Synchronous Versus Asynchronous Calls
7.4.3 Synchronous Versus Asynchronous Calls
Web-method calls are synchronous by default. That is, the caller waits while the call is sent over the network and while the result is calculated and sent back. When dealing with the Internet, this can be a lengthy process a fraction of a second to several seconds. There are times when it would be useful for the client to go off and perform some other processing while waiting for the web method to complete. Fortunately, the autogenerated proxy class created by either Visual Studio .NET or wsdl.exe provides a way to do this. Ex am ple 7- 4 shows client code that calls the SayHello web method asynchronously. Example 7-4. Calling SayHello asynchronously Private myService As New HelloWebService Private Sub btnInvoke_Click _ ByVal sender As System.Object, _ ByVal e As System.EventArgs _ Handles btnInvoke.Click Assumes that there is a text box named txtName. myService.BeginSayHellotxtName.Text, AddressOf MySayHelloCallback, _ Nothing 339 End Sub Private Sub MySayHelloCallbackByVal ar As IAsyncResult Assumes that there is a text box named txtResult. txtResult.Text = myService.EndSayHelloar End Sub For each web method, the web-service proxy class exposes two additional methods, named Begin WebMethodName and End WebMethodName , where WebMethodName is the name of the web method to be invoked. The purpose of the Begin WebMethodName method is to invoke the web method and immediately return. This allows client code to continue executing while the web service processes the request. When the response arrives from the web service, it is held until the client calls the End WebMethodName method. If the client calls the End WebMethodName method before the web methods response arrives, the client blocks until the response arrives. If the client calls the End WebMethodName method after the response arrives, the method immediately returns with the response value. The parameters of the Begin WebMethodName method are exactly the same as the parameters of the web-method call itself, plus two additional parameters: callback If the client would like to be notified when the web methods response has arrived, it can pass a delegate reference in this parameter. The type of this parameter is AsyncCallback, which is defined in the System namespace as: Public Delegate Sub AsyncCallbackByVal ar As IAsyncResult To pass a callback function to the Begin WebMethodName method, write a method having the appropriate signature and then use the AddressOf operator to create a delegate from that method, as shown in Ex am ple 7- 4 . If the client does not want to be notified when the web methods response has arrived, it can pass Nothing in this parameter. asyncState If the client would like to provide some arbitrary additional information to the callback function, it can do so by assigning a value to the asyncState parameter. This parameter is of type Object, so any value can be passed. The value passed in this parameter is passed on to the callback function when the web-method call returns. If no additional information needs to be passed to the callback function, pass Nothing in this parameter. The Begin WebMethodName method has no return value. When the callback function is called, it receives a reference to an object that implements IAsyncResult defined in the System namespace. The AsyncState property of this object holds the value passed in the asyncState parameter of the Begin Web - MethodName method. The first parameter of the End WebMethodName method is the IAsyncResult reference obtained in the callback function. Alternatively, the client can pass Nothing in this parameter if not using a callback function. The End WebMethodName method may have additional parameters, depending on whether the corresponding web method has any ByRef parameters. If there are any such parameters, they appear here in the signature of the End WebMethodName method. In addition, the return value of the End WebMethodName method is the return value of the web method. 3407.5 Web-Service Discovery
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