The selection service

It is quite common in GUI applications to have a UI element displaying a collection of items that a user can select (“selection providers”), while other parts of the application must react to changes in the selection (“selection listeners”).

Ideally, the listeners would not have a direct dependency on the UI object. This is especially important in extensible envisage applications, where a plugin might need to react to a selection change, but we do not want to expose the internal organization of the application to external developers.

This package defines a selection service that manages the communication between providers and listener.

The SelectionService object

The SelectionService object is the central manager that handles the communication between selection providers and listener.

Selection providers are components that wish to publish information about their current selection for public consumption. They register to a selection service instance when they first have a selection available (e.g., when the UI showing a list of selectable items is initialized), and un-register as soon as the selection is not available anymore (e.g., the UI is destroyed when the windows is closed).

Selection listeners can query the selection service to get the current selection published by a provider, using the provider unique ID.

The service acts as a broker between providers and listeners, making sure that they are notified when the selection event is fired.

Selection providers

Any object can become a selection provider by implementing the ISelectionProvider interface, and registering to the selection service.

Selection providers must provide a unique ID provider_id, which is used by listeners to request its current selection.

Whenever its selection changes, providers fire a selection event. The content of the event is an instance implementing ISelection that contains information about the selected items. For example, a ListSelection object contains a list of selected items, and their indices.

Selection providers can also be queried directly about their current selection using the get_selection method, and can be requested to change their selection to a new one with the set_selection method.

Registration

Selection providers publish their selection by registering to the selection service using the add_selection_provider method. When the selection is no longer available, selection providers should un-register through remove_selection_provider.

Typically, selection providers are UI objects showing a list or tree of items, they register as soon as the UI component is initialized, and un-register when the UI component disappears (e.g., because their window has been closed). In more complex applications, the registration could be done by a controller object instead.

Selection listeners

Selection listeners request information regarding the current selection of a selection provider given their provider ID. The SelectionService supports two distinct use cases:

1. Passively listening to selection changes: listener connect to a specific provider and are notified when the provider’s selection changes.

2. Actively querying a provider for its current selection: the selection service can be used to query a provider using its unique ID.

Passive listening

Listeners connect to the selection events for a given provider using the connect_selection_listener method. They need to provide the unique ID of the provider, and a function (or callable) that is called to send the event. This callback function takes one argument, an implementation of the ISelection that represents the selection.

It is possible for a listener to connect to a provider ID before it is registered. As soon as the provider is registered, the listener will receive a notification containing the provider’s initial selection.

To disconnect a listener use the methods disconnect_selection_listener.

Active querying

In other instances, an element of the application only needs the current selection at a specific time. For example, a toolbar button could open dialog representing a user action based on what is currently selected in the active editor.

The get_selection method calls the corresponding method on the provider with the given ID and returns an ISelection instance.

Setting a selection

Finally, it is possible to request a provider to set its selection to a given set of objects with set_selection. The main use case for this method is multiple views of the same list of objects, which need to keep their selection synchronized.

If the items specified in the arguments are not available in the provider, a ProviderNotRegisteredError is raised, unless the optional keyword argument ignore_missing is set to True.