// 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.views;

/// A View is an interface that a component implements to offer Scenic view(s)
/// to its clients.
///
/// A Scenic view contains a tree of Scenic graph nodes which is used to render
/// a graphical user interface, such as a module, shell, or on-screen keyboard.
/// Other Scenic views can be embedded within this tree of nodes.  This creates
/// a scene graph describing the UI of the entire system, rooted at the
/// top-most view.  Different processes can contribute branches to the scene
/// graph, and their content will be rendered together in a shared space.
///
/// # Offering a View
///
/// A component that wishes to offer a |View| can do so in one of two ways:
///
/// 1. Expose a |View| interface as a service. This is usually done by components
///    that provide a single view, or have a clearly defined "main" view.
///    In this case, the component may obtain relevant properties for configuring
///    the view using services that may be available in its namespace, such as:
///    - |fuchsia.intl.PropertyProvider|
///    - |fuchsia.accessibility.PropertyProvider|
///
/// 2. Offer a domain-specific interface that provides |View|s using a
///    `request<View>` parameter.  In this case, the component may obtain relevant
///    properties for configuring the view using services that may be provided by its
///    client as part of the request.
///
///    For example, an encyclopedia component might offer a method to expose article views:
///
///      GetArticleView(string article_id,
///                     fuchsia.intl.PropertyProvider intl_properties,
///                     fuchsia.accessibility.PropertyProvider accessibility_properties,
///                     request<View> view_request);
///
///    This latter case is probably less common, as controlling domain-specific
///    views tends to be the job of the component that creates them.
///
/// # Presenting a View
///
/// A client of the |View| interface will:
///
/// 1. Launch (or bind to) the component that provides the interface.
/// 2. Connect to the component's |View| interface.
/// 3. Call |Present()| to give the |View| an attachment point into the scene graph
///    via the |view_token|.  Subsequent calls to |Present()| will generate an error
///    and cause the connection to be closed.
///
/// When the client no longer needs the View, it should disconnect from the
/// interface.
///
/// NOTE: The client owns the |View| instance and must retain it for the
/// lifetime of the UI that it displays. If the |View| instance is destroyed,
/// the connection will be dropped and all UI content will be destroyed.
///
/// # Implementing a View
///
/// On the implementation side, a component that exposes the |View| interface
/// has the following responsibilities:
///
/// * When |Present()| is called, create a root for the |View|'s content in the
///   Scenic scene graph by passing the provided |view_token|.
/// * Provide graphical content for the view and attach it to the root.
/// * Adjust the appearance and/or contents of the view's content based on
///   relevant internationalization and/or accessibility properties as described
///   above.
/// * Handle user interface events such as touches, key presses, and
///   |fuchsia.ui.gfx.ViewProperty| changes using other Scenic interfaces such
///   as |fuchsia.ui.Scenic| and |fuchsia.ui.SessionListener|.
///
/// See also: |fuchsia.intl.PropertyProvider|, |fuchsia.accessibility.PropertyProvider|.
///
/// TODO(SCN-1198): Migrate all implementations of |ViewProvider| to use |View|.
[Discoverable]
protocol View {
    /// DEPRECATED
    [Transitional]
    Present(ViewToken view_token, ViewConfig initial_config);

    /// Provides the View with an attachment point to Scenic's scene graph.
    ///
    /// When |Present()| is called the View's implementation should create a
    /// View resource within Scenic by providing it with the |view_token| (using
    /// a |fuchsia.ui.gfx.CreateResourceCmd| and |fuchsia.ui.gfx.ViewArgs|).
    ///
    /// Then the implementation should attach its graphical content to the
    /// newly-created View resource using a |fuchsia.ui.gfx.AddChildCmd|.
    ///
    /// If the implementation already owns a View resource (because |Present()|
    /// had already been called before), then it should terminate the connection
    /// with an error.
    ///
    /// TODO(SCN-1271): Allow re-parenting |View|s with a new |Present()| call.
    /// TODO(SCN-1343): Rename to |Present()|.
    [Transitional]
    Present2(ViewToken view_token);

    /// DEPRECATED
    [Transitional]
    SetConfig(ViewConfig config);
};

/// A ViewRef is a kernel object that identifies a unique View
/// across the system. When two ViewRefs refer to the same View,
/// they have identical KOIDs.
///
/// We use a ViewRef to identify a View, to validate a View, and to
/// signal invalidation.
///
/// For example, accessibility accepts a ViewRef to validate that
/// a client has a valid View, and subsequently uses that ViewRef
/// to group the semantic nodes associated with that View.
///
/// Another example: We use ViewRefs to create a FocusChain
/// (TBD), which identifies Views considered as "in-focus" down
/// the View hierarchy. When a View is destroyed, Scenic signals
/// to all ViewRef holders that the ViewRef is now invalid.
///
/// Scenic creates and associates a ViewRef for each View. Scenic
/// creates a copy of a ViewRef upon request by that View. The
/// clone is transferrable, cloneable, and waitable, but not
/// signalable (latter is reserved for Scenic).
struct ViewRef {
    handle<event> reference;
};