我理解事件的目的,特别是在创建用户界面的上下文中。我认为这是创建事件的原型:
public void EventName(object sender, EventArgs e);
事件处理程序做什么,为什么需要它们,以及如何创建一个?
我理解事件的目的,特别是在创建用户界面的上下文中。我认为这是创建事件的原型:
public void EventName(object sender, EventArgs e);
事件处理程序做什么,为什么需要它们,以及如何创建一个?
当前回答
下面是一个代码示例,可能会有所帮助:
using System;
using System.Collections.Generic;
using System.Text;
namespace Event_Example
{
// First we have to define a delegate that acts as a signature for the
// function that is ultimately called when the event is triggered.
// You will notice that the second parameter is of MyEventArgs type.
// This object will contain information about the triggered event.
public delegate void MyEventHandler(object source, MyEventArgs e);
// This is a class which describes the event to the class that receives it.
// An EventArgs class must always derive from System.EventArgs.
public class MyEventArgs : EventArgs
{
private string EventInfo;
public MyEventArgs(string Text) {
EventInfo = Text;
}
public string GetInfo() {
return EventInfo;
}
}
// This next class is the one which contains an event and triggers it
// once an action is performed. For example, lets trigger this event
// once a variable is incremented over a particular value. Notice the
// event uses the MyEventHandler delegate to create a signature
// for the called function.
public class MyClass
{
public event MyEventHandler OnMaximum;
private int i;
private int Maximum = 10;
public int MyValue
{
get { return i; }
set
{
if(value <= Maximum) {
i = value;
}
else
{
// To make sure we only trigger the event if a handler is present
// we check the event to make sure it's not null.
if(OnMaximum != null) {
OnMaximum(this, new MyEventArgs("You've entered " +
value.ToString() +
", but the maximum is " +
Maximum.ToString()));
}
}
}
}
}
class Program
{
// This is the actual method that will be assigned to the event handler
// within the above class. This is where we perform an action once the
// event has been triggered.
static void MaximumReached(object source, MyEventArgs e) {
Console.WriteLine(e.GetInfo());
}
static void Main(string[] args) {
// Now lets test the event contained in the above class.
MyClass MyObject = new MyClass();
MyObject.OnMaximum += new MyEventHandler(MaximumReached);
for(int x = 0; x <= 15; x++) {
MyObject.MyValue = x;
}
Console.ReadLine();
}
}
}
其他回答
我同意KE50的观点,除了我认为'event'关键字是'ActionCollection'的别名,因为事件包含了要执行的操作的集合(例如。委托)。
using System;
namespace test{
class MyTestApp{
//The Event Handler declaration
public delegate void EventAction();
//The Event Action Collection
//Equivalent to
// public List<EventAction> EventActions=new List<EventAction>();
//
public event EventAction EventActions;
//An Action
public void Hello(){
Console.WriteLine("Hello World of events!");
}
//Another Action
public void Goodbye(){
Console.WriteLine("Goodbye Cruel World of events!");
}
public static void Main(){
MyTestApp TestApp = new MyTestApp();
//Add actions to the collection
TestApp.EventActions += TestApp.Hello;
TestApp.EventActions += TestApp.Goodbye;
//Invoke all event actions
if (TestApp.EventActions!= null){
//this peculiar syntax hides the invoke
TestApp.EventActions();
//using the 'ActionCollection' idea:
// foreach(EventAction action in TestApp.EventActions)
// action.Invoke();
}
}
}
}
发布者:事件发生的地方。发布者应该指定类使用哪个委托,并生成必要的参数,将这些参数和自身传递给委托。
订阅者:响应发生的地方。订阅者应指定响应事件的方法。这些方法应该采用与委托相同类型的参数。订阅者然后将此方法添加到发布者的委托。
因此,当事件在publisher中发生时,delegate将收到一些事件参数(数据等),但publisher不知道所有这些数据将会发生什么。订阅者可以在自己的类中创建方法来响应发布者类中的事件,这样订阅者就可以响应发布者的事件。
这实际上是一个事件处理程序的声明——一个在触发事件时被调用的方法。要创建一个事件,你可以这样写:
public class Foo
{
public event EventHandler MyEvent;
}
然后你可以像这样订阅这个事件:
Foo foo = new Foo();
foo.MyEvent += new EventHandler(this.OnMyEvent);
OnMyEvent()的定义如下:
private void OnMyEvent(object sender, EventArgs e)
{
MessageBox.Show("MyEvent fired!");
}
每当Foo触发MyEvent,那么你的OnMyEvent处理程序将被调用。
你不必总是使用EventArgs的实例作为第二个参数。如果你想包含额外的信息,你可以使用一个从EventArgs派生的类(根据惯例EventArgs是基类)。例如,如果你查看WinForms中的Control上定义的一些事件,或者WPF中的FrameworkElement上定义的一些事件,你可以看到将附加信息传递给事件处理程序的事件示例。
下面是一个代码示例,可能会有所帮助:
using System;
using System.Collections.Generic;
using System.Text;
namespace Event_Example
{
// First we have to define a delegate that acts as a signature for the
// function that is ultimately called when the event is triggered.
// You will notice that the second parameter is of MyEventArgs type.
// This object will contain information about the triggered event.
public delegate void MyEventHandler(object source, MyEventArgs e);
// This is a class which describes the event to the class that receives it.
// An EventArgs class must always derive from System.EventArgs.
public class MyEventArgs : EventArgs
{
private string EventInfo;
public MyEventArgs(string Text) {
EventInfo = Text;
}
public string GetInfo() {
return EventInfo;
}
}
// This next class is the one which contains an event and triggers it
// once an action is performed. For example, lets trigger this event
// once a variable is incremented over a particular value. Notice the
// event uses the MyEventHandler delegate to create a signature
// for the called function.
public class MyClass
{
public event MyEventHandler OnMaximum;
private int i;
private int Maximum = 10;
public int MyValue
{
get { return i; }
set
{
if(value <= Maximum) {
i = value;
}
else
{
// To make sure we only trigger the event if a handler is present
// we check the event to make sure it's not null.
if(OnMaximum != null) {
OnMaximum(this, new MyEventArgs("You've entered " +
value.ToString() +
", but the maximum is " +
Maximum.ToString()));
}
}
}
}
}
class Program
{
// This is the actual method that will be assigned to the event handler
// within the above class. This is where we perform an action once the
// event has been triggered.
static void MaximumReached(object source, MyEventArgs e) {
Console.WriteLine(e.GetInfo());
}
static void Main(string[] args) {
// Now lets test the event contained in the above class.
MyClass MyObject = new MyClass();
MyObject.OnMaximum += new MyEventHandler(MaximumReached);
for(int x = 0; x <= 15; x++) {
MyObject.MyValue = x;
}
Console.ReadLine();
}
}
}
//This delegate can be used to point to methods
//which return void and take a string.
public delegate void MyDelegate(string foo);
//This event can cause any method which conforms
//to MyEventHandler to be called.
public event MyDelegate MyEvent;
//Here is some code I want to be executed
//when SomethingHappened fires.
void MyEventHandler(string foo)
{
//Do some stuff
}
//I am creating a delegate (pointer) to HandleSomethingHappened
//and adding it to SomethingHappened's list of "Event Handlers".
myObj.MyEvent += new MyDelegate (MyEventHandler);