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.

API Reference

apptools.selection Package

Users of the apptools.selection package can access the objects that are part of the public API through the convenience apptools.selection.api.

selection_service Module

class apptools.selection.selection_service.SelectionService

Bases: traits.has_traits.HasTraits

The selection service connects selection providers and listeners.

The selection service is a register of selection providers, i.e., objects that publish their current selection.

Selections can be requested actively, by explicitly requesting the current selection in a provider (get_selection(id)()), or passively by connecting selection listeners.

add_selection_provider(provider)

Add a selection provider.

The provider is identified by its ID. If a provider with the same ID has been already registered, an IDConflictError is raised.

Arguments:
provider – ISelectionProvider
The selection provider added to the internal registry.
connect_selection_listener(provider_id, func)

Connect a listener to selection events from a specific provider.

The signature if the listener callback is func(i_selection). The listener is called:

  1. When a provider with the given ID is registered, with its initial selection as argument, or
  2. whenever the provider fires a selection event.

It is perfectly valid to connect a listener before a provider with the given ID is registered. The listener will remain connected even if the provider is repeatedly connected and disconnected.

Arguments:
provider_id – str
The selection provider ID.
func – callable(i_selection)
A callable object that is notified when the selection changes.
disconnect_selection_listener(provider_id, func)

Disconnect a listener from a specific provider.

Arguments:
provider_id – str
The selection provider ID.
func – callable(provider_id, i_selection)
A callable object that is notified when the selection changes.
get_selection(provider_id)

Return the current selection of the provider with the given ID.

If a provider with that ID has not been registered, a ProviderNotRegisteredError is raised.

Arguments:
provider_id – str
The selection provider ID.
Returns:
selection – ISelection
The current selection of the provider.
has_selection_provider(provider_id)

Has a provider with the given ID been registered?

remove_selection_provider(provider)

Remove a selection provider.

If the provider has not been registered, a ProviderNotRegisteredError is raised.

Arguments:
provider – ISelectionProvider
The selection provider added to the internal registry.
set_selection(provider_id, items, ignore_missing=False)

Set the current selection in a provider to the given items.

If a provider with the given ID has not been registered, a ProviderNotRegisteredError is raised.

If ignore_missing is True, items that are not available in the selection provider are silently ignored. If it is False (default), a ValueError should be raised.

Arguments:
provider_id – str
The selection provider ID.
items – list
List of items to be selected.
ignore_missing – bool
If False (default), the provider raises an exception if any of the items in items is not available to be selected. Otherwise, missing elements are silently ignored, and the rest is selected.

i_selection_provider Module

class apptools.selection.i_selection_provider.ISelectionProvider

Bases: traits.has_traits.Interface

Source of selections.

get_selection()

Return the current selection.

Returns:
selection – ISelection
Object representing the current selection.
provider_id = Str()

Unique ID identifying the provider.

selection = Event

Event triggered when the selection changes. The content of the event is an ISelection instance.

set_selection(items, ignore_missing=False)

Set the current selection to the given items.

If ignore_missing is True, items that are not available in the selection provider are silently ignored. If it is False (default), an ValueError should be raised.

Arguments:
items – list
List of items to be selected.
ignore_missing – bool
If False (default), the provider raises an exception if any of the items in items is not available to be selected. Otherwise, missing elements are silently ignored, and the rest is selected.

is_selection Module

class apptools.selection.i_selection.IListSelection

Bases: apptools.selection.i_selection.ISelection

Selection for ordered sequences of items.

indices = List

Indices of the selected objects in the selection provider.

items = List

Selected objects.

class apptools.selection.i_selection.ISelection

Bases: traits.has_traits.Interface

Collection of selected items.

is_empty()

Is the selection empty?

provider_id = Str

ID of the selection provider that created this selection object.

list_selection Module

class apptools.selection.list_selection.ListSelection

Bases: traits.has_traits.HasTraits

Selection for ordered sequences of items.

This is the default implementation of the IListSelection interface.

classmethod from_available_items(provider_id, selected, all_items)

Create a list selection given a list of all available items.

Fills in the required information (in particular, the indices) based on a list of selected items and a list of all available items.

Note

  • The list of available items must not contain any duplicate items.
  • It is expected that selected is populated by items in all_items.
indices = List

Indices of the selected objects in the selection provider.

is_empty()

Is the selection empty?

items = List

Selected objects.

provider_id = Str

ID of the selection provider that created this selection object.

errors Module

exception apptools.selection.errors.IDConflictError(provider_id)

Bases: exceptions.Exception

Raised when a provider is added and its ID is already registered.

exception apptools.selection.errors.ListenerNotConnectedError(provider_id, listener)

Bases: exceptions.Exception

Raised when a listener that was never connected is disconnected.

exception apptools.selection.errors.ProviderNotRegisteredError(provider_id)

Bases: exceptions.Exception

Raised when a provider is requested by ID and not found.