FW:Use the OnReadyStateChange Property in Visual Basic and Visual C/C++

Link:http://www.nedcomp.nl/support/origdocs/xml4/extracted/dom_howdoi_0rj7.aspx

The onreadystatechange property and event notification was intended primarily for use by scripting clients (VBScript, Jscript). It can, however, also be used in Win32 applications.

This topic discusses the implementation details necessary to use onreadystatechangenotification in applications written using Microsoft Visual Basic and Microsoft Visual C/C++.

Background Information for OnReadyStateChange events

The onreadystatechange callback function was not implemented as a COM automation event in the IXMLHTTPRequest and IServerXMLHTTPRequest components. This is because these components are heavily used in scripting environments, many of which do not support COM events. The onreadystatechange callback function was intended to be easy to use when working with scripting clients such as VBScript and JScript.

Because the onreadystatechange property was not implemented through COM-based automation events, using this callback functionality in Visual Basic and Visual C/C++ applications can be awkward for developers using those tools. Several options for implementing onreadystatechange, however, allow you to choose a way to work around this awkwardness.

Using OnReadyStateChange in Visual Basic applications

In Visual Basic, you can use any of the following approaches to design applications that support onreadystatechange events.

1. Use a timer control to poll the readyState property. When the data is ready, turn the timer off.
2. Use a DomDocument object to load the XML and handle the state using the WithEventskeyword.

   Note   If you are using the IXMLHTTPRequest and IServerXMLHTTPRequestcomponents is to first post your XML data to a Web server, this option will not work for you.

3. Create a wrapper class and create a procedure to handle the event within the class module. Set the procedure to be the default, and bind the class to theonreadystatechangeevent to either the IXMLHTTPRequest or IServerXMLHTTPRequest component, depending on which component you are using with your application.

The following sample application demonstrates each of these three approaches.

To demonstrate OnReadyStateChange in a Visual Basic application

1. Open Microsoft® Visual Basic® 6.0. In the New Project dialog box, double-clickStandard EXE.
2. On the Project menu, click References.
3. In the Available References list, select Microsoft XML,v4.0, and then click OK.
4. Add four command buttons to Form1 and set the caption of each button as follows:

   Control
   Caption

   Command1
   Fail

   Command2
   Polling using Timer

   Command3
   Using Class Wrapper

   Command4
   Using DOMDocument

5. Add a timer control to Form1.
6. Copy and paste the following code into Form1.

   Option ExplicitPublic XMLHttpRequest As MSXML2.XMLHTTP40Public WithEvents XMLDom As MSXML2.DOMDocument40Private Function FunctionReadyStateChange()    Debug.Print XMLHttpRequest.readyStateEnd FunctionPrivate Sub Command1_Click()    FailedOnReadyStateEnd SubPrivate Sub Command2_Click()    TimerResolutionEnd SubPrivate Sub Command3_Click()    ClassResolutionEnd SubPrivate Sub Command4_Click()    DOMResolutionEnd SubPrivate Sub FailedOnReadyState()On Error GoTo FailedState    If Not XMLHttpRequest Is Nothing Then Set XMLHttpRequest = Nothing    Set XMLHttpRequest = New MSXML2.XMLHTTP40    ' Assign the wrapper class object to onreadystatechange.    XMLHttpRequest.OnReadyStateChange = FunctionReadyStateChange    ' Get some stuff asynchronously.    XMLHttpRequest.open "GET", "http://localhost/test.xml", True    XMLHttpRequest.send    Exit SubFailedState:    MsgBox Err.Number & ": " & Err.DescriptionEnd SubPrivate Sub TimerResolution()    If Not XMLHttpRequest Is Nothing Then Set XMLHttpRequest = Nothing    Timer1.Interval = 1    Set XMLHttpRequest = New MSXML2.XMLHTTP40    ' Get some stuff asynchronously.    XMLHttpRequest.open "GET", "http://localhost/test.xml", True    XMLHttpRequest.sendEnd SubPrivate Sub ClassResolution()    If Not XMLHttpRequest Is Nothing Then Set XMLHttpRequest = Nothing    Dim MyOnReadyStateWrapper As MyReadyStateHandler    Set XMLHttpRequest = New MSXML2.XMLHTTP40    ' Create an instance of the wrapper class.    Set MyOnReadyStateWrapper = New MyReadyStateHandler    ' Assign the wrapper class object to onreadystatechange.    XMLHttpRequest.OnReadyStateChange = MyOnReadyStateWrapper    ' Get some stuff asynchronously.    XMLHttpRequest.open "GET", "http://localhost/test.xml", True    XMLHttpRequest.sendEnd SubPrivate Sub DOMResolution()    If Not XMLHttpRequest Is Nothing Then Set XMLHttpRequest = Nothing    If Not XMLDom Is Nothing Then Set XMLDom = Nothing    Set XMLDom = New MSXML2.DOMDocument40    XMLDom.async = True    XMLDom.Load "http://localhost/test.xml"End SubPrivate Sub Timer1_Timer()    Debug.Print XMLHttpRequest.readyState    If XMLHttpRequest.readyState = 4 Then        MsgBox "Done"        Timer1.Interval = 0    End IfEnd SubPrivate Sub XMLDom_onreadystatechange()    Debug.Print XMLDom.readyState    If XMLDom.readyState = 4 Then        MsgBox "Done"    End IfEnd Sub

7. From the Project menu, click Add Class Module.
8. Change the name of the new class module from "Class1" to "MyReadyStateHandler"
9. Paste the following code into the class module:

   Option ExplicitSub OnReadyStateChange()    Debug.Print Form1.XMLHttpRequest.readyState    If Form1.XMLHttpRequest.readyState = 4 Then        MsgBox "Done"    End IfEnd Sub

10. In the sample code added in the previous step, highlight the procedure name "OnReadyStateChange" by selecting it in the Code window.
11. From the Tools menu, click Procedure Attributes.

    In the Procedure Attributes dialog, the Name combo box should show "OnReadyStateChange."

12. Click Advanced.
13. In Procedure ID, select "(Default)" from the available options.
14. Click OK.
15. Save the class module (MyReadyStateHandler.cls) to file.
16. Open Notepad and paste the following XML into it

      This is to test the onreadystatechange event on the XMLHTTPRequest or DOMDocument  This is due to the event not being declared in the type library 

17. Save the file as test.xml to your IIS localhost directory. For example, this folder might be C:Inetpubwwwroot for a typical default installation of IIS with Windows 2000.
18. In Visual Basic, from the Run menu, click Start to run the application.
19. Try the following command options to observe the different approaches to using theonreadystatechange event within Visual Basic.
    1. To force a ready state failure, click Fail.
    2. To view the polling resolution, click Polling using Timer.
    3. To view the wrapper class solution, click Using Class Wrapper.
    4. To view the DOMDocument approach, click Using DomDocument.
20. For each of the code paths in the previous step, you can place brake-points at various places to step through the code.

Remarks

For step 19a, the following error message should appear indicating the failure:

424: Object required

For steps 19b, 19c and 19d, return values for onreadystatechange should appear in the Immediate window while the document loads. When the document completes loading, a message box saying "Done" should then appear.

Using OnReadyStateChange in Visual C/C++ applciations

In C++, use connection points to trap all XMLDOMDocument events. The DISPID of the connection point container is DIID_XMLDOMDocumentEvents. The DISPID for the connection point to trap the readystatechange event is DISPID_XMLDOMEVENT_ONREADYSTATECHANGE.

For a C/C++ example, see onreadystatechange Event.