// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example chat
    \ingroup qtgrpc-examples
    \examplecategory {Application Examples}
    \meta tag {network,protobuf,grpc,serialization,tcp,ssl,tls,quick}
    \title QtGrpc Chat
    \brief A chat application to share messages of any kind in a chat room.

    The \e Chat example demonstrates advanced usage of the QtGrpc client API.
    The server enables users to register and authenticate, allowing them to
    join the \e ChatRoom. Once joined, users can share various message types in
    the \e ChatRoom, such as text messages, images, user activity or any other
    files from their disk with all other participants.

    \inlineimage {chat_room.webp} {Mobile chat window}
    \inlineimage {chat_login.webp} {Login window}

    \section1 Running the example

    \list
        \li Ensure that the \c{qtgrpc_chat_server} is running and successfully
            listening.
        \li If you are on the same machine as the server, the default
            \c{localhost} address should suffice when running the
            \c{qtgrpc_chat_client}. If you are using a device other than the
            one hosting the server, specify the correct IP address of the host
            running the server in the Settings dialog.
        \li Ensure that the \c{GRPC_CHAT_USE_EMOJI_FONT} CMake option is
            enabled on the client to build with a smooth emoji experience 🚀.
    \endlist

    \image {chat_settings.webp} {Connection settings window img}

    To run the example from \l{\QC Documentation}{Qt Creator}, open the
    \uicontrol Welcome mode and select the example from \uicontrol Examples.
    For more information, see \l{\QC: Tutorial: Build and run}.

    \section1 Relevant modules and classes.
    This example introduces the following Qt modules and classes.

    \list
        \li Communication through long-lived \l{QGrpcBidiStream}.
        \li Using the QtGrpc client from a worker \l{QThread}{thread}.
        \li Using the QtProtobufQtCoreTypes module in the protobuf schema.
        \li Secure communication through \l{Secure Sockets Layer (SSL) Classes}{SSL}.
        \li Visualizing QtProtobuf messages in a QML ListView.
    \endlist

    \section1 Protobuf Schema

    The Protobuf schema defines the structure of messages and services used in
    the chat application. The schema is split into two files:

    \snippet chat/proto/qtgrpcchat.proto proto-1

    The \c{qtgrpcchat.proto} file specifies the QtGrpcChat service, which provides
    two RPC methods:

    \list
        \li \c{Register}: Registers a user with the provided \c Credentials.
            The server stores and verifies users from a database in plain text.
        \li \c{ChatRoom}: Establishes a bidirectional stream for exchanging
            \c{ChatMessage}(s) between all connected clients. The server
            broadcasts all incoming messages to other connected clients.
    \endlist

    \snippet chat/proto/chatmessages.proto proto-2

    The \c{chatmessages.proto} file defines \c{ChatMessage}, which is a
    \l{https://en.wikipedia.org/wiki/Tagged_union}{tagged union (also known as
    a sum type)}. It represents all the individual messages that can be sent
    through the \c{ChatRoom} streaming RPC. Every \c{ChatMessage} must include
    a \c{username} and \c{timestamp} to identify the sender.

    You include the \c{QtCore/QtCore.proto} import to enable the types of the
    QtProtobufQtCoreTypes module, allowing seamless conversion between
    QtCore-specific types and their Protobuf equivalents.

    \snippet chat/proto/chatmessages.proto proto-3

    \c{FileMessage} is one of the supported message types for the
    \c{ChatMessage} sum type. It allows wrapping any local file into a message.
    The optional \c{Continuation} field ensures reliable delivery by handling
    large file transfers in chunks.

    \note For more details on using the \c{ProtobufQtCoreTypes} module in your
    Protobuf schema and application code, see \l{Qt Core usage}.

    \section1 Server

    \note The server application described here uses the \gRPC library.

    The server application uses the asynchronous \gRPC
    \l{https://grpc.io/docs/languages/cpp/callback/}{callback API}. This allows
    us to benefit from the performance advantages of the async API without the
    complexity of manually managing completion queues.

    \snippet chat/server/main.cpp server-1

    You declare the \c{QtGrpcChatService} class, which subclasses the
    \c{CallbackService} of the generated \c{QtGrpcChat} service.

    \snippet chat/server/main.cpp server-2

    Also override the virtual functions to implement the functionality for the
    two \gRPC methods provided by the service:

    \list
        \li The \c{Register} method verifies and stores users in a plain text
            database.
        \li The \c{ChatRoom} method checks credentials provided in the metadata
            against the database. If successful, it establishes a bidirectional
            stream for communication.
    \endlist

    \snippet chat/server/main.cpp server-3

    The service implementation tracks all active clients that connect or
    disconnect through the \c{ChatRoom} method. This enables the \c{broadcast}
    functionality, which shares messages with all connected clients. To reduce
    storage and overhead, the \c{ChatMessage} is wrapped in a \c{shared_ptr}.

    \snippet chat/server/main.cpp server-4

    The \c{startSharedWrite} method is a member function of the
    \c{ChatRoomReactor}. If the reactor is currently writing, the message is
    buffered in a queue. Otherwise, a write operation is initiated. There is a
    single and unique message shared between all clients.
    Each copy of the \c{response} message increases the \c{use_count}. Once all
    clients have finished writing the message, and its \c{use_count} drops to 0 its
    resources are freed.

    \snippet chat/server/main.cpp server-5

    This snippet is part of the \c{ChatRoomReactor::OnReadDone} virtual method.
    Each time this method is called, a new message has been received from the
    client. The message is broadcast to all other clients, skipping the sender.

    \snippet chat/server/main.cpp server-6

    This snippet is part of the \c{ChatRoomReactor::OnWriteDone} virtual
    method. Each time this method is called, a message has been written to the
    client. If there are buffered messages in the queue, the next message is
    written. Otherwise, \c{m_response} is reset to signal that no write
    operation is in progress. A lock is used to protect against contention with
    the \c{broadcast} method.

    \section1 Client

    The client application uses the provided Protobuf schema to communicate
    with the server. It provides both front-end and back-end capabilities for
    registering users and handling the long-lived bidirectional stream of the
    \c{ChatRoom} \gRPC method. This enables the visualization and communication
    of \c{ChatMessage}s.

    \section2 Setup

    \snippet chat/client/CMakeLists.txt client-setup-1

    First, generate the source files from the Protobuf schema. Since the
    \c{qtgrpcchat.proto} file does not contain any \c{message} definitions,
    only \l{qt_add_grpc}{qtgrpcgen} generation is required. Also provide the
    \c{PROTO_INCLUDES} of the \c{ProtobufQtCoreTypes} module to ensure the
    \c{"QtCore/QtCore.proto"} import is valid.

    \snippet chat/client/CMakeLists.txt client-setup-2

    Ensure that the independent \c{qtgrpc_chat_client_proto} target is
    publicly linked against its dependencies, including the
    \c{ProtobufQtCoreTypes} module. The application target is then linked
    against this library.

    \section2 Backend Logic

    The backend of the application is built around four crucial elements:

    \list
        \li \c{ChatEngine}: A QML-facing singleton that manages the application
            logic.
        \li \c{ClientWorker}: A worker object that provides \gRPC client
            functionality asynchronously.
        \li \c{ChatMessageModel}: A custom \c{QAbstractListModel} for handling
            and storing \c{ChatMessage}s.
        \li \c{UserStatusModel}: A custom \c{QAbstractListModel} for managing
            user activity.
    \endlist

    \snippet chat/client/chatengine.h client-1

    The snippet above shows some of the \c{Q_INVOKABLE} functionality that is
    called from QML to interact with the server.

    \snippet chat/client/clientworker.h client-2a
    \snippet chat/client/clientworker.h client-2b

    The slots provided by the \c{ClientWorker} somewhat mirror the API exposed by the
    \c{ChatEngine}. The \c{ClientWorker} operates in a dedicated thread to
    handle expensive operations, such as transmitting or receiving large files,
    in the background.

    \snippet chat/client/chatengine.cpp client-3
    \dots 0

    In the \c{ChatEngine} constructor, assign the \c{ClientWorker} to its
    dedicated worker thread and continue handling and forwarding its signals to
    make them available on the QML side.

    \snippet chat/client/chatengine.cpp client-4a
    \dots 0
    \snippet chat/client/clientworker.cpp client-4b

    This demonstrates how the \c{ChatEngine} interacts with the
    \c{ClientWorker} to register users. Since the \c{ClientWorker} runs in its
    own thread, it is important to use \l{QMetaObject::}{invokeMethod} to call
    its member functions safely.

    In the \c{ClientWorker}, you check whether the client is uninitialized or if
    the host URI has changed. If either condition is met, call
    \c{initializeClient}, which creates a new QGrpcHttp2Channel. Since this
    is an expensive operation, minimize its occurrences.

    To handle the \c{Register} RPC, use the
    \l{QGrpcCallOptions::}{setDeadlineTimeout} option to guard against server
    inactivity. It is generally recommended to set a deadline for unary RPCs.

    \snippet chat/client/clientworker.cpp client-5a
    \dots
    \snippet chat/client/clientworker.cpp client-5b

    When logging into the \c{ChatRoom},you can use the
    \l{QGrpcCallOptions::}{setMetadata} option to provide user credentials, as
    required by the server for authentication. The actual call and connection
    setup are handled in the \c{connectStream} method.

    \snippet chat/client/clientworker.cpp client-6a
    \dots
    \snippet chat/client/clientworker.cpp client-6b
    \dots
    \snippet chat/client/clientworker.cpp client-6c
    \dots

    To implement basic reconnection logic in case the stream finishes abruptly
    while you are still connected. This is done by simply calling
    \c{connectStream} again with the \c{QGrpcCallOptions} from the initial
    call. This ensures that all required connections are also updated.

    \note Android’s
    \l{https://developer.android.com/training/monitoring-device-state/doze-standby}{Doze/App-Standby}
    mode can be triggered, e.g., by using the FileDialog or switching to
    another app. This mode shuts down network access, closing all active
    QTcpSocket connections and causing the stream to be
    \l{QGrpcBidiStream::}{finished}. You can address this issue with the
    reconnection logic.

    \snippet chat/client/clientworker.cpp client-6d
    \dots 8
    \snippet chat/client/clientworker.cpp client-6e
    \dots 8
    \snippet chat/client/clientworker.cpp client-6f

    When messages are received, the \c{ClientWorker} performs some
    pre-processing, such as saving the \c{FileMessage} content, so that the
    \c{ChatEngine} only needs to focus on the models. Use the \c{ContentFields}
    enum to safely check the \c{oneof content} field of our ChatMessage sum type.

    \snippet chat/client/chatengine.cpp client-7a
    \dots 0
    \snippet chat/client/clientworker.cpp client-7b

    When sending messages, the \c{ChatEngine} creates properly formatted
    requests. For example, the \c{sendText} method accepts a \c{QString} and
    uses the \c{createMessage} function to generate a valid message with the
    \c{username} and \c{timestamp} fields set. The client is then invoked to
    send the message, and a copy is enqueued into our own \c{ChatMessageModel}.

    \section2 QML Frontend

    \snippet chat/client/Main.qml client-qml-1

    The following imports are used in the QML code:

    \list
        \li \c{QtGrpc}: Provides QtGrpc QML functionality, such as the
            \l{QtGrpc::}{StatusCode}.
        \li \c{QtGrpcChat}: Our application module, which includes components
            like the \c{ChatEngine} singleton.
        \li \c{QtGrpcChat.Proto}: Provides QML access to our generated \l{Qt
            Protobuf QML Types}{protobuf types}.
    \endlist

    \snippet chat/client/Main.qml client-qml-2

    \c{Main.qml} handles core signals emitted by the \c{ChatEngine}. Most
    of these signals are handled globally and are visualized in any state of
    the application.

    \snippet chat/client/LoginView.qml client-qml-3a
    \dots
    \snippet chat/client/LoginView.qml client-qml-3b
    \dots 8
    \snippet chat/client/LoginView.qml client-qml-3c
    \dots 12
    \snippet chat/client/LoginView.qml client-qml-3d
    \dots 16
    \snippet chat/client/LoginView.qml client-qml-3e

    The generated message types from the protobuf schema are accessible in QML
    as they're \l{QML_VALUE_TYPE}s (a \e{camelCase} version of the message
    definition). The \c{LoginView.qml} uses the \c{credentials} value type
    property to initiate the \c{login} on the \c{ChatEngine}.

    \snippet chat/client/ChatView.qml client-qml-4a
    \dots 12
    \snippet chat/client/ChatView.qml client-qml-4b
    \dots 16
    \snippet chat/client/ChatView.qml client-qml-4c
    \dots 12
    \snippet chat/client/ChatView.qml client-qml-4d
    \dots 16
    \snippet chat/client/ChatView.qml client-qml-4e

    In \c{ChatView.qml}, the ListView displays messages in the \c{ChatRoom}.
    This is slightly more complex, as you need to handle the \c{ChatMessage} sum
    type conditionally.

    You can use a DelegateChooser, which allows us to select the appropriate
    delegate based on the type of message. Use the default
    \c{whatThis} role in the model, which provides the message type for each
    \c{ChatMessage} instance. The \c{DelegateBase} component then accesses the
    \c{display} role of the model, making the chatMessage data available for
    rendering.

    \snippet chat/client/ChatMessages/TextDelegate.qml client-qml-5

    Here is one of the components that visualizes the \c{TextMessage} type. It
    uses the \c{textMessage} value type from the protobuf module to visualize
    the text.

    \snippet chat/client/ChatView.qml client-qml-6a
    \dots 20
    \snippet chat/client/ChatView.qml client-qml-6b

    The Chat client provides various access points in sending
    messages like:

    \list
        \li Accepting files Dropped onto the application.
        \li <Ctrl + V> to send anything stored in the QClipboard.
        \li <Ctrl + Enter> to send the message from the \c{inputField}
        \li Clicking the send button for the \c{inputField}
        \li Selecting files through a FileDialog
    \endlist

    \section1 SSL

    To secure communication between the server and clients, SSL/TLS encryption
    is used. This requires the following at a minimum:

    \list
        \li \e{Private Key}: contains the server's private key, which is used
            to establish secure connections. It must be kept confidential and
            should never be shared.
        \li \e{Certificate}: contains the server's public certificate, which
            is shared with clients to verify the server's identity. It is
            typically signed by a Certificate Authority (CA) or can be
            self-signed for testing purposes.
        \li Optional \e{Root CA Certificate}: If you are using a custom
            Certificate Authority (CA) to sign your server certificate, the
            root CA certificate is required on the client side to validate the
            server's certificate chain. This ensures the client can trust the
            server's certificate, as the custom CA's root certificate is not
            pre-installed in the client's trust store like those of public CAs.
    \endlist

    Use \l{https://www.openssl.org/}{OpenSSL} to create these files and set
    up our \gRPC communication to use SSL/TLS.

    \snippet chat/server/main.cpp server-ssl

    You provide the \e{Private Key} and \e{Certificate} to the \gRPC server.
    With that, you can construct the \c{SslServerCredentials} to enable TLS on the
    server-side. In addition to secure communication, also allow unencrypted
    access.

    The server listens on the following addresses:
    \list
        \li HTTPS : \c{0.0.0.0:65002}
        \li HTTP  : \c{0.0.0.0:65003}
    \endlist

    The server binds to \c{0.0.0.0} to listen on all network interfaces,
    allowing access from any device on the same network.

    \snippet chat/client/clientworker.cpp client-ssl

    The client loads the \e{Root CA Certificate}, as you self-signed the CA.
    This certificate is used to create the QSslCertificate. It is important to
    provide the \c{"h2"} protocol with
    \l{QSslConfiguration::}{setAllowedNextProtocols}, as you are using HTTP/2.

    \section1 Source files

    \sa {All Qt Examples}
*/

