学步园

Prerequisites

Software Requirements

• The FetchProgress and FetchComplete events only work properly in MDAC 2.5 or later. You can download the latest version of the Microsoft Data Access Components from the following Microsoft Web site:
  Microsoft Data Access Components (http://msdn.microsoft.com/library/default.asp?url=/downloads/list/dataaccess.asp)
• If you are developing your application in Microsoft Visual Studio 6.0, then you must have service pack 3 or later installed. You can install the latest service pack for Microsoft Visual Studio 6.0 from the following Microsoft Web site:
  Visual Studio Product Updates (http://msdn.microsoft.com/vstudio/downloads/updates.asp)

Coding Requirements

• When you open the recordset, you must specify adAsyncFetch as a Recordset Option.
• You must use client-side cursors, because the FetchProgress and FetchComplete events are returned by the ActiveX Data Objects (ADO) client cursor engine.
• In Visual Basic, you must declare the Recordset at module level using Dim with the WithEvents keyword.

Back to the top

Properties

There are two recordset properties that affect the asynchronous behavior of ADO. These properties should be set prior to opening the recordset.

• Initial Fetch Size determines how many records are fetched synchronously before the asynchronous thread is created. This allows for a small recordset to be created without the additional overhead of creating a thread. This is set to 50 by default. To guarantee that FetchProgress and FetchComplete are called, set this value to 0.
• Background Fetch Size determines how many records are fetched between calls to the FetchProgress event. This is set to 15 by default.

Back to the top

The Events

FetchProgress

FetchProgress has four Parameters:

• Progress is the number of records currently in the recordset.

  The first time FetchProgress is called, Progress is equal to Initial Fetch Size plus Background Fetch Size. For each additional call, Progress equals the previous value plus Background Fetch Size.

• MaxProgress is the maximum value expected to be returned.

  MaxProgress is not equal to the actual number of records that will be returned. ADO has to fetch the records in order to get this value. This means MaxProgress is only ever a best guess. MaxProgress usually equals Progress plus Background Fetch Size.

  FetchProgess is always called before calling FetchComplete. In this case, Progress and MaxProgress are equal to each other. This MaxProgress equals the actual number of records retrieved.

• The value of adStatus determines if any errors have occurred. This value is normally adStatusOK.

  You may disable the FetchProgress event by setting the value of adStatus to the value adStatusUnwantedEvent.

• pRecordset is a reference to the recordset itself.

FetchComplete

FetchComplete has three Parameters:

• If adStatus is adStatusErrorsOccurred, then you can check pError to determine what error has occurred. This can happen if your code calls the Cancel method before the query is done executing, for example.
• The value of adStatus determines if any errors have occurred.

  You may disable the FetchComplete event by setting the value of adStatus to the value adStatusUnwantedEvent.

• pRecordset is a reference to the recordset itself.

Back to the top

Sample Code

The following sample demonstrates how to use the FetchProgress and FetchComplete events in Visual Basic.

The sample uses an ODBC Datasource named Pubs to connect to the Pubs database that comes with SQL Server.

1. In Microsoft Visual Basic, create a new Standard EXE. Form1 is added to the project by default.
2. On the Project menu, click to select References, and then select Microsoft ActiveX Data Objects Library.
3. On the Project menu, click to select Components, and then select Microsoft DataGrid Control 6.0 (OLEDB).
4. Place a DataGrid, a Textbox, and a CommandButton onto Form1.
5. Add the following code to Form1's Code Window:

      Option Explicit
   
      Const strConn = "DSN=Pubs"
      Const strDefaultSQL = "SELECT * FROM Titles"
    
      Dim cn As ADODB.Connection
      Dim WithEvents rs As ADODB.Recordset
   
      Private Sub Form_Load()
         Command1.Caption = "Go"   
         Text1.Text = strDefaultSQL
      
         Set cn = New ADODB.Connection
         cn.Open strConn
      End Sub
   
      Private Sub Command1_Click()
         Dim strSQL As String
         strSQL = Text1.Text
      
         Set rs = New ADODB.Recordset
         With rs
            .CursorLocation = adUseClient
         
            .Properties("Initial Fetch Size") = 2
            .Properties("Background Fetch Size") = 4
         
            Debug.Print "Start"
            Debug.Print "Initial Fetch Size: " & _
                        .Properties("Initial Fetch Size")
            Debug.Print "Background Fetch Size" & _
                        .Properties("Background Fetch Size")
         
            .Open strSQL, cn, , , adAsyncFetch
         End With
      End Sub
   
      Private Sub rs_FetchProgress(ByVal Progress As Long, _
                                   ByVal MaxProgress As Long, _
                                   adStatus As ADODB.EventStatusEnum, _
                                   ByVal pRecordset As ADODB.Recordset)
                                   
         Debug.Print "Fetch: " & Progress & _
                     "  Max: " & MaxProgress
         
      End Sub
   
      Private Sub rs_FetchComplete(ByVal pError As ADODB.Error, _
                                   adStatus As ADODB.EventStatusEnum, _
                                   ByVal pRecordset As ADODB.Recordset)
                                
         If adStatus <> adStatusOK Then
            Debug.Print "Failed"
            Debug.Print "Error: " & pError.Number & " - " & pError.Description
         Else
            Set DataGrid1.DataSource = pRecordset
            Debug.Print "Done"
         End If
      
      End Sub
   					

6. Change strConn to a valid connection string for your database, and change strDefaultSQL to a valid SQL query that returns records from your database.
7. Run the code. Click the Go button to start reading the Recordset. On the View menu, click to select Immediate Window. Visual Basic's Immediate window displays the progress of the asynchronous query.

For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base: