
| lv2:ExtensionData

### Instance ui:makeSONameResident

DEPRECATED

DEPRECATED

This feature was intended to support UIs that link against toolkit libraries which may not be unloaded during the lifetime of the host. This is better achieved by using the appropriate flags when linking the UI, e.g. `gcc -Wl,-z,nodelete`.

Type | lv2:Feature
owl:deprecated | true

### Instance ui:noUserResize

If a UI requires this feature it indicates that it does not make sense to let the user resize the main widget, and the host should prevent that. This feature may not make sense for all UI types. The data pointer for the LV2_Feature for this feature should always be set to NULL.

Type | lv2:Feature

### Instance ui:parent

The parent for the UI.

This feature can be used to pass a parent (e.g. a widget, container, canvas, etc.) the UI should be a child of. The format of data pointer of this feature is determined by the UI type, and is generally the same type as the LV2_Widget the UI would return (e.g. for a GtkUI the data would be a pointer to a GtkWidget which is a GtkContainer). This is particularly useful for cross-toolkit embedding, where the parent often must be known at construction time for embedding to work correctly. UIs should not require this feature unless it is absolutely necessary for them to work at all.

Type | lv2:Feature

### Instance ui:peakProtocol

peak measurement for a period of audio

This port protocol defines a way for the host to send continuous peak measurements of the audio signal at a certain port to the UI. The intended use is visualisation, e.g. an animated meter widget that shows the level of the audio input or output.

A contiguous sequence of audio samples for which a peak value has been computed is called a measurement period.

The rules for this protocol are:

Port Type | lv2:AudioPort
Feature Data | None.
Update Frequency | The host SHOULD call port_event() at regular intervals. The measurement periods used for calls to port_event() for the same port SHOULD be contiguous (i.e. the measurement period for one call should begin right after the end of the measurement period for the previous call ends) unless the UI has removed and re-added the port subscription between those calls. However, UIs MUST NOT depend on either the regularity of the calls or the contiguity of the measurement periods; hosts may change the call rate or skip calls for performance or other reasons. Measurement periods for different calls to port_event() for the same port MUST NOT overlap.
Update Format | A single LV2UI_Peak_Data object.
Options | None.
Write Format | None.
Write Effect | None.

Type | ui:PortProtocol

### Instance ui:portMap

A feature for accessing the index of a port by symbol. This makes it possible to implement and distribute UIs separately from the plugin binaries they control. This feature corresponds to the LV2UI_Port_Index struct, which should be passed with the URI LV2_UI__portIndex.

Type | lv2:Feature

### Instance ui:portSubscribe

A feature for dynamically subscribing to updates from a port. This makes it possible for a UI to explicitly request a particular style of update from a port at run-time, in a more flexible and powerful way than listing subscriptions statically allows. This feature corresponds to the LV2UI_Port_Subscribe struct, which should be passed with the URI LV2_UI__portSubscribe.

Type | lv2:Feature

### Instance ui:resize

A feature that allows the UI to notify the host about its current size, or request a size change. This feature corresponds to the LV2UI_Resize struct, which should be passed with the URI LV2_UI__resize. This struct may also be provided by the UI as extension data using the same URI, in which case it is used by the host to request that the UI change its size.

Type | lv2:Feature
| lv2:ExtensionData

### Instance ui:showInterface

An interface for showing and hiding a window for a UI. This allows UIs to gracefully degrade to separate windows when the host is unable to embed the UI widget for whatever reason. When the UI's extension_data() is called with this URI (LV2_UI__showInterface), it should return a pointer to an LV2UI_Show_Interface.

Type | lv2:ExtensionData

### Instance ui:touch

A feature to notify the host that the user has grabbed a particular port control. This is useful for automation, so the host can allow the user to take control of a port, even if that port would otherwise be automated (much like grabbing a physical morotised fader). It can also be used for MIDI learn or in any other situation where the host needs to do something with a particular control and it would be convenient for the user to select it directly from the plugin UI. This feature corresponds to the LV2UI_Touch struct, which should be passed with the URI LV2_UI__touch.

Type | lv2:Feature

## History

Version 2.20 (2015-07-25)
  - Improve documentation.
  - Add missing property labels.
Version 2.18 (2014-08-08)
  - Fix identifier typos in documentation.
  - Add show interface so UIs can gracefully degrade to separate windows if hosts can not use their widget directly.
Version 2.16 (2014-01-04)
  - Fix LV2_UI_INVALID_PORT_INDEX identifier in documentation.
Version 2.14 (2013-03-18)
  - Add idle interface so native UIs and foreign toolkits can drive their event loops.
  - Add ui:updateRate property.
Version 2.12 (2012-12-01)
  - Fix incorrect linker flag in ui:makeSONameResident documentation.
Version 2.10 (2012-10-14)
  - Add types for WindowsUI, CocoaUI, and Gtk3UI.
  - Use consistent label style.
  - Add missing LV2_SYMBOL_EXPORT declaration for lv2ui_descriptor prototype.
Version 2.8 (2012-04-17)
  - Add support for referring to ports by symbol.
  - Add ui:parent and ui:resize.
  - Add ui:portMap for accessing ports by symbol, allowing for UIs to be distributed separately from plugins.
  - Add touch feature to notify the host that the user has grabbed a control.
  - Merge with unified LV2 package.
  - Add port protocols and a dynamic notification subscription mechanism, for more flexible communication, and audio port metering without control port kludges.
Version 2.4 (2011-11-21)
  - Install header to URI-based system path.
  - Add Qt4 and X11 widget types.
  - Deprecate ui:makeSONameResident.
  - Make ui.ttl a valid OWL 2 DL ontology.
  - Add pkg-config file.
Version 2.2 (2011-05-26)
  - Add build system (for installation).
  - Convert documentation to HTML and use lv2:documentation.
  - Use lv2:Specification to be discovered as an extension.
Version 2.0 (2010-10-06)
  - Initial release.