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That is pretty much all of the implementation
for the stack itself, but since we're working with Frames
for a GUI engine, this is the ideal place to handle the
graphical parts of the stack as well. The next section of
code positions the new Frame in the viewable area of the
screen, makes the newly pushed Frame visible, and hides
the Frame's border.
m_objFrameStack(m_iFramesOnStack).Left
= _
FRAME_X_OFFSET
m_objFrameStack(m_iFramesOnStack).Top = _
FRAME_Y_OFFSET
m_objFrameStack(m_iFramesOnStack).ZOrder 0
m_objFrameStack(m_iFramesOnStack).Visible = True
m_objFrameStack(m_iFramesOnStack).BorderStyle = _
vbBSNone
The last item of business is to update
our stack item counter and refresh the form to make sure
our Frame is shown where it's supposed to be.
m_iFramesOnStack = m_iFramesOnStack
+ 1
objForm.Refresh
Now adding a frame to the stack is as
simple as this:
PushFrame frmMain, "fraTitle"
And the stack method positions and displays
the frame for you.
Go through the Form code and add the
appropriate PushFrame calls in the Click event
handlers for all of the relevant CommandButtons.
For example, the single Button on fraListing should push
fraTitle on the stack.
Popping Frames
Now that you're able to push frames
onto the stack, you need to be able to pop them back off.
PopFrame takes two input parameters: the form on which the
Frames in the stack reside and the number of Frames to pop.
The first order of business is to make
sure we're not trying to pop more frames than the stack
holds. This example simply exits the function if it's attempted,
but you could easily add functionality to pop all but the
last frame or to pass back an error value.
If m_iFramesOnStack - intFramesToPop
<= 0 Then
Exit Function
End If
Next the Frame being popped is set to
non-visible:
m_objFrameStack(m_iFramesOnStack - 1).Visible
= _
False
The stack array is resized:
ReDim Preserve m_objFrameStack(m_iFramesOnStack
- _
intFramesToPop - 1)
And our stack element counter is updated:
m_iFramesOnStack = m_iFramesOnStack -
_
intFramesToPop
Finally, the Frame now at the top of
the stack is made visible and the Form is refreshed:
m_objFrameStack(m_iFramesOnStack - 1).Visible
= _
True
objForm.Refresh
Now that you have a full pop method,
implement it in your project by adding a call to it in the
Back button's Click event handler like this:
Private Sub cmdBack_Click()
PopFrame Me, 1
End Sub
The Stack In Action
You stack is now fully functional, but
to make it a little more demonstrative, it would be nice
to be able to see the stack's contents at any time. To do
this, I created a method in the module called StackEnum
that simply iterates through the stack array backwards,
adding the Frame name to a string. Once it has iterated
all the Frames, it pops up a MsgBox with the stack's contents.
Implement this method by adding a call to StackEnum in the
Click event handler for the cmdStackEnum button.
The last remaining item of business
is to add the first Frame, fraListing, to the stack
at startup so the screen doesn't start out blank. Add a
call to PushFrame in Form_Load and run the project.
Test out the application by moving through
a few screens in whatever order you feel like then take
a look at the stack by clicking on the 'Show Stack'
button. You will get a list of all the frames you've viewed
in order with the most recent at the top, something like
Figure 2.
One thing that I haven't demonstrated
here, and that you may have already caught, is that we when
we back through the stack, we may well pass through the
same Frame multiple times, but it's quite possible that
it should have different data displayed each time. How do
we tell the Frame to populate with the right data?
Adding support for additional data is
not as difficult as one might think. There's no rule that
says the stack can only hold the Frame. You can add a dimension
to the stack and use the second dimension to hold, for example,
the ID of the data that should be displayed.
Of course this means adding a
bit more code to the PushFrame method, but it doesn't require
a great deal of effort and you can customize it to your
applications needs in many ways. I leave this implementation
to your own creativity and imagination.
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