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Erlang OTP: Distributed & Fault-Tolerant Systems Programming · 课时

使用 GenEvent 处理事件

学习使用 GenEvent 创建解耦的事件处理系统，让发布者和订阅者能够异步交互。

使用 GenEvent 处理事件 是 CoddyKit 上的免费 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课时。 这是第 2 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Why Event Handling?

Imagine you have many parts of your application that need to react to something happening, like a user logging in.

If each part directly calls every other part, your code becomes tightly coupled and hard to change. This is where event handling comes in!

Introducing GenEvent

Erlang's OTP (Open Telecom Platform) provides a powerful behavior called gen_event for building decoupled event handling systems.

  • It implements the publisher-subscriber pattern.
  • Publishers send events without knowing who will receive them.
  • Subscribers (called handlers) register to receive specific events.
  • This promotes loose coupling and makes systems more flexible.

The Event Manager

At the heart of gen_event is an event manager. This is a special Erlang process that acts as a central hub:

  • It receives events from publishers.
  • It dispatches these events to all registered handlers.

You start an event manager just like other OTP behaviors:

-module(event_manager_starter).
-export([start/0]).

start() ->
    % Start a local event manager named 'my_app_events'
    {ok, Pid} = gen_event:start_link({local, my_app_events}),
    io:format("Event manager started with PID: ~p~n", [Pid]),
    ok.

Event Handlers: The Listeners

An event handler is an Erlang module that implements the gen_event behavior. It defines callback functions that the event manager will call when an event occurs.

Think of handlers as specialized listeners, each interested in certain types of events or performing a specific action when an event is received.

Creating a Simple Handler

Here's the basic structure for an event handler module. The key callback is handle_event/2, where you process the incoming event.

-module(my_event_handler).
-behaviour(gen_event).

-export([init/1, handle_event/2, terminate/2]).

init(Args) ->
    io:format("Handler initialized with args: ~p~n", [Args]),
    {ok, []}. % Returns initial state

handle_event(Event, State) ->
    io:format("Handler received event: ~p~n", [Event]),
    % Here you'd process the event
    {ok, State}. % Return new state

terminate(_Args, _State) ->
    io:format("Handler terminating.~n", []),
    ok.

Adding Handlers to the Manager

After starting your event manager and defining your handler, you need to tell the manager about your handler. This is done using gen_event:add_handler/3.

You can add multiple handlers to the same manager, and each will receive the events.

-module(handler_adder).
-export([add_to_manager/1]).

add_to_manager(ManagerName) ->
    % The handler module name (my_event_handler)
    % and any initial arguments for its init/1 function ([] in this case)
    gen_event:add_handler(ManagerName, my_event_handler, []),
    io:format("Handler 'my_event_handler' added to '~p'.~n", [ManagerName]),
    ok.

Notifying the Manager (Publishing Events)

Once handlers are registered, any part of your application can send an event to the manager using gen_event:notify/2. The manager will then forward this event to all active handlers.

The publisher doesn't know (or care) how many handlers there are, or what they do. This is the power of decoupling!

-module(event_publisher).
-export([send_alert/2]).

send_alert(ManagerName, Message) ->
    Event = {alert, Message, os:timestamp()},
    io:format("Publisher sending event: ~p to manager '~p'.~n", [Event, ManagerName]),
    gen_event:notify(ManagerName, Event).

Full GenEvent Demo

Let's put it all together! This runnable example starts a manager, adds a simple handler, and then sends a few events. Watch the handler react!

-module(gen_event_demo).
-export([start/0]).

% --- Inline Handler Module for Demo ---
% In a real app, this would be a separate .erl file.
my_demo_handler() ->
    % Compile and load a temporary handler module
    code:delete(demo_handler_impl),
    code:purge(demo_handler_impl),
    {ok, _} = compile:forms([
        '-module(demo_handler_impl).',
        '-behaviour(gen_event).',
        '-export([init/1, handle_event/2, terminate/2]).',
        'init(Args) -> {ok, Args}.',
        'handle_event(Event, State) ->',
        '    io:format("*** Handler received: ~p~n", [Event]),',
        '    {ok, State}.',
        'terminate(_Args, _State) -> ok.'
    ], []),
    demo_handler_impl.

% --- Main Application Logic ---
start() ->
    ManagerName = my_system_events,
    HandlerModule = my_demo_handler(),

    io:format("--- Starting GenEvent Demo ---~n"),

    % 1. Start the event manager
    {ok, _ManagerPid} = gen_event:start_link({local, ManagerName}),
    io:format("Manager '~p' started.~n", [ManagerName]),

    % 2. Add the handler to the manager
    gen_event:add_handler(ManagerName, HandlerModule, []), 
    io:format("Handler '~p' added.~n", [HandlerModule]),

    % 3. Publish some events
    io:format("Sending first event...~n"),
    gen_event:notify(ManagerName, {user_logged_in, "Alice"}),
    timer:sleep(100), % Give time for event processing

    io:format("Sending second event...~n"),
    gen_event:notify(ManagerName, {sensor_reading, 25.5}),
    timer:sleep(100),

    io:format("Sending third event...~n"),
    gen_event:notify(ManagerName, {error_alert, "Disk_full"}),
    timer:sleep(100),

    % 4. Stop the manager (optional, for clean exit)
    gen_event:stop(ManagerName),
    io:format("--- GenEvent Demo Finished ---~n"),
    ok.

Advanced GenEvent Usage

gen_event is quite flexible:

  • Handler State: Handlers can maintain their own internal state, updated with each event.
  • Multiple Handlers: A single event manager can have many handlers, each processing events independently.
  • Removing Handlers: Handlers can be dynamically removed using gen_event:delete_handler/3.
  • Synchronous Calls: While primarily asynchronous, gen_event:call/2 allows synchronous calls to handlers (though less common for pure eventing).

GenEvent Quick Check

You've learned about GenEvent's core components and how they interact. Let's test your understanding!

Recap: GenEvent Essentials

Great job! You've learned how gen_event helps build flexible, decoupled systems:

  • Event Manager: The central hub that dispatches events.
  • Event Handlers: Modules that subscribe to and process events.
  • Publishers: Send events to the manager using gen_event:notify/2.
  • Decoupling: Key benefit, allowing parts of your system to evolve independently.

Next, you'll explore even more advanced OTP behaviors!

常见问题解答

「使用 GenEvent 处理事件」课时是免费的吗？

是的 — 「使用 GenEvent 处理事件」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程的其余内容，请升级到 CoddyKit PRO。 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程共包含 4 节课。

「使用 GenEvent 处理事件」这节课中我会学到什么？

学习使用 GenEvent 创建解耦的事件处理系统，让发布者和订阅者能够异步交互。 你通过在浏览器中直接运行的动手代码来练习 Erlang OTP: Distributed & Fault-Tolerant Systems Programming，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 2 节课，共 4 节。

「使用 GenEvent 处理事件」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课中编写并运行代码吗？

能。每节 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

  1. 使用 GenStatem 管理状态
  2. 使用 GenEvent 处理事件
  3. 自定义 OTP 行为
  4. 热代码替换与在线升级
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