MATLAB event 和 listener 简介

文章目录

Event and Listener Concepts 概念
- The Event Model 模型
- Limitations
- Default Event Data
- - Customize Event Data
- Events Only in Handle Classes
- Property-Set and Query Events
Events and Listeners Syntax 创建方法
- Components to Implement
- Name Events
- Trigger Events
- Listen to Events
- - Use addlistener for Persistent Listeners
  - Use handle.listener to Decouple Listener and Source
  - Callback Function
  - Define Listener
  - Remove Listeners
- Define Event-Specific Data

本节内容都来自与官网，稍加翻译

Event and Listener Concepts 概念

The Event Model 模型

Events represent changes or actions that occur within objects. For example,

Modification of class data 修改了类的数据
Execution of a method 执行了一个方法
Querying or setting a property value 查询或设置了一个属性值
Destruction of an object 销毁一个对象

Basically, any activity that you can detect programmatically can generate an event and communicate information to other objects. 所有的活动都能够唤起一个event并且把信息传递给其他的对象

MATLAB® classes define a process that communicates the occurrence of events to other objects that respond to the events. The event model works this way:

MATLAB定义了一个把event发生的信息传递给其他对象的流程，event模型按照以下方式工作

A handle class declares a name used to represent an event. Name Events 声明一个名称来表示event
After creating an object of the event-declaring class, attach listener to that object. Control Listener Lifecycle 在创建一个能检测event的类的对象后，附加给它一个listener
A call to the handle class notify method broadcasts a notice of the event to listeners. The class user determines when to trigger the event. Trigger Events 调用notify函数，把event发生的通知广播给各listener，什么时候唤起event由class的使用者决定
Listeners execute a callback function when notified that the event has occurred. Specifying Listener Callbacks listener在侦测到event发生的信号后调用callback function
You can bind listeners to the lifecycle of the object that defines the event, or limit listeners to the existence and scope of the listener object. Control Listener Lifecycle 你能够把listener绑定到定义了event的对象的生命周期上，或把listener的声明周期限制为listener object的生命周期

The following diagram illustrates the event model.

Limitations

There are certain limitations to the use of events:

The event source cannot guarantee that listeners exist when triggering the event. event源不能保证所有listener能够在它被触发时还存在
A listener cannot prevent other listeners from being notified that the event occurred. 一个listener不能够阻止其他event侦听event的发生
The order in which listeners execute is not defined. listener执行的顺序没有定义
Listeners should not modify the event data object passed to the listener callback, because other listeners are passed this same handle object. listener不应该修改传送给callback function的event data，因为其它listener也被传送了相同的handle object

Default Event Data

Events provide information to listener callbacks by passing an event data argument to the callback function. By default, MATLAB passes an event.EventData object to the listener callback. This object has two properties:

EventName — The event name as defined in the class event block
Source — The object that is the source of the event

MATLAB passes the source object to the listener callback in the required event data argument. Use the source object to access any of the object’s public properties from within your listener callback function.

Customize Event Data

You can create a subclass of the event.EventData class to provide additional information to listener callback functions. The subclass would define properties to contain the additional data and provide a method to construct the derived event data object so it can be passed to the notify method.

Define Event-Specific Data provides an example showing how to customize this data.

Events Only in Handle Classes

You can define events only in handle classes. This restriction exists because a value class is visible only in a single MATLAB workspace so no callback or listener can have access to the object that triggered the event. The callback could have access to a copy of the object. However, accessing a copy is not useful because the callback cannot access the current state of the object that triggered the event or effect any changes in that object.

Comparison of Handle and Value Classes provides general information on handle classes.

Events and Listeners Syntax shows the syntax for defining a handle class and events.

Property-Set and Query Events

There are four predefined events related to properties:

PreSet — Triggered just before the property value is set, before calling its set access method
PostSet — Triggered just after the property value is set
PreGet — Triggered just before a property value query is serviced, before calling its get access method
PostGet — Triggered just after returning the property value to the query

These events are predefined and do not need to be listed in the class events block.

When a property event occurs, the callback is passed an event.PropertyEvent object. This object has three properties:

EventName — The name of the event described by this data object
Source — The source object whose class defines the event described by the data object
AffectedObject — The object whose property is the source for this event (that is, AffectedObject contains the object whose property was either accessed or modified).

You can define your own property-change event data by subclassing the event.EventData class. The event.PropertyEvent class is a sealed subclass of event.EventData.

See Listen for Changes to Property Values for a description of the process for creating property listeners.

See The PostSet Event Listener for an example.

See Property Access Methods for information on methods that control access to property values.

Events and Listeners Syntax 创建方法

Components to Implement

Implementation of events and listeners involves these components: 创建events和listeners包含如下部分：

Specification of the name of an event in a handle class — Name Events. 命名event
A function or method to trigger the event when the action occurs — Trigger Events. 唤起event
Listener objects to execute callback functions in response to the triggered event — Listen to Events. 侦听event
Default or custom event data that the event passes to the callback functions — Define Event-Specific Data. event data传递给callback function

Name Events

Define an event by declaring an event name inside an events block. For example, this class creates an event called ToggledState:

classdef ToggleButton < handlepropertiesState = falseendevents %定义eventToggledStateend
end

注意是handle类

Trigger Events

The OnStateChange method calls notify to trigger the ToggledState event. Pass the handle of the object that is the source of the event and the name of the event to notify.

classdef ToggleButton < handlepropertiesState = falseendeventsToggledState %定义eventendmethodsfunction OnStateChange(obj,newState)if newState ~= obj.Stateobj.State = newState;%修改了State属性，唤起ToggledStatenotify(obj,'ToggledState');endendend
end

Listen to Events

After the call to notify triggers an event, MATLAB® broadcasts a message to all listeners that are defined for that event and source object. There are two ways to create listeners: using the handle class addlistener or listener method.

在notify触发了event之后，MATLAB把信息传送给所有订阅了这个event的listener

Use addlistener for Persistent Listeners

If you want the listener to persist beyond the normal variable scope, use addlistener to create it. The event source object holds a reference to the listener object. When the event source object is destroyed, MATLAB destroys the listener.

This code defines a listener for the ToggleState event: 定义一个event ‘ToggledState’ 的 listener

lh = addlistener(obj,'ToggleState',@RespondToToggle.handleEvnt);

addlistener has these arguments:

obj — The object that is the source of the event
ToggleState — The event name passed as a char vector
@RespondToToggle.handleEvnt — A function handle to the callback function (see the following definition Define Listener).

Use handle.listener to Decouple Listener and Source

Use the listener method to create listeners when you want to manage the lifecycle of the listener and do not want a coupling between the event source and listener object. MATLAB does not destroy listeners created with listener when the event source is destroyed. However, your code must keep the listener object handle in scope when creating listeners using listener. 如果想控制listener的生命周期，不想把listener的生命周期绑定在event source上，使用listener方法，在event source被销毁后，listener不会被销毁

The listener method requires the same arguments as addlistener: the event-naming object, the event name, and a function handle to the callback. listener returns the handle to the listener object. listener使用方法：

lh = listener(obj,'EventName',@callbackFunction)

For example, this code uses the ToggleState event discussed previously: 范例：

lh = listener(obj,'ToggleState',@RespondToToggle.handleEvnt)

Callback Function

The listener callback function must accept a minimum of two arguments, which MATLAB automatically passes to the callback. Here are the required arguments:

The source of the event — that is, obj in the call to addlistener or event.listener.
An event.EventData object or a subclass of event.EventData, such as the ToggleEventData object described in, Define Event-Specific Data.

Define the callback function to accept the source object and event data arguments.

function callbackFunction(src,evtdata)...
end

For more information on callback syntax, see Listener Callback Syntax.

Define Listener

The RespondToToggle class defines objects that listen for the ToggleState event defined in the ToggleButton class.

classdef RespondToToggle < handlemethodsfunction obj = RespondToToggle(toggle_button_obj)addlistener(toggle_button_obj,'ToggledState',@RespondToToggle.handleEvnt); %给类添加一个listener，能够响应event 'ToggledState'endendmethods (Static)function handleEvnt(src,~) %这时callback functionif src.Statedisp('ToggledState is true')elsedisp('ToggledState is false')endendend
end

注意是handle类
The class RespondToToggle adds the listener in its constructor. In this case, the class defines the callback (handleEvnt) as a static method that accepts the two required arguments:

src — The handle of the object triggering the event (that is, a ToggleButton object)
evtdata — An event.EventData object

For example, this code creates objects of both classes:

tb = ToggleButton;
rtt = RespondToToggle(tb);

Whenever you call the OnStateChange method of the ToggleButton object, notify triggers the event. For this example, the callback displays the value of the State property:

tb.OnStateChange(true)
ToggledState is true
tb.OnStateChange(false)
ToggledState is false

Remove Listeners

Remove a listener object by calling delete on its handle. For example, if the class RespondToToggle saved the listener handle as a property, you could delete the listener.

classdef RespondToToggle < handlepropertiesListenerHandle % Property for listener handleendmethodsfunction obj = RespondToToggle(toggle_button_obj)hl = addlistener(toggle_button_obj,'ToggledState',@RespondToToggle.handleEvnt);obj.ListenerHandle = hl; % Save listener handleendendmethods (Static)function handleEvnt(src,~)if src.Statedisp('ToggledState is true')elsedisp('ToggledState is false')endendend
end

With this code change, you can remove the listener from an instance of the RespondToToggle class. For example: 当代码改变时，你能够从RespondToToggle的一个对象中删除listener

tb = ToggleButton; %定义一个触发event的对象
rtt = RespondToToggle(tb); %定义一个侦测了它的对象

The object rtt is listening for the ToggleState event triggered by object tb. To remove the listener, call delete on the property containing the listener handle.
rtt 侦测了 tb 的 event ToggleState，如果想删除listener，调用delete

delete(rtt.ListenerHandle)

To deactivate a listener temporarily, see Temporarily Deactivate Listeners.

Define Event-Specific Data

Suppose that you want to pass the state of the toggle button as a result of the event to the listener callback. You can add more data to the default event data by subclassing the event.EventData class and adding a property to contain this information. Then you can pass this object to the notify method.

Note

To save and load objects that are subclasses of event.EventData, such as ToggleEventData, enable the ConstructOnLoad class attribute for the subclass.

classdef (ConstructOnLoad) ToggleEventData < event.EventDatapropertiesNewStateendmethodsfunction data = ToggleEventData(newState)data.NewState = newState;endend
end

The call to notify can use the ToggleEventData constructor to create the necessary argument.

evtdata = ToggleEventData(newState);
notify(obj,'ToggledState',evtdata);