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17.7.2 Event accessors

王朝other·作者佚名  2006-05-15
窄屏简体版  字體: |||超大  

[Note: Event declarations typically omit event-accessor-declarations, as in

the Button example above. One

situation for doing so involves the case in which the storage cost of one

field per event is not acceptable. In such

cases, a class can include event-accessor-declarations and use a private

mechanism for storing the list of event

handlers. Similarly, in cases where the handling of an event requires

access to external resources, event accessors

may be used to manage these resources. end note]

The event-accessor-declarations of an event specify the executable

statements associated with adding and

removing event handlers.

The accessor declarations consist of an add-accessor-declaration and a

remove-accessor-declaration. Each

accessor declaration consists of the token add or remove followed by a

block. The block associated with an addaccessor-

declaration specifies the statements to execute when an event handler is

added, and the block associated

with a remove-accessor-declaration specifies the statements to execute when

an event handler is removed.

Each add-accessor-declaration and remove-accessor-declaration corresponds

to a method with a single value

parameter of the event type, and a void return type. The implicit parameter

of an event accessor is named

value. When an event is used in an event assignment, the appropriate event

accessor is used. Specifically, if the

assignment operator is += then the add accessor is used, and if the

assignment operator is ?= then the remove

accessor is used. In either case, the right-hand operand of the assignment

operator is used as the argument to the

event accessor. The block of an add-accessor-declaration or a

remove-accessor-declaration must conform to the

rules for void methods described in §17.5.8. In particular, return

statements in such a block are not permitted

to specify an expression.

Since an event accessor implicitly has a parameter named value, it is a

compile-time error for a local variable or

constant declared in an event accessor to have that name.

C# LANGUAGE SPECIFICATION

250

[Example: In the example

class Control: Component

{

// Unique keys for events

static readonly object mouseDownEventKey = new object();

static readonly object mouseUpEventKey = new object();

// Return event handler associated with key

protected Delegate GetEventHandler(object key) {?}

// Add event handler associated with key

protected void AddEventHandler(object key, Delegate handler) {?}

// Remove event handler associated with key

protected void RemoveEventHandler(object key, Delegate handler) {?}

// MouseDown event

public event MouseEventHandler MouseDown {

add { AddEventHandler(mouseDownEventKey, value); }

remove { RemoveEventHandler(mouseDownEventKey, value); }

}

// MouseUp event

public event MouseEventHandler MouseUp {

add { AddEventHandler(mouseUpEventKey, value); }

remove { RemoveEventHandler(mouseUpEventKey, value); }

}

// Invoke the MouseUp event

protected void OnMouseUp(MouseEventArgs args) {

MouseEventHandler handler;

handler = (MouseEventHandler)GetEventHandler(mouseUpEventKey);

if (handler != null)

handler(this, args);

}

}

the Control class implements an internal storage mechanism for events. The

AddEventHandler method

associates a delegate value with a key, the GetEventHandler method returns

the delegate currently associated

with a key, and the RemoveEventHandler method removes a delegate as an

event handler for the specified

event. Presumably, the underlying storage mechanism is designed such that

there is no cost for associating a

null delegate value with a key, and thus unhandled events consume no

storage. end example]

 
 
 
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