Recipe 8.1 Accelerate the Load Time of Forms
8.1.1 Problem
The first time you open a form in
your application, it seems to take forever to load. Is there any way
to accelerate this?
8.1.2 Solution
You can radically improve the time it takes to load a form for the
first time by preloading your forms when the database is initially
opened. You can also decrease the load time for subsequent loadings
by hiding instead of closing forms. This solution shows you how to
improve form load time using these techniques.Load the 08-01.MDB database. Note the time it
takes for the switchboard form to appear (see Figure 8-1). Make sure that the "Preload
and keep loaded forms" checkbox is unchecked; if
it's checked, uncheck it, close the database, and
start over. Now press one of the command buttons, such as the Orders
button, and note how long it takes Access to initially load the form.
Close the form.
Figure 8-1. The 08-01.MDB switchboard form
Now check the "Preload and keep loaded
forms" checkbox on the switchboard form and close
the database. Reload the database and again note the time it takes
for the switchboard form to appear. Load the Orders form, again
recording the form load time.You'll see that the switchboard form now takes
longer to appear but that the subsequent form load time is
significantly shorter. That's because checking the
"Preload and keep loaded forms"
checkbox and reloading the database flips an internal switch that
causes the application to preload its forms (in a hidden state) as
the switchboard form is loaded by Access. This lengthens the time it
takes for the switchboard form to appear initially. However, because
the Orders form is now preloaded, it takes less time for it to appear
when you press the Orders command button.
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- Create a table for storing the names of
the forms you wish to preload. This table (zstblPreloadForms in the
sample database) should have a single field, FormName, with a
datatype of Text. Switch to datasheet view (see Figure 8-2) and add a row for each form in your
application that you wish to preload.
Figure 8-2. Store the list of preloaded forms in the zstblPreloadForms table
- Create a switchboard form or edit your
existing one. - Set the form's AutoCenter property to Yes.
- Add the following code to the declarations section at the top of the
form's module (replacing the values with the actual
names for your table and splash form, if you've used
different names):Private Const acbPreloadTable = "zstblPreloadForms"
Private Const acbSplashForm = "frmSplash" - Create a new event procedure for the form's Open
event. (If you're unsure of how to do this, see
How Do I Create an Event
Procedure? in the Preface of this book.) - Add the following code to the event procedure:
Private Sub Form_Open(Cancel As Integer)
You can also copy this code from the frmSwitchboard1 form
' Preload forms.
Dim db As DAO.Database
Dim rst As DAO.Recordset
Dim varFormName As Variant
On Error GoTo HandleErr
DoCmd.OpenForm acbSplashForm
Set db = CurrentDb( )
' Preload the forms listed in zstblPreloadForms.
Set rst = db.OpenRecordset(acbPreloadTable, dbOpenSnapshot)
Do While Not rst.EOF
varFormName = rst("FormName")
If Not IsNull(varFormName) Then
DoCmd.OpenForm FormName:=varFormName, _
WindowMode:=acHidden, OpenArgs:="StayLoaded"
End If
rst.MoveNext
Loop
ExitHere:
DoCmd.Close acForm, acbSplashForm
If Not rst Is Nothing Then
rst.Close
End If
Set rst = Nothing
Exit Sub
HandleErr:
MsgBox "Error " & Err.Number & ": " & Err.Description, , "Form Open"
Resume ExitHere
End Sub
(not the frmSwitchboard form) in
08-01.MDB . (The frmSwitchboard1 version of the
form always preloads forms, thus eliminating all the code associated
with the "Preload and keep loaded
forms" checkbox.) - Create an event procedure for the
switchboard form's Close event. Add this code to the
event procedure:Private Sub Form_Close( )
' Unload preloaded forms
Dim db As DAO.Database
Dim rst As DAO.Recordset
Dim varFormName As Variant
On Error GoTo HandleErr
Set db = CurrentDb( )
' Unload the forms listed in zstblPreloadForms
Set rst = db.OpenRecordset(acbPreloadTable, dbOpenSnapshot)
Do Until rst.EOF
varFormName = rst("FormName")
If Not IsNull(varFormName) Then
DoCmd.Close acForm, varFormName
End If
rst.MoveNext
Loop
ExitHere:
If Not rst Is Nothing Then
rst.Close
End If
Set rst = Nothing
Exit Sub
HandleErr:
MsgBox "Error " & Err.Number & ": " & Err.Description, , "Form Open"
Resume ExitHere
End Sub - Create the following functions in a
global module (or import the basStayLoaded module from
08-01.MDB ):Public Function acbOpenForm(strFormName As String, _
fStayLoaded As Boolean) As Boolean
' Open specified form and pass it the
' StayLoaded argument.
On Error GoTo acbOpenFormErr
If fStayLoaded Then
DoCmd.OpenForm strFormName, OpenArgs:="StayLoaded"
Else
DoCmd.OpenForm strFormName
End If
acbOpenFormExit:
Exit Function
acbOpenFormErr:
MsgBox "Error " & Err.Number & ": " & Err.Description, _
vbOKOnly + vbCritical, "acbOpenForm"
Resume acbOpenFormExit
End Function
Public Function acbCloseForm(frmToClose As Form)
' If StayLoaded is True, hide the form instead of closing it.
On Error GoTo acbCloseFormErr
If InStr(frmToClose.OpenArgs, "StayLoaded") > 0 Then
frmToClose.Visible = False
Else
DoCmd.Close acForm, frmToClose.Name
End If
acbCloseFormExit:
Exit Function
acbCloseFormErr:
MsgBox "Error " & Err.Number & ": " & Err.Description, _
vbOKOnly + vbCritical, "acbCloseForm"
Resume acbCloseFormExit
End Function - Throughout your application, when you create code that opens a form
and you wish to load that form only once, call the
acbOpenForm function from Step 8. If you wish to
open a form from code, you can use this syntax:Call acbOpenForm("formname", True)
You can also
call the function directly from an event property. In this case,
enter the following in the event property:=acbOpenForm("formname", True)
For those forms that you don't wish to keep loaded,
change the second parameter of acbOpenForm to
False. - For each form you are preloading or
loading with the acbOpenForm function, add a
command button with the caption
"Close". Enter the following in the
event property for the button's Click event:=acbCloseForm(Form)
Don't place any quotes around the
Form argument. - Make a copy of the form created in Step 2
and name it frmSplash. This is what's known as a
"splash form." Open frmSplash in
design view and remove all the command button controls. Also remove
all the code behind the form for this copy. In the area where the
command buttons used to be, add a label control that contains an
initialization message. For example, the label on frmSplash has the
attributes shown in Table 8-1. frmSplash is shown
in form view in Figure 8-3.
Property | Value |
---|---|
Name | lblMessage |
Caption | Initializing... |
BackStyle | Transparent |
BorderStyle | Transparent |
FontName | Arial |
FontSize | 14 |
TextAlign | Center |
Figure 8-3. The splash form, frmSplash
- Open the switchboard form created in Step 2. Open the
form's module and add the following constants to the
declarations section of the module:Const acbcPreloadTable = "zstblPreloadForms"
Change "zstblPreloadForms" to the
Const acbcSplashForm = "frmSplash"
name of your table from Step 1. Change
"frmSplash" to the name of your
form from Step 11. - Select Tools Startup to open
the database Startup dialog (see Figure 8-4).
Select the switchboard form from Step 2 in the Display Form/Page
field.
Figure 8-4. The database Startup dialog
- Close the database and reload it to test your startup procedure and
switchboard form.
8.1.3 Discussion
Access forms are stored as binary data in
hidden system tables in your database. When you load a form, Access
reads data from the system tables to recreate and display that form.
This takes time. The solution described here improves the application
load time of forms by preloading them when the database is first
loaded. This means that the initial application load time will be
slower, but users are more tolerant of a long application load time
because it is a one-time commitment. As with most performance
optimizations, the benefits of this technique are especially
noticeable on slow machines.
Prior to
Access 95, you had to use an AutoExec macro to
initiate some action upon database startup; in recent versionsyou can
use the Startup dialog to specify a form to be opened when the
database is loaded. This solution takes advantage of the Startup
properties, but you also could have used an
AutoExec macro.When the switchboard form opens, the
form's Open event is triggered and the code attached
to the Open event is executed. Unfortunately, when the Open event
procedure is called, the form has not had time to paint itself, so
users normally see nothing during the Open event procedure. To remedy
this, we created a "splash" form to
display during the potentially lengthy process. You
don't have to make the splash form the same size as
the switchboard form, but in this case, we made the two forms very
similar in appearance.The code to preload the forms is shown here:
Set rst = db.OpenRecordset(acbcPreloadTable)
Do While Not rst.EOF
varFormName = rst("FormName")
If Not IsNull(varFormName) Then
DoCmd.OpenForm FormName:=varFormName, _
WindowMode:=acHidden, OpenArgs:="StayLoaded"
End If
rst.MoveNext
Loop
Each record from the zstblPreloadForms table is read and the named
form is loaded in hidden mode. In addition, the
form's OpenArgs parameter
is passed the string "StayLoaded".
You can use the OpenArgs parameter of
OpenForm to pass a custom string to a form, much as you pass
parameters to a function. This OpenArgs
parameter will be used later to decide what to do when the preloaded
form is closed.Once the forms have been loaded in a hidden state, you
don't need to do anything special to make them
appear. Access is smart enough to make a hidden form visible when you
attempt to load it, which makes working with invisible forms easy.
However, we include wrapper functions for opening and closing your
application's forms in case you want some forms to
be treated differently. For example, you may not wish to preload and
keep all your forms loaded, because they will take up memory.Like the Form_Open event procedure
attached to the switchboard form, the
acbOpenForm function passes the string
"StayLoaded" to a form via its
OpenArgs argument when you pass
True as the function's second
parameter. Closing the application form is then handled by
acbCloseForm , which is called by the Click event
of each form's Close button. This function
determines whether to close or hide the form by checking its OpenArgs
property, which was passed to the form when it was opened:
If InStr(frmToClose.OpenArgs, "StayLoaded") > 0 Then
frmToClose.Visible = False
Else
DoCmd.Close acForm, frmToClose.Name
End If
For forms that you do not wish to preload, don't add
them to zstblPreloadForms. For forms that you wish to close normally
when the Close button is pressed, open them using the following
syntax:
=acbOpenForm("formname", False)
If you have enough memory, you may wish to
preload all forms and not close them until the application exits. In
some situations, however, you may wish to be more selective. By using
the preload technique and the acbOpenForm and
acbCloseForm functions throughout your
application, you can easily change your mind or customize form
preloading and form hiding for different requirements.We did not remove from each sample form
the Close button and control box provided by the system. This means
that you can use one of these alternate mechanisms to bypass the
application-defined Close button (and the
acbCloseForm function) and close the form
instead of hiding it. Thus, you may wish to set the CloseButton and
ControlBox properties of your forms to No to prevent the use of these
mechanisms.You
may wish to make zstblPreloadForms a hidden table. You can adjust the
hidden property of an object by selecting View Properties.
Benchmarking 101Benchmarking different scenarios is a painstaking process. Because Windows includes a hard disk cache and because Access itself caches data, it's difficult to get fair and accurate timings. Because of caching, the order in which you time things does matter. Avoid jumping to conclusions without repeating the readings several times in different orders. Also, there is no reliable programmatic way to measure the time a form takes to load. Although you can set timers at each of the form's events, Access does some things internally after the last loading event has fired. You will find that the only accurate way to test a form's loading time is to manually test and average the form load using a stopwatch. |