// 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.
///
///
/// A component that wishes to offer a |View| can do so in one of two ways:
///
/// 1. Expose a |View| interface. This is usually done by components that
///    provide a single view, or have a clearly defined "main" view.
/// 2. Offer a domain-specific interface that provides |View|s using a
///    |request<View>| parameter. For example, an encyclopedia component
///    might offer a method to expose article views:
///
///      GetArticleView(string article_id, 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.
///
/// 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| its initial configuration and 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.
/// 4. Optionally, while the View is attached, call |SetConfig()| again to
///    modify any presentation parameters as needed.
///
/// 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.
///
///
/// 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 whenever
///   its |ViewConfig| changes.
/// * 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|.
///
///  TODO(SCN-1198): Migrate all implementations of |ViewProvider| to use |View|.
[Discoverable]
protocol View {
    /// 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|.  The
    /// initial configuration describing the properties of this content is
    /// provided in |initial_config|.
    ///
    /// 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.
    Present(ViewToken view_token, ViewConfig initial_config);

    /// Updates the View's configuration after it has been attached.
    ///
    /// If |Present()| has not been called yet, then the implementation should
    /// terminate the connection with an error.
    SetConfig(ViewConfig config);
};