Access Cookbook, 2nd Edition [Electronic resources] نسخه متنی

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Access Cookbook, 2nd Edition [Electronic resources] - نسخه متنی

Ken Getz; Paul Litwin; Andy Baron

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Recipe 2.11 Open Multiple Instances of a Form



2.11.1 Problem


In an application, you have
a form showing information about a customer. You would like to be
able to open another copy of the form so you could move to a
different row, compare values, perhaps copy from one row to another,
or just look at more than one customer's record at
once. As far as you can tell, you can have only one open copy of a
form at a time.


2.11.2 Solution


In older
versions of Access, you were limited to having only a single copy of
a form open at any time. Starting with Access 95, you can open
multiple instances of a form, under complete program control.
There's no user interface for this functionality,
however, so you must write code to make it happen. This solution
demonstrates how to create, handle, and delete multiple instances of
a form using the New keyword and user-defined
collections.

Follow these steps to convert your own forms to allow for multiple
instances:

  1. Add two buttons to your form, with captions like Create New Instance
    (named cmdViewAnother in the example) and Delete All Extra Instances
    (named cmdCloseAll in the example).

  2. Add the following code to
    the Click event procedure of the Create New Instance button:

    Private Sub cmdViewAnother_Click( )
    Call acbAddForm
    End Sub
  3. Add the following code to the Click event procedure of the Delete All
    Extra Instances button:

    Private Sub cmdCloseAll_Click( )
    Call acbRemoveAllForms
    End Sub
  4. Add the following code to
    the Close event procedure for the form:

    Private Sub Form_Close( )
    Call acbRemoveForm(Me)
    End Sub
  5. Import the module basMultiInstance from

    02-11.MDB .


To see this functionality in action, load and run frmCustomers from

02-11.MDB . Once it's open,
click View Another Customer. This will create a new instance of the
original form, with its own set of properties and current row. You
can create as many new forms as you like and move from row to row on
any or all of them. When you're done, click Close
All Extra Copies, which will run code to delete all the extra forms.
Figure 2-18 shows the original form, along with
three extras.


Figure 2-18. Clones of frmCustomers with their own current rows



2.11.3 Discussion


Working with multiple instances of forms
requires three skills: creating the new forms, storing their
references, and deleting them. All three topics center around
user-defined collections. These collections allow you to add and
delete items at will, based on either their position in the
collection or a string value that uniquely identifies each element.
This example uses each form's hWnd property (its
window handle) to identify the form in the collection.

In Access, each
form stored in the database can be viewed as its own
"class" of form:
it's an object that you can replicate in memory,
using the New keyword. The following statement
will create a new instance of the form named frmCustomers:

Set frm = New Form_frmCustomers

Form_frmCustomers
is the object type, and its name originates from its type (Form)
concatenated with the actual class name (frmCustomers). Once
you've executed this line of code, frm refers to a
new, invisible form. You can set its properties, if you like, or make
it visible with the following statement:

frm.Visible = True

If you want to refer to your new
form later, you'll need to store a reference to it
somewhere. In the example code, we used a user-defined collection.
When you create a new instance of the form, the code adds that form
reference to the collection so you can find the form, under program
control, when you need to refer to it again.


Life Span of a Form


The variables that refer to the newly created forms must have a life
span longer than that of the procedure that created the forms. In
this case, the form references are stored in a module-level
collection, so their lifetime is the same as the database itself.
When you create a new instance of a form, if the variable referring
to that form goes out of scope, Access destroys the new form
instance. Because you'll want your forms to hang
around longer than that, make sure your form variables have a static,
module, or global scope.

In this example, the

acbAddForm subroutine creates and stores the new
form references. As it creates a new form (when requested to do so by
that button click on frmCustomers), it adds the form reference to the
collection of forms. A collection's Add method
allows you to add the item and optionally store a unique string value
describing the value at the same time. In this case, the code stores
the form's hWnd property, converted to a string, as
its unique identifier. The code also increments a variable that keeps
track of the number of instances and places the new form at a
convenient location before making it visible. This is the

acbAddForm subroutine:

Private colForms As New Collection
Private mintForm As Integer
Const acbcOffsetHoriz = 75
Const acbcOffsetVert = 375
Public Sub acbAddForm( )
Dim frm As Form
Set frm = New Form_frmCustomers
' You have to convert the key to a string, so tack a " onto
' the hWnd (which uniquely identifies each form instance)
' to convert it to a string.
colForms.Add Item:=frm, Key:=frm.Hwnd & "
' Build up the caption for each new instance.
mintForm = mintForm + 1
frm.Caption = frm.Caption & " " & mintForm
' The numbers used here are arbitrary and are really useful
' only for this simple example.
frm.SetFocus
DoCmd.MoveSize mintForm * acbcOffsetHoriz, mintForm * acbcOffsetVert
' Finally, set this form to be visible.
frm.Visible = True
End Sub
Sub acbRemoveForm(frm As Form)
' All the forms call this from their Close events. Since
' the main form isn't in the collection at all, it'll cause
' an error. Just disregard that.
On Error Resume Next
colForms.Remove frm.Hwnd & "
Err.Clear
End Sub

Eventually
you'll want to close down all the extra instances of
your form. This is quite simple: once you delete the form reference,
Access will close the form for you. Therefore, in reaction to the
Close All Instances button you created on your form, Access runs this
subroutine:

Public Sub acbRemoveAllForms( )
Dim varForm As Variant
' Reset the static variables.
mintForm = 0
For Each varForm In colForms
colForms.Remove 1
Next varForm
End Sub

This
subroutine first resets the total number of instances back to 0, then
walks through the collection of form instances one at a time,
removing the first item each time. Because Access renumbers the
collection each time you remove an item, this is the simplest way to
remove all the items.

To keep
things neat, we instructed you to attach to the
form's Close event code that removes the specific
form from the collection of forms when you close that form. Though
this example doesn't need that functionality, you
may find that in other situations you do need your collection to
reflect accurately the forms that are currently loaded (if you want
to list all the open forms, for example). There is one wrinkle here,
however: when you ask the application to close all extra instances,
Access closes each form, one by one. This, in turn, triggers the
Close event for each of those forms. The Close event calls code
attached to that event that attempts to remove the form from the
collection of forms, but that form has already been removed from the
collection. Therefore, the

acbRemoveForm
subroutine, shown here, disables error handling; attempting to remove
an already removed form won't trigger an error.

Public Sub acbRemoveForm(frm As Form)
' All the forms call this from their Close events. Since
' the main form isn't in the collection at all, it'll cause
' an error. Just disregard that.
On Error Resume Next
colForms.Remove frm.hWnd & "
Err.Clear
End Sub

Extra instances of forms aren't really treated
exactly the same as their originals. For example, all the copies of a
form share the same name as the original, so if you attempt to use
the syntax:

Forms!frmCustomers

or:

Forms("frmCustomers")

to refer to an instance of a form, you'll be able to
access only the original form. Access does add each instance to the
Forms collection, but you can access them only by their ordinal
positions in the collection. If you loop through the Forms collection
to close all open forms, the code will close the instances, too.

Form instances have their own properties and their own current rows,
but any changes you make to a form instance are not saved. That is,
all instances of a form other than the original are read-only.
That's not to say that the data on the form is
read-onlyit's the

design
of the form instance that's read-only.

You'll find multiple instances of forms to be a
useful addition to your programming arsenal. They allow users to view
multiple rows with their forms in Form View (the proverbial
"have their cake and eat it, too"
situation), and from there to copy/cut/paste data from one row to
another. Your responsibility as the developer is to carefully manage
the creation, storage, and deletion of these forms, because the
Access user interface provides no help.


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