[no0000130]wpf 4.5使用标记扩展订阅事件 -ag凯发k8国际

自从我上次写到关于标记扩展的时候已经有一段时间了...... visual studio 11 developer preview的发布给wpf带来了一些新功能，让我有理由再次使用它们。我要在这里讨论的功能可能不是最令人印象深刻的，但它填补了以前版本的空白：支持事件标记扩展。

到目前为止，可以在xaml中使用标记扩展来为属性赋值，但我们无法做到这一点来订阅事件。在wpf 4.5中，现在有可能。所以这里是我们可以用它做的一个小例子...

当使用mvvm模式时，我们通常通过绑定机制将viewmodel的命令与视图的控件关联起来。这种方法通常运行良好，但它有一些缺点：

• 它在viewmodel中引入了很多样板代码
• 并不是所有的控件都有一个command属性（实际上大多数不是），当这个属性存在时，它只对应于控件的一个事件（例如点击一个按钮）。没有真正简单的方法将其他事件“绑定”到viewmodel的命令

能够直接将事件绑定到viewmodel方法会很好，如下所示：

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使用onclickviewmodel中定义的方法：

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public void onclick(object sender, eventargs e) {     messagebox.show("hello world!"); }

那么，现在可以！这是一个概念证明...下面eventbindingextension显示的类首先获取datacontext控件，然后在该对象上查找指定的方法datacontext，并最终返回此方法的委托：

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using system; using system.collections.generic; using system.componentmodel; using system.diagnostics; using system.linq; using system.reflection; using system.windows; using system.windows.markup;     public class eventbindingextension : markupextension     {         public eventbindingextension() { }         public eventbindingextension(string eventhandlername)         {             this.eventhandlername = eventhandlername;         }         [constructorargument("eventhandlername")]         public string eventhandlername { get; set; }         public override object providevalue(iserviceprovider serviceprovider)         {             if (string.isnullorempty(eventhandlername))                 throw new argumentexception("the eventhandlername property is not set", "eventhandlername");             var target = (iprovidevaluetarget)serviceprovider.getservice(typeof(iprovidevaluetarget));             eventinfo eventinfo = target.targetproperty as eventinfo;             if (eventinfo == null)                 throw new invalidoperationexception("the target property must be an event");                           object datacontext = getdatacontext(target.targetobject);             if (datacontext == null)                 throw new invalidoperationexception("no datacontext found");             var handler = gethandler(datacontext, eventinfo, eventhandlername);             if (handler == null)                 throw new argumentexception("no valid event handler was found", "eventhandlername");             return handler;         }         #region helper methods         static object gethandler(object datacontext, eventinfo eventinfo, string eventhandlername)         {             type dctype = datacontext.gettype();             var method = dctype.getmethod(                 eventhandlername,                 getparametertypes(eventinfo));             if (method != null)             {                 if (method.isstatic)                     return delegate.createdelegate(eventinfo.eventhandlertype, method);                 else                     return delegate.createdelegate(eventinfo.eventhandlertype, datacontext, method);             }             return null;         }         static type[] getparametertypes(eventinfo eventinfo)         {             var invokemethod = eventinfo.eventhandlertype.getmethod("invoke");             return invokemethod.getparameters().select(p => p.parametertype).toarray();         }         static object getdatacontext(object target)         {             var depobj = target as dependencyobject;             if (depobj == null)                 return null;             return depobj.getvalue(frameworkelement.datacontextproperty)                 ?? depobj.getvalue(frameworkcontentelement.datacontextproperty);         }         #endregion     }

这个类可以像上面例子中所示的那样使用。

现在，这个标记扩展有一个令人讨厌的限制：datacontext必须在调用之前设置providevalue，否则将无法找到事件处理程序方法。一种ag凯发k8国际的解决方案可能是订阅datacontextchanged事件以在datacontext设置后查找方法，但同时我们仍然需要返回一些内容......并且我们不能返回null，因为它会导致异常（因为您无法订阅具有空处理程序的事件）。所以我们需要返回一个从事件签名中动态生成的假处理程序。它让事情变得更加困难......但它仍然是可行的。

这是实现这种改进的第二个版本：

？

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using system; using system.collections.generic; using system.componentmodel; using system.diagnostics; using system.linq; using system.reflection; using system.reflection.emit; using system.windows; using system.windows.markup;     public class eventbindingextension : markupextension     {         private eventinfo _eventinfo;         public eventbindingextension() { }         public eventbindingextension(string eventhandlername)         {             this.eventhandlername = eventhandlername;         }         [constructorargument("eventhandlername")]         public string eventhandlername { get; set; }         public override object providevalue(iserviceprovider serviceprovider)         {             if (string.isnullorempty(eventhandlername))                 throw new argumentexception("the eventhandlername property is not set", "eventhandlername");             var target = (iprovidevaluetarget)serviceprovider.getservice(typeof(iprovidevaluetarget));             var targetobj = target.targetobject as dependencyobject;             if (targetobj == null)                 throw new invalidoperationexception("the target object must be a dependencyobject");             _eventinfo = target.targetproperty as eventinfo;             if (_eventinfo == null)                 throw new invalidoperationexception("the target property must be an event");             object datacontext = getdatacontext(targetobj);             if (datacontext == null)             {                 subscribetodatacontextchanged(targetobj);                 return getdummyhandler(_eventinfo.eventhandlertype);             }             var handler = gethandler(datacontext, _eventinfo, eventhandlername);             if (handler == null)             {                 trace.traceerror(                     "eventbinding: no suitable method named '{0}' found in type '{1}' to handle event '{2'}",                     eventhandlername,                     datacontext.gettype(),                     _eventinfo);                 return getdummyhandler(_eventinfo.eventhandlertype);             }             return handler;                       }         #region helper methods         static delegate gethandler(object datacontext, eventinfo eventinfo, string eventhandlername)         {             type dctype = datacontext.gettype();             var method = dctype.getmethod(                 eventhandlername,                 getparametertypes(eventinfo.eventhandlertype));             if (method != null)             {                 if (method.isstatic)                     return delegate.createdelegate(eventinfo.eventhandlertype, method);                 else                     return delegate.createdelegate(eventinfo.eventhandlertype, datacontext, method);             }             return null;         }         static type[] getparametertypes(type delegatetype)         {             var invokemethod = delegatetype.getmethod("invoke");             return invokemethod.getparameters().select(p => p.parametertype).toarray();         }         static object getdatacontext(dependencyobject target)         {             return target.getvalue(frameworkelement.datacontextproperty)                 ?? target.getvalue(frameworkcontentelement.datacontextproperty);         }         static readonly dictionary _dummyhandlers = new dictionary();         static delegate getdummyhandler(type eventhandlertype)         {             delegate handler;             if (!_dummyhandlers.trygetvalue(eventhandlertype, out handler))             {                 handler = createdummyhandler(eventhandlertype);                 _dummyhandlers[eventhandlertype] = handler;             }             return handler;         }         static delegate createdummyhandler(type eventhandlertype)         {             var parametertypes = getparametertypes(eventhandlertype);             var returntype = eventhandlertype.getmethod("invoke").returntype;             var dm = new dynamicmethod("dummyhandler", returntype, parametertypes);             var il = dm.getilgenerator();             if (returntype != typeof(void))             {                 if (returntype.isvaluetype)                 {                     var local = il.declarelocal(returntype);                     il.emit(opcodes.ldloca_s, local);                     il.emit(opcodes.initobj, returntype);                     il.emit(opcodes.ldloc_0);                 }                 else                 {                     il.emit(opcodes.ldnull);                 }             }             il.emit(opcodes.ret);             return dm.createdelegate(eventhandlertype);         }         private void subscribetodatacontextchanged(dependencyobject targetobj)         {             dependencypropertydescriptor                 .fromproperty(frameworkelement.datacontextproperty, targetobj.gettype())                 .addvaluechanged(targetobj, targetobject_datacontextchanged);         }         private void unsubscribefromdatacontextchanged(dependencyobject targetobj)         {             dependencypropertydescriptor                 .fromproperty(frameworkelement.datacontextproperty, targetobj.gettype())                 .removevaluechanged(targetobj, targetobject_datacontextchanged);         }         private void targetobject_datacontextchanged(object sender, eventargs e)         {             dependencyobject targetobj = sender as dependencyobject;             if (targetobj == null)                 return;             object datacontext = getdatacontext(targetobj);             if (datacontext == null)                 return;             var handler = gethandler(datacontext, _eventinfo, eventhandlername);             if (handler != null)             {                 _eventinfo.addeventhandler(targetobj, handler);             }             unsubscribefromdatacontextchanged(targetobj);         }         #endregion     }

所以这是我们可以做的事情，这要感谢这个新的wpf功能。我们也可以设想一个行为系统，类似于我们可以对附加属性进行的操作，例如在事件发生时执行标准操作。有很多可能的应用程序，我把它留给你找到它们.

from:https://www.thomaslevesque.com/2011/09/23/wpf-4-5-subscribing-to-an-event-using-a-markup-extension/

转载于:https://www.cnblogs.com/chary/p/no0000130.html

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