Run VBA Code Before Next Slide Becomes Visible - vba

Question

After clicking Next, I want to run some code before the next slide becomes visible on screen. The code will control some third-party software. This will take between 1 and 3 seconds to complete. In an ideal world, the previous slide's content would remain visible while the code is running and then PowerPoint would advance to the next slide.
I have used PowerPoint's Application.SlideShowNextSlide event using the 'Application Object Events' method described here: http://youpresent.co.uk/powerpoint-application-events-in-vba/ This works, the event triggers as expected.
Despite the documentation saying:
Occurs immediately before the transition to the next slide
...my tests suggest that the code runs after the slide is displayed. Using the test code below, this is what I see on my screen:
The next slide is displayed together with all its content
After approximately 1 second, all content is hidden
After about 5 seconds, all content becomes visible
Private Sub App_SlideShowNextSlide(ByVal Wn As SlideShowWindow)
' Checking this works before spending time on the desired functionality
' Will Select/Case (.SlideID) to ensure code runs on the correct slide
Dim oSld As Slide
Dim oShp As Shape
Dim stoptime As Date
Set oSld = Wn.View.Slide
' Test #6: See if hiding the slide's content, running the code, and
' then revealing it is a solution. Unfortunately, all the content is
' already visible before this loop is executed.
'
For Each oShp In oSld.Shapes
oShp.Visible = False
Next
' Waste time (code would go here)
'
stoptime = Now() + #12:00:05 AM#
Do Until Now() > stoptime
DoEvents
Loop
' Reveal content of the slide
'
For Each oShp In oSld.Shapes
oShp.Visible = True
Next
End Sub
I've tried the Sleep API instead of a loop. I've tried making a WScript.Shell call with bWaitOnReturn = true. Neither prevents the content of the slide being rendered before/while the code is being executed.
It is possible to stop the old slide being replaced by the new one until some code has run? Or, stop the content of the next slide being seen while a code block does its job?

Answer 1

Okay. I've made something that works well enough for my needs. It isn't perfect but I'm putting it here in case it acts as a good starting point for others:
The important thing for me is that code must run immediately before the slide is displayed. It's not good enough to run code some seconds prior. This would be apparent to the audience.
So, the key to this is that in the App_SlideShowNextClick event, the passed variable nEffect is Nothing when there are no animations left to complete on that slide. Testing for that alerts you to the point where PowerPoint is about to move to the next slide. Running code at this point, and finishing it with
ActivePresentation.SlideShowWindow.View.Next to trigger the move, works for me.
This is what I did:
Set-up the 'Application Object Events' method of triggering events (see original question for more on this)
Add something unnecessary (e.g., text) within the slide before the one you need code to have been run before it's displayed
[Colour|Color] or bury it such that it won't bother your audience
Add an animation effect to it e.g., 'Disappear'
Ensure this animation is the final animation on this slide
Design some way of knowing when you want the code to run (e.g., using SlideID)
' Module variable
Private mGoingForward As Boolean
Private mPreviousSlideNumber as Integer
Private Sub App_SlideShowNextSlide(ByVal Wn As SlideShowWindow)
' In which direction is the user moving through the slideshow?
' This is used in App_SlideShowNextClick
'
mGoingForward = Wn.View.Slide.SlideNumber > mPreviousSlideNumber
End Sub
Private Sub App_SlideShowNextClick(ByVal Wn As SlideShowWindow, ByVal nEffect As Effect)
' nEffect is nothing when there are no further effects to
' perform on the current slide i.e., it's about to move to
' the next slide
'
If nEffect Is Nothing Then
' This is the last time we are going to be on this slide, so update this
'
mPreviousSlideNumber = Wn.View.Slide.SlideNumber
' We are now at the very end of the current slide. The next click will
' move you to the next slide
'
If [ condition when you want the code to run e.g., Wn.View.Slide.SlideID ] Then
' Run your code here
' If necessary, use something like the following to delay execution
' until everything's done
'Dim oShell As Object
'Dim waitOnReturn As Boolean: waitOnReturn = True
'Dim windowStyle As Integer: windowStyle = 0
'oShell.Run cmd, windowStyle, waitOnReturn
' In my situation, I don't have to consider when a user
' moves backwards in the presentation. However, if you don't
' do something like this, you'll never be able to move back. It may
' be possible to possible to detect this without relying on a
' module variable
'
If mGoingForward Then ActivePresentation.SlideShowWindow.View.Next
End If
End If
End Sub