5.记录和宏 | 5. Records and Macros
5记录和宏
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。
