Tutorial 2: Programming Beyond Visual Basic
Using the Windows API

Tutorial 2: Programming Beyond VB Using Windows API
Understanding the Windows API
Working with the API Viewer
Monitoring Mouse Movement with GetCursorPos
Keeping a Window Always on Top by Using SetWindowPos
Source

Monitoring Mouse Movement with GetCursorPos

Now that you've seen how to use the API Viewer to work more easily with the Windows API, let's put the function GetCursorPos to work in a real-world example.

GetCursorPos reports the position of the mouse pointer anywhere onscreen. Normally, a Visual Basic application is limited to reporting the position of the mouse pointer with the boundaries of itself, so the application has no idea where the cursor is when the mouse pointer is moved beyond a Visual Basic application's form. However, windows always knows where the mouse pointer is and reports the location through the GetCursorPos API function.

The project prjAPIStuff.vbp, which is available with the source text, illustrates this use of . Within the project, in the module modAPIStuff is a user-defined function, ReportMousePos(), which returns a string that reports back the location of the mouse pointer in friendly language. The function is a wrapper function for GetCursorPos. A wrapper function is one that primarily uses the functionality of another function, as if the function were wrapping itself around another. Listing 1 shows the structure of ReportMousePos().

01  Public Function ReportMousePos() As String
02
03      Dim MyPointAPI As POINTAPI 
04      Dim l As Long  
05      Dim strReport As String 
06
07      'Call the API function
08      l = GetCursorPos(MyPointAPI)
09
10      'Make a report string
11      strReport = "X=" & CStr(MyPointAPI.X) & ", "
12      strReport = strReport & "Y= " & CStr
        (MyPointAPI.Y)
13
14      'Return the value of the report string
15      If l > 0 Then
16          ReportMousePos = strReport
17      Else
18          ReportMousePos = "Error"
19      End If
20
21  End Function

Notice that line 3 initializes POINTAPI. The actual declaration of the type and the GetCursorPos function were done in the General Declarations section of the module. Line 8 calls the GetCursorPos function, using the type instance MyPointAPI as an argument. Windows passes back the x and y location of the mouse pointer to the elements of MyPointAPI. If the value of l (the return buffer) is greater than 0, the call was successful. Lines 11 and 12 concatenate a string that reports back the pixel location of the mouse pointer with some "x:" and "y:" labeling. If the operation was successful (line 15), the reporting string is returned by the ReportMousePos() function. Otherwise, it returns an error string (line 18).

This function is called from within a Timer control's Timer() event procedure. The Timer() event procedure assigns the function's return value to the Caption property of a Label control. The event procedure is as follows:

Private Sub Timer1_Timer()
    lblMousePos.Caption = ReportMousePos()
End Sub

GetCursorPos does not report back the mouse pointer position unless called. Therefore, the Timer control is used to poll Windows every 1/10 of a second for the location of the mouse pointer. For more information about timers, see the source text in Chapter 16: "Working with Time and Timers."