Read Summary (11) detail events

What is an event?
An event is a member of a class.If the type defines an event, it can notify other objects of a specific event (such as the Click Event of the button ). An event is a member of the interaction type.
Events are based on the delegation,Events are encapsulated delegates.

I. publisher and subscriber Modes

Understanding this figure is very important for understanding the principles of events:

1. The publisher class defines the event members.
2. subscriber class registers the method to be called when an event member is triggered (Event ProcessingProgram).
3. When the publisher triggers an event, all event handlers in the list will be called.

Ii. Actual cases

This is an example of email Arrival notification. When an email arrives, A newmail event will be triggered, and both fax and pager registered to this event will receive notifications, and process the email in your own way.
CodeAs follows:

 Namespace  Eventdemo2 {  Class Program {  Static   Void Main ( String  [] ARGs) {mailmanager mm = New  Mailmanager (); fax Fax = New  Fax (mm); pager Pager = New  Pager (mm); mm. simulatenewmail (  "  A  " , "  B  " , "  Hi, how are you?  "  ); Console. writeline (); console. writeline (  "  * ******* Pager undo register ********  "  ); Pager. unregister (mm); mm. simulatenewmail (  "  B  " ,"  A  " , "  I'm fine, and you?  "  ); Console. readkey ();}}  //  Publisher class      Internal   Class  Mailmanager {  //  1. Declare the event          Public   Event Eventhandler <newmaileventargs> Newmail;  //  2. trigger event 1-Method of triggering event          Protected   Virtual   Void  Onnewmail (newmaileventargs e ){  If (Newmail! = Null  ) {Newmail (  This  , E );}}  //  3. trigger event 2-convert input to expected event         Public   Void Simulatenewmail ( String   From , String To, String  Subject) {newmaileventargs E = New Newmaileventargs ( From  , To, subject); onnewmail (e );}}  //  Custom class, which transmits data by extending eventargs  // The information sent to the event recipient is contained here.      Internal   Class  Newmaileventargs: eventargs {  Private   Readonly   String  _ From, _ to, _ subject;  Public Newmaileventargs ( String   From , String To, String  Subject) {_ from =From  ; _ = To; _ subject = Subject ;}  Public   String  From {  Get { Return  _ From ;}}  Public   String  To {  Get {Return  _ ;}}  Public   String  Subject {  Get { Return  _ Subject ;}}}  //  Subscriber Class 1      Internal   Class  Fax {  Public  Fax (mailmanager mm) {mm. newmail + = New Eventhandler <newmaileventargs> (faxmsg ); //  Register an event handler  //  Mm. newmail + = faxmsg;  //  Equivalent to the above  }  Public   Void  Unregister (mailmanager mm) {mm. newmail -= New Eventhandler <newmaileventargs> (faxmsg ); // Cancel event handler  }  Private   Void Faxmsg ( Object Sender, newmaileventargs E) //  Note: The return type and signature of the handler must be consistent with the return type and signature of the event Delegate.  {Console. writeline (  "  Faxing mail message:  "  ); Console. writeline (  "  From: {0}, To = {1}, subject = {2} "  , E. From, E. To, E. Subject );}}  //  Subscriber Category 2      Internal   Class  Pager {  Public  Pager (mailmanager mm) {mm. newmail + = Pagermsg; //  Register an event handler  }  Public   Void Unregister (mailmanager mm) {mm. newmail -= Pagermsg; //  Cancel event handler  }  Private   Void Pagermsg ( Object  Sender, newmaileventargs e) {console. writeline (  "  Pager mail message:  "  ); Console. writeline (  " From: {0}, To = {1}, subject = {2}  "  , E. From, E. To, E. Subject );}}} 

Shows the program output result.

Note: 1, + = and-= are the only operators allowed to be used in an event, representing the registration of the event handler and the cancellation of the event handler.
2. Do not mistakenly think that an event is a type, and an event is a type member. Therefore, it cannot use an object creation expression (new expression) to create its object.
3. The call event is similar to the call delegate (similar to the call method), but note that its parameters must match the event Delegate.
4. There are multiple methods to subscribe to events (register an event handler), such as the instance method, static method, anonymous method, or Lambda expression. The most common method is to use a delegated instance. For example:
BTN. Click + = new eventhandler (btn_click); // equivalent to: BTN. Click + = btn_click;
5. eventargs, the second parameter of the eventhandler delegate, cannot transmit any data by default. To be designed to transmit data, you must customize a class inherited from eventargs, use private fields to save the data to be passed.