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213 lines
9.4 KiB
Markdown
213 lines
9.4 KiB
Markdown
Utility Client Example Application
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==================================
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Initial Setup
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-------------
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### Step 1
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A basic program loop that prompts the user for a command and then processes it. In this tutorial we will modify this program to perform tasks and retrieve data from a remote server over a WebSocket connection.
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#### Build
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`clang++ step1.cpp`
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#### Code so far
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```cpp
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#include <iostream>
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#include <string>
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int main() {
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bool done = false;
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std::string input;
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while (!done) {
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std::cout << "Enter Command: ";
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std::getline(std::cin, input);
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if (input == "quit") {
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done = true;
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} else if (input == "help") {
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std::cout
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<< "\nCommand List:\n"
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<< "help: Display this help text\n"
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<< "quit: Exit the program\n"
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<< std::endl;
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} else {
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std::cout << "Unrecognized Command" << std::endl;
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}
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}
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return 0;
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}
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```
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### Step 2
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_Add WebSocket++ includes and set up an endpoint type._
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WebSocket++ includes two major object types. The endpoint and the connection. The
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endpoint creates and launches new connections and maintains default settings for
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those connections. Endpoints also manage any shared network resources.
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The connection stores information specific to each WebSocket session.
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> **Note:** Once a connection is launched, there is no link between the endpoint and the connection. All default settings are copied into the new connection by the endpoint. Changing default settings on an endpoint will only affect future connections.
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Connections do not maintain a link back to their associated endpoint. Endpoints do not maintain a list of outstanding connections. If your application needs to iterate over all connections it will need to maintain a list of them itself.
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WebSocket++ endpoints are built by combining an endpoint role with an endpoint config. There are two different types of endpoint roles, one each for the client and server roles in a WebSocket session. This is a client tutorial so we will use the client role `websocketpp::client` which is provided by the `<websocketpp/client.hpp>` header.
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> ###### Terminology: Endpoint Config
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> WebSocket++ endpoints have a group of settings that may be configured at compile time via the `config` template parameter. A config is a struct that contains types and static constants that are used to produce an endpoint with specific properties. Depending on which config is being used the endpoint will have different methods available and may have additional third party dependencies.
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The endpoint role takes a template parameter called `config` that is used to configure the behavior of endpoint at compile time. For this example we are going to use a default config provided by the library called `asio_client`, provided by `<websocketpp/config/asio_no_tls_client.hpp>`. This is a client config that uses boost::asio to provide network transport and does not support TLS based security. Later on we will discuss how to introduce TLS based security into a WebSocket++ application, more about the other stock configs, and how to build your own custom configs.
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Combine a config with an endpoint role to produce a fully configured endpoint. This type will be used frequently so I would recommend a typedef here.
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`typedef websocketpp::client<websocketpp::config::asio_client> client`
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#### Build
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Adding WebSocket++ has added a few dependencies to our program that must be addressed in the build system. Firstly, the WebSocket++ and Boost library headers must be in the include search path of your build system. How exactly this is done depends on where you have the WebSocket++ headers installed and what build system you are using.
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In addition to the new headers, boost::asio depends on the `boost_system` shared library. This will need to be added (either as a static or dynamic) to the linker. Refer to your build environment documentation for instructions on linking to shared libraries.
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`clang++ step2.cpp -lboost_system`
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#### Code so far
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```cpp
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#include <websocketpp/config/asio_no_tls_client.hpp>
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#include <websocketpp/client.hpp>
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#include <iostream>
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#include <string>
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typedef websocketpp::client<websocketpp::config::asio_client> client;
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int main() {
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bool done = false;
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std::string input;
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while (!done) {
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std::cout << "Enter Command: ";
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std::getline(std::cin, input);
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if (input == "quit") {
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done = true;
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} else if (input == "help") {
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std::cout
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<< "\nCommand List:\n"
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<< "help: Display this help text\n"
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<< "quit: Exit the program\n"
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<< std::endl;
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} else {
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std::cout << "Unrecognized Command" << std::endl;
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}
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}
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return 0;
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}
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```
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### Step 3
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_Create endpoint wrapper object that handles initialization and setting up the background thread._
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In order to process user input while network processing occurs in the background we are going to use a separate thread for the WebSocket++ processing loop. This leaves the main thread free to process foreground user input. In order to enable simple RAII style resource management for our thread and endpoint we will use a wrapper object that configures them both in its constructor.
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> ###### Terminology: websocketpp::lib namespace
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> WebSocket++ is designed to be used with a C++11 standard library. As this is not universally available in popular build systems the Boost libraries may be used as polyfills for the C++11 standard library in C++98 build environments. The `websocketpp::lib` namespace is used by the library and its associated examples to abstract away the distinctions between the two. `websocketpp::lib::shared_ptr` will evaluate to `std::shared_ptr` in a C++11 environment and `boost::shared_ptr` otherwise.
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>
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> This tutorial uses the `websocketpp::lib` wrappers because it doesn't know what the build environment of the reader is. For your applications, unless you are interested in similar portability, are free to use the boost or std versions of these types directly.
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>
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>[TODO: link to more information about websocketpp::lib namespace and C++11 setup]
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Within the `websocket_endpoint` constructor several things happen:
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First, we set the endpoint logging behavior to silent by clearing all of the access and error logging channels. [TODO: link to more information about logging]
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```cpp
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m_endpoint.clear_access_channels(websocketpp::log::alevel::all);
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m_endpoint.clear_error_channels(websocketpp::log::elevel::all);
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```
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Next, we initialize the transport system underlying the endpoint and sets it to perpetual mode. In perpetual mode the endpoint's processing loop will not exit automatically when it has no connections. This is important because we want this endpoint to remain active while our application is running and process requests for new WebSocket connections on demand as we need them. Both of these methods are specific to the asio transport. They will not be necessary or present in endpoints that use a non-asio config.
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```cpp
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m_endpoint.init_asio();
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m_endpoint.start_perpetual();
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```
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Finally, we launch a thread to run the `run` method of our client endpoint. While the endpoint is running it will process connection tasks (read and deliver incoming messages, frame and send outgoing messages, etc). Because it is running in perpetual mode, when there are no connections active it will wait for a new connection.
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```cpp
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m_thread.reset(new websocketpp::lib::thread(&client::run, &m_endpoint));
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```
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#### Build
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Now that our client endpoint template is actually instantiated a few more linker dependencies will show up. In particular, WebSocket clients require a cryptographically secure random number generator. WebSocket++ is able to use either boost_random or the C++11 standard library <random> for this purpose.
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##### Clang (C++98 & boost)
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`clang++ step3.cpp -lboost_system -lboost_random`
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##### Clang (C++11)
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`clang++ -std=c++0x -stdlib=libc++ step3.cpp -lboost_system -D_WEBSOCKETPP_CPP11_STL_`
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##### G++ (C++98 & Boost)
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`g++ step3.cpp -lboost_system -lboost_random`
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##### G++ v4.6+ (C++11)
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`g++ -std=c++0x step3.cpp -lboost_system -D_WEBSOCKETPP_CPP11_STL_`
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#### Code so far
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```cpp
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#include <websocketpp/config/asio_no_tls_client.hpp>
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#include <websocketpp/client.hpp>
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#include <websocketpp/common/thread.hpp>
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#include <websocketpp/common/memory.hpp>
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#include <iostream>
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#include <string>
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typedef websocketpp::client<websocketpp::config::asio_client> client;
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class websocket_endpoint {
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public:
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websocket_endpoint () {
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m_endpoint.clear_access_channels(websocketpp::log::alevel::all);
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m_endpoint.clear_error_channels(websocketpp::log::elevel::all);
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m_endpoint.init_asio();
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m_endpoint.start_perpetual();
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m_thread.reset(new websocketpp::lib::thread(&client::run, &m_endpoint));
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}
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private:
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client m_endpoint;
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websocketpp::lib::shared_ptr<websocketpp::lib::thread> m_thread;
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};
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int main() {
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bool done = false;
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std::string input;
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websocket_endpoint endpoint;
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while (!done) {
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std::cout << "Enter Command: ";
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std::getline(std::cin, input);
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if (input == "quit") {
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done = true;
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} else if (input == "help") {
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std::cout
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<< "\nCommand List:\n"
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<< "help: Display this help text\n"
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<< "quit: Exit the program\n"
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<< std::endl;
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} else {
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std::cout << "Unrecognized Command" << std::endl;
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}
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}
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return 0;
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}
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``` |