5.记录和宏 | 5. Records and Macros

较大的程序通常被编写为一个文件集合，在各个部分之间有一个定义良好的接口。

5.1更大的例子分成几个文件

为了说明这一点，上一节中的信使示例分为以下五个文件：

• mess_config.hrl配置数据的头文件

• mess_interface.hrl

客户端和信使之间的接口定义

• user_interface.erl用户界面的函数

• mess_client.erl

的客户端的函数。

• mess_server.erl

的服务器端的函数。

这样做时，shell、客户端和服务器之间的消息传递接口将被清理，并使用记录.同时，宏介绍：

%%%----FILE mess_config.hrl----

%%% Configure the location of the server node,
-define(server_node, messenger@super).

%%%----END FILE----

%%%----FILE mess_interface.hrl----

%%% Message interface between client and server and client shell for
%%% messenger program 

%%%Messages from Client to server received in server/1 function.
-record(logon,{client_pid, username}).
-record(message,{client_pid, to_name, message}).
%%% {'EXIT', ClientPid, Reason}  (client terminated or unreachable.

%%% Messages from Server to Client, received in await_result/0 function 
-record(abort_client,{message}).
%%% Messages are: user_exists_at_other_node, 
%%%               you_are_not_logged_on
-record(server_reply,{message}).
%%% Messages are: logged_on
%%%               receiver_not_found
%%%               sent  (Message has been sent (no guarantee)
%%% Messages from Server to Client received in client/1 function
-record(message_from,{from_name, message}).

%%% Messages from shell to Client received in client/1 function
%%% spawn(mess_client, client, [server_node(), Name])
-record(message_to,{to_name, message}).
%%% logoff

%%%----END FILE----

%%%----FILE user_interface.erl----

%%% User interface to the messenger program
%%% login(Name)
%%%     One user at a time can log in from each Erlang node in the
%%%     system messenger: and choose a suitable Name. If the Name
%%%     is already logged in at another node or if someone else is
%%%     already logged in at the same node, login will be rejected
%%%     with a suitable error message.

%%% logoff()
%%%     Logs off anybody at that node

%%% message(ToName, Message)
%%%     sends Message to ToName. Error messages if the user of this 
%%%     function is not logged on or if ToName is not logged on at
%%%     any node.

-module(user_interface).
-export([logon/1, logoff/0, message/2]).
-include("mess_interface.hrl").
-include("mess_config.hrl").

logon(Name) ->
    case whereis(mess_client) of 
        undefined ->
            register(mess_client, 
                     spawn(mess_client, client, [?server_node, Name]));
        _ -> already_logged_on
    end.

logoff() ->
    mess_client ! logoff.

message(ToName, Message) ->
    case whereis(mess_client) of % Test if the client is running
        undefined ->
            not_logged_on;
        _ -> mess_client ! #message_to{to_name=ToName, message=Message},
             ok
end.

%%%----END FILE----

%%%----FILE mess_client.erl----

%%% The client process which runs on each user node

-module(mess_client).
-export([client/2]).
-include("mess_interface.hrl").

client(Server_Node, Name) ->
    {messenger, Server_Node} ! #logon{client_pid=self(), username=Name},
    await_result(),
    client(Server_Node).

client(Server_Node) ->
    receive
        logoff ->
            exit(normal);
        #message_to{to_name=ToName, message=Message} ->
            {messenger, Server_Node} ! 
                #message{client_pid=self(), to_name=ToName, message=Message},
            await_result();
        {message_from, FromName, Message} ->
            io:format("Message from ~p: ~p~n", [FromName, Message])
    end,
    client(Server_Node).

%%% wait for a response from the server
await_result() ->
    receive
        #abort_client{message=Why} ->
            io:format("~p~n", [Why]),
            exit(normal);
        #server_reply{message=What} ->
            io:format("~p~n", [What])
    after 5000 ->
            io:format("No response from server~n", []),
            exit(timeout)
    end.

%%%----END FILE---

%%%----FILE mess_server.erl----

%%% This is the server process of the messenger service

-module(mess_server).
-export([start_server/0, server/0]).
-include("mess_interface.hrl").

server() ->
    process_flag(trap_exit, true),
    server([]).

%%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...]
server(User_List) ->
    io:format("User list = ~p~n", [User_List]),
    receive
        #logon{client_pid=From, username=Name} ->
            New_User_List = server_logon(From, Name, User_List),
            server(New_User_List);
        {'EXIT', From, _} ->
            New_User_List = server_logoff(From, User_List),
            server(New_User_List);
        #message{client_pid=From, to_name=To, message=Message} ->
            server_transfer(From, To, Message, User_List),
            server(User_List)
    end.

%%% Start the server
start_server() ->
    register(messenger, spawn(?MODULE, server, [])).

%%% Server adds a new user to the user list
server_logon(From, Name, User_List) ->
    %% check if logged on anywhere else
    case lists:keymember(Name, 2, User_List) of
        true ->
            From ! #abort_client{message=user_exists_at_other_node},
            User_List;
        false ->
            From ! #server_reply{message=logged_on},
            link(From),
            [{From, Name} | User_List]        %add user to the list
    end.

%%% Server deletes a user from the user list
server_logoff(From, User_List) ->
    lists:keydelete(From, 1, User_List).

%%% Server transfers a message between user
server_transfer(From, To, Message, User_List) ->
    %% check that the user is logged on and who he is
    case lists:keysearch(From, 1, User_List) of
        false ->
            From ! #abort_client{message=you_are_not_logged_on};
        {value, {_, Name}} ->
            server_transfer(From, Name, To, Message, User_List)
    end.
%%% If the user exists, send the message
server_transfer(From, Name, To, Message, User_List) ->
    %% Find the receiver and send the message
    case lists:keysearch(To, 2, User_List) of
        false ->
            From ! #server_reply{message=receiver_not_found};
        {value, {ToPid, To}} ->
            ToPid ! #message_from{from_name=Name, message=Message}, 
            From !  #server_reply{message=sent} 
    end.

%%%----END FILE---

5.2头文件

如上所示，一些文件具有扩展名.hrl。这些是.erl通过以下方式包含在文件中的头文件：

-include("File_Name").

例如：

-include("mess_interface.hrl").

在上面的例子中，文件是从messenger示例中所有其他文件的同一目录中获取的。（*手册*）。

.hrl文件可以包含任何有效的Erlang代码，但最常用于记录和宏定义。

5.3记录

记录定义为：

-record(name_of_record,{field_name1, field_name2, field_name3, ......}).

例如：

-record(message_to,{to_name, message}).

这相当于：

{message_to, To_Name, Message}

创建记录最好用一个例子来说明：

#message_to{message="hello", to_name=fred)

这就产生了：

{message_to, fred, "hello"}

请注意，在创建记录时，您不必担心将值分配给记录的各个部分的顺序。使用记录的优点是，通过将其定义放在头文件中，您可以方便地定义易于更改的界面。例如，如果要向记录添加新字段，则只需更改使用新字段的代码，而不是记录引用的每个位置。如果您在创建记录时忽略了一个字段，它将获取原子的值undefined。（*手册*）

与记录模式匹配与创建记录非常相似。例如，在case或者里面receive：

#message_to{to_name=ToName, message=Message} ->

这与：

{message_to, ToName, Message}

5.4宏

另一件已被添加到信使是一个宏。该文件mess_config.hrl包含定义：

%%% Configure the location of the server node,
-define(server_node, messenger@super).

此文件包含在mess_server.erl*

-include("mess_config.hrl").

?server_nodein的每一次出现mess_server.erl现在都被替换掉了messenger@super。

在生成服务器进程时也使用宏：

spawn(?MODULE, server, [])

这是一个标准宏（即由系统定义，而不是由用户定义）。?MODULE总是由当前模块的名称替换（即-module文件起始位置附近的定义）。有更多先进的方式来使用宏，例如参数（*手动*）。

messenger示例中的三个Erlang（.erl）文件分别编译为目标代码文件（.beam）。Erlang系统在执行代码时将这些文件加载​​并链接到系统中。在这种情况下，它们只是放在我们当前的工作目录中（即，您已完成“cd”到的位置）。有些方法可以将.beam文件放入其他目录中。

在信使的例子中，没有任何关于发送消息的假设。它可以是任何有效的Erlang术语。