事件模式与观察者模式

来源：互联网发布：java catch return 编辑：程序博客网时间：2024/05/22 08:22

学习C#的事件模式时Delegate和event这二个概念比较难理解，因为这二个都被设计成关键字，想进一步查看其定义和实现就没有下文了，不过再难也得硬着头皮弄明白，我以前好长时间都是似懂非懂的状态，后来在学习了观察者设计模式和IL后利用这二者的知识来理解就比较容易。下面我就用观察者设计模式和IL来分析一下事件模式中Delegate和event这二个关键字。

首先，我们来了解一下delegate究竟是什么东西，先来看一下下面的程序（为了演示，代码尽量简略）。

Using System;

public class Test

{

public delegatevoid aaa();

public static void Main(String[]args) {

aaaa=new aaa(show);

a();

}

public static voidshow()

{

Console.WriteLine("hi");

}

以上代码用csc.exe编译后再用ILDASM反编译成文本文件：

.class public auto ansibeforefieldinit Test extends [mscorlib]System.Object

{

.class auto ansisealed nestedpublicaaa extends[mscorlib]System.MulticastDelegate

{

.method public hidebysig specialnamertspecialname instance void .ctor(object 'object', native int'method') runtime managed

{

} // end of method aaa::.ctor

.method public hidebysig newslotvirtual instance void Invoke() runtime managed

{

} //end of method aaa::Invoke

.method public hidebysig newslotvirtual instance class[mscorlib]System.IAsyncResult BeginInvoke(class [mscorlib]System.AsyncCallback callback,

object 'object')runtime managed

{

} // end of method aaa::BeginInvoke

.method public hidebysig newslotvirtual instance void EndInvoke(class [mscorlib]System.IAsyncResultresult) runtime managed

{

} // end of method aaa::EndInvoke

} // end of class aaa

.method public hidebysig static void Main(string[] args) cil managed

{

.entrypoint

.maxstack 2

.locals init (class Test/aaa V_0)

IL_0000: nop

IL_0001: ldnull

//将show方法的指针推送到计算堆栈上

IL_0002: ldftn void Test::show()

IL_0008: newobj instance voidTest/aaa::.ctor(object,

native int)

IL_000d: stloc.0

IL_000e: ldloc.0

IL_000f: callvirt instance voidTest/aaa::Invoke()

IL_0014: nop

IL_0015: ret

} // end of method Test::Main

.method public hidebysig static void show() cil managed

{

.maxstack 8

IL_0000: nop

IL_0001: ldstr "hi"

IL_0006: call void[mscorlib]System.Console::WriteLine(string)

IL_000b: nop

IL_000c: ret

} // end of method Test::show

.method public hidebysig specialnamertspecialname

instance void .ctor() cil managed

{

.maxstack 8

IL_0000: ldarg.0

IL_0001: call instance void[mscorlib]System.Object::.ctor()

IL_0006: ret

} // end of method Test::.ctor

} // end of class Test

从反编译出来IL指令中可以看到delegate关键字修饰的aaa由编译器转换为一个继承自System.MulticastDelegate的类，然后在main方法中以方法show的指针作参数构造一个aaa的实例，后面再通过该实例调用Invoke方法，相信至此对delegate关键字应该明白其作用了。

要了解事件模式的话，光是了解delegate还不够，我们还得看看event是什么玩艺：

using System;

public class Test {

public static voidMain(String[] args) {

EventTest et = newEventTest();

et.OnPropertyChanged+=show;

et.Age=2;

}

public static void show()

{

Console.WriteLine("event trigged");

}

public class EventTest

{

public delegate voidPropertyChangedHandler();

public eventPropertyChangedHandler OnPropertyChanged;

private int age;

public int Age

{

get { return age; }

set

{

age = value;

if(OnPropertyChanged!=null)

{

OnPropertyChanged();

}

学过MVVM模式的朋友对以上代码是否有似曾相识的感觉，用csc.exe编译后再用ILDASM反编译成文本文件：

.class public auto ansi beforefieldinit Test extends[mscorlib]System.Object

{

.method public hidebysig staticvoid Main(string[] args) cil managed

{

.entrypoint

.maxstack 3

.locals init (class EventTestV_0)

IL_0000: nop

IL_0001: newobj instance voidEventTest::.ctor()

IL_0006: stloc.0

IL_0007: ldloc.0

IL_0008: ldnull

IL_0009: ldftn void Test::show()

IL_000f: newobj instance void EventTest/PropertyChangedHandler::.ctor(object,native int)

IL_0014: callvirt instance void EventTest::add_OnPropertyChanged(classEventTest/PropertyChangedHandler)

IL_0019: nop

IL_001a: ldloc.0

IL_001b: ldc.i4.2

IL_001c: callvirt instance void EventTest::set_Age(int32)

IL_0021: nop

IL_0022: ret

} // end of method Test::Main

.method public hidebysig staticvoid show() cil managed

{

.maxstack 8

IL_0000: nop

IL_0001: ldstr "event trigged"

IL_0006: call void [mscorlib]System.Console::WriteLine(string)

IL_000b: nop

IL_000c: ret

} // end of method Test::show

.method public hidebysigspecialname rtspecialname instancevoid .ctor() cil managed

{

.maxstack 8

IL_0000: ldarg.0

IL_0001: call instance void [mscorlib]System.Object::.ctor()

IL_0006: ret

} // end of method Test::.ctor

} // end of class Test

.class public auto ansi beforefieldinitEventTest extends [mscorlib]System.Object

{

.class auto ansi sealed nestedpublic PropertyChangedHandler extends[mscorlib]System.MulticastDelegate

{

.method public hidebysigspecialname rtspecialname instancevoid .ctor(object 'object', native int 'method') runtime managed

{

} // end of methodPropertyChangedHandler::.ctor

.method public hidebysig newslotvirtual instance void Invoke() runtime managed

{

} // end of methodPropertyChangedHandler::Invoke

.method public hidebysig newslotvirtual instance class[mscorlib]System.IAsyncResult

BeginInvoke(class[mscorlib]System.AsyncCallback callback, object 'object') runtime managed

{

} // end of methodPropertyChangedHandler::BeginInvoke

.method public hidebysig newslotvirtual instance void EndInvoke(class [mscorlib]System.IAsyncResultresult) runtime managed

{

} // end of methodPropertyChangedHandler::EndInvoke

} // end of classPropertyChangedHandler

.field private classEventTest/PropertyChangedHandlerOnPropertyChanged

.field private int32 age

.method public hidebysigspecialname instance void add_OnPropertyChanged(classEventTest/PropertyChangedHandler 'value') cil managed

{

.maxstack 3

.locals init (classEventTest/PropertyChangedHandler V_0,

classEventTest/PropertyChangedHandler V_1,

classEventTest/PropertyChangedHandler V_2,

bool V_3)

IL_0000: ldarg.0

IL_0001: ldfld class EventTest/PropertyChangedHandler EventTest::OnPropertyChanged

IL_0006: stloc.0

IL_0007: ldloc.0

IL_0008: stloc.1

IL_0009: ldloc.1

IL_000a: ldarg.1

IL_000b: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Combine(class[mscorlib]System.Delegate,

class [mscorlib]System.Delegate)

IL_0010: castclass EventTest/PropertyChangedHandler

IL_0015: stloc.2

IL_0016: ldarg.0

IL_0017: ldflda class EventTest/PropertyChangedHandler EventTest::OnPropertyChanged

IL_001c: ldloc.2

IL_001d: ldloc.1

IL_001e: call !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<classEventTest/PropertyChangedHandler>(!!0&, !!0, !!0)

IL_0023: stloc.0

IL_0024: ldloc.0

IL_0025: ldloc.1

IL_0026: ceq

IL_0028: ldc.i4.0

IL_0029: ceq

IL_002b: stloc.3

IL_002c: ldloc.3

IL_002d: brtrue.s IL_0007

IL_002f: ret

} // end of methodEventTest::add_OnPropertyChanged

.method public hidebysigspecialname instance void

remove_OnPropertyChanged(classEventTest/PropertyChangedHandler 'value') cil managed

{

.maxstack 3

.locals init (classEventTest/PropertyChangedHandler V_0,

classEventTest/PropertyChangedHandler V_1,

classEventTest/PropertyChangedHandler V_2,

bool V_3)

IL_0000: ldarg.0

IL_0001: ldfld class EventTest/PropertyChangedHandler EventTest::OnPropertyChanged

IL_0006: stloc.0

IL_0007: ldloc.0

IL_0008: stloc.1

IL_0009: ldloc.1

IL_000a: ldarg.1

IL_000b: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Remove(class [mscorlib]System.Delegate,

class [mscorlib]System.Delegate)

IL_0010: castclass EventTest/PropertyChangedHandler

IL_0015: stloc.2

IL_0016: ldarg.0

IL_0017: ldflda class EventTest/PropertyChangedHandler EventTest::OnPropertyChanged

IL_001c: ldloc.2

IL_001d: ldloc.1

IL_001e: call !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<classEventTest/PropertyChangedHandler>(!!0, !!0, !!0)

IL_0023: stloc.0

IL_0024: ldloc.0

IL_0025: ldloc.1

IL_0026: ceq

IL_0028: ldc.i4.0

IL_0029: ceq

IL_002b: stloc.3

IL_002c: ldloc.3

IL_002d: brtrue.s IL_0007

IL_002f: ret

} // end of methodEventTest::remove_OnPropertyChanged

.method public hidebysigspecialname instance int32 get_Age() cil managed

{

.maxstack 1

.locals init (int32 V_0)

IL_0000: nop

IL_0001: ldarg.0

IL_0002: ldfld int32 EventTest::age

IL_0007: stloc.0

IL_0008: br.s IL_000a

IL_000a: ldloc.0

IL_000b: ret

} // end of methodEventTest::get_Age

.method public hidebysigspecialname instance void set_Age(int32'value') cil managed

{

.maxstack 2

.locals init (bool V_0)

IL_0000: nop

IL_0001: ldarg.0

IL_0002: ldarg.1

IL_0003: stfld int32 EventTest::age

IL_0008: ldarg.0

IL_0009: ldfld class EventTest/PropertyChangedHandlerEventTest::OnPropertyChanged

IL_000e: ldnull

IL_000f: ceq

IL_0011: stloc.0

IL_0012: ldloc.0

IL_0013: brtrue.s IL_0023

IL_0015: nop

IL_0016: ldarg.0

IL_0017: ldfld class EventTest/PropertyChangedHandler EventTest::OnPropertyChanged

IL_001c: callvirt instance void EventTest/PropertyChangedHandler::Invoke()

IL_0021: nop

IL_0022: nop

IL_0023: ret

} // end of methodEventTest::set_Age

.method public hidebysigspecialname rtspecialname instance void .ctor() cil managed

{

.maxstack 8

IL_0000: ldarg.0

IL_0001: call instance void [mscorlib]System.Object::.ctor()

IL_0006: ret

} // end of methodEventTest::.ctor

.eventEventTest/PropertyChangedHandler OnPropertyChanged

{

.addon instancevoid EventTest::add_OnPropertyChanged(class EventTest/PropertyChangedHandler)

.removeoninstance void EventTest::remove_OnPropertyChanged(classEventTest/PropertyChangedHandler)

} // end of eventEventTest::OnPropertyChanged

.property instance int32 Age()

{

.get instance int32EventTest::get_Age()

.set instance voidEventTest::set_Age(int32)

} // end of propertyEventTest::Age

} // end of class EventTest

用event声明事件后编译器在编译时给类中加了二个方法：

add_OnPropertyChanged(class EventTest/PropertyChangedHandler 'value')、

remove _OnPropertyChanged(class EventTest/PropertyChangedHandler 'value')。

在这二个方法中有二个调用:

System.Delegate::Combine(class [mscorlib]System.Delegate, class[mscorlib]System.Delegate)、

System.Delegate::Remove(class[mscorlib]System.Delegate, class[mscorlib]System.Delegate)，

查一下MSDN这二个调用是干什么的：

Combine方法将两个委托的调用列表连接在一起，Remove方法从一个委托的调用列表中移除另一个委托的最后一个调用列表。

委托里有个调用列表！

挖到这里，学过观察者模式的朋友是否有似曾相识的感觉：观察者模式中被观察者也有一个订阅者列表，有Add和Remove方法。事件模式不就是观察者模式吗？，不过在C#中用delegate来实现观察者模式中的更新状态的接口和订阅者列表，用event关键字实现的订阅者的添加和删除。

理解了事件模式和观察者模式的关系，对MVVM模式中的Notification类也就不难理解了。

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