// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

library fuchsia.ui.focus;

using fuchsia.ui.views;

/// A FocusChain tracks the status of the View hierarchy as View focus changes.
///
/// Reception. Only certain components may receive a FocusChain, as it
/// captures global information about the scene graph.
resource table FocusChain {
    /// The `focus_chain` is reported in order of dominance in the View
    /// hierarchy; each adjacent pair of elements represents a
    /// parent-child relationship.
    ///
    /// The `focus_chain` MAY be unset when `FocusChain` message is received, if
    /// the message is sent very early in the scene setup, before the first
    /// view is available.
    ///
    /// When `focus_chain` is set, however, the root View is always present
    /// and occupies slot 0 in the `focus_chain`.  The newly-focused View
    /// receives a `fuchsia.ui.input.FocusEvent` and occupies the final slot
    /// in the vector.
    ///
    /// If a View gets destroyed, a `FocusChain` holder that listens will
    /// receive a `ZX_EVENTPAIR_PEER_CLOSED` signal on the corresponding
    /// `ViewRef`.
    ///
    /// ## Invalidation.
    ///
    /// A FocusChain is invalid if any one if its ViewRefs is
    /// invalid.
    1: vector<fuchsia.ui.views.ViewRef> focus_chain;
};

/// A FocusChainListener receives an updated FocusChain when focus changes.
protocol FocusChainListener {
    /// Sent when a focus change occurs.  Since `focus_chain` may contain an
    /// empty update, every handler MUST respond to the message even
    /// if its contents are not immediately useful.
    OnFocusChange(FocusChain focus_chain) -> ();
};

/// A FocusChainListenerRegistry allows listening to FocusChain updates.
[Discoverable]
protocol FocusChainListenerRegistry {
    Register(FocusChainListener listener);
};