一个gen_server服务器在运行周期里面保持了一系列运行状态，erlang根据运行状态来决定是否停止该服务，今天说的是常规方法停服务的方法，至于由异常引起的服务停止，又要分为我们有没有提前做过trap_exit处理，改日再说。 先看一个简单的服务器，什么也不处理，只是启动起来。
先看一个简单的服务器，什么也不处理，只是启动起来。
%%%-------------------------------------------------------------------
%%% @author SummerGao
%%% @copyright (C) 2022, <COMPANY>
%%% @doc
%%%
%%% @end
%%% Created : 25. 5月 2022 0:01
%%%-------------------------------------------------------------------
-module(test).
-author("SummerGao").
-behaviour(gen_server).
%% API
-export([start_link/0]).
%% gen_server callbacks
-export([init/1,
handle_call/3,
handle_cast/2,
handle_info/2,
terminate/2,
code_change/3]).
-define(SERVER, ?MODULE).
-record(state, {}).
%%%===================================================================
%%% API
%%%===================================================================
%%--------------------------------------------------------------------
%% @doc
%% Starts the server
%%
%% @end
%%--------------------------------------------------------------------
-spec(start_link() ->
{ok, Pid :: pid()} | ignore | {error, Reason :: term()}).
start_link() ->
gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).
%%%===================================================================
%%% gen_server callbacks
%%%===================================================================
%%--------------------------------------------------------------------
%% @private
%% @doc
%% Initializes the server
%%
%% @spec init(Args) -> {ok, State} |
%% {ok, State, Timeout} |
%% ignore |
%% {stop, Reason}
%% @end
%%--------------------------------------------------------------------
-spec(init(Args :: term()) ->
{ok, State :: #state{}} | {ok, State :: #state{}, timeout() | hibernate} |
{stop, Reason :: term()} | ignore).
init([]) ->
{ok, #state{}}.
%%--------------------------------------------------------------------
%% @private
%% @doc
%% Handling call messages
%%
%% @end
%%--------------------------------------------------------------------
-spec(handle_call(Request :: term(), From :: {pid(), Tag :: term()},
State :: #state{}) ->
{reply, Reply :: term(), NewState :: #state{}} |
{reply, Reply :: term(), NewState :: #state{}, timeout() | hibernate} |
{noreply, NewState :: #state{}} |
{noreply, NewState :: #state{}, timeout() | hibernate} |
{stop, Reason :: term(), Reply :: term(), NewState :: #state{}} |
{stop, Reason :: term(), NewState :: #state{}}).
handle_call(_Request, _From, State) ->
{reply, ok, State}.
%%--------------------------------------------------------------------
%% @private
%% @doc
%% Handling cast messages
%%
%% @end
%%--------------------------------------------------------------------
-spec(handle_cast(Request :: term(), State :: #state{}) ->
{noreply, NewState :: #state{}} |
{noreply, NewState :: #state{}, timeout() | hibernate} |
{stop, Reason :: term(), NewState :: #state{}}).
handle_cast(_Request, State) ->
{noreply, State}.
%%--------------------------------------------------------------------
%% @private
%% @doc
%% Handling all non call/cast messages
%%
%% @spec handle_info(Info, State) -> {noreply, State} |
%% {noreply, State, Timeout} |
%% {stop, Reason, State}
%% @end
%%--------------------------------------------------------------------
-spec(handle_info(Info :: timeout() | term(), State :: #state{}) ->
{noreply, NewState :: #state{}} |
{noreply, NewState :: #state{}, timeout() | hibernate} |
{stop, Reason :: term(), NewState :: #state{}}).
handle_info(_Info, State) ->
{noreply, State}.
%%--------------------------------------------------------------------
%% @private
%% @doc
%% This function is called by a gen_server when it is about to
%% terminate. It should be the opposite of Module:init/1 and do any
%% necessary cleaning up. When it returns, the gen_server terminates
%% with Reason. The return value is ignored.
%%
%% @spec terminate(Reason, State) -> void()
%% @end
%%--------------------------------------------------------------------
-spec(terminate(Reason :: (normal | shutdown | {shutdown, term()} | term()),
State :: #state{}) -> term()).
terminate(_Reason, _State) ->
ok.
%%--------------------------------------------------------------------
%% @private
%% @doc
%% Convert process state when code is changed
%%
%% @spec code_change(OldVsn, State, Extra) -> {ok, NewState}
%% @end
%%--------------------------------------------------------------------
-spec(code_change(OldVsn :: term() | {down, term()}, State :: #state{},
Extra :: term()) ->
{ok, NewState :: #state{}} | {error, Reason :: term()}).
code_change(_OldVsn, State, _Extra) ->
{ok, State}.
%%%===================================================================
%%% Internal functions
%%%===================================================================
停止服务的几种方法
1、异步非阻塞式停止服务器
handle_cast(stop, State) ->
{stop, normal, State};
stop() ->
gen_server:cast(?MODULE, stop).
在erlang虚拟机中用c(test)编译后，通过test:start_link()运行服务，通过whereis(test)可以查看到服务已经成功启动并注册了test这个名字。我们通过test:stop()来停止服务，执行成功，通过whereis(test)查看得到undefined结果，说明服务已经被停止，这种停止方法可行。
2、同步阻塞式停止服务器
将stop函数改为：
stop() ->
gen_server:call(?MODULE, stop).
同时用handle_call处理这条消息:
handle_call(stop,_From, State) ->
{stop, normal, shutdown_ok, State};
同上面编译运行测试的方法，这样的停止服务器也是ok的。调用 stop/0 并且成功停止后调用函数会得到 shutdown_ok的返回值。
3、当一个模块注册的是global名字时，发送stop消息时，要注意模块名称的一致，具体如下:
start_link() ->
gen_server:start_link({global, ?MODULE}, ?MODULE, [], []).
stop() ->
gen_server:call({global, ?MODULE}, stop).
另外一个区别就是，通过注册名称查看进程id时，需要用到global:whereis_name/1函数（关于local和global的区别和适用场合，可以参考erlang doc中的说明）。上面的停止服务器的机制依然正确。
因为游戏中很多应用和功能都是一个gen_server类型的服务器，在结束时需要做很多收尾工作，如状态的收集统计，结果存储和消息广播等等，我们可以在捕获到stop消息时来做这些工作，希望对刚接触erlang游戏开发的同学有一点点启发和帮助，这就是今天我目的。
