[DUG] Updating a property's memory address doesn't cause a redraw

[Dennis Chuah]

Paul,

Hi.  You cannot treat Delphi properties as simple pointers to data. 
Properties are always accessed through setter and getter methods, either 
explicitly defined or implicitly through compiler magic.  The best way to 
achieve what you are doing is to pass your routine a component reference and 
the name of the property.  Use RTTI to set the value - Google this, there 
are plenty of code examples out there.  Keep in mind also setting properties 
in most cases only invalidates the component.  You will need to explicitly 
refresh it to force the redraw.

Regards,
Dennis.

-----Original Message----- 
From: Paul Lyons
Sent: Tuesday, May 28, 2013 1:02 AM
To: delphi at listserver.123.net.nz
Subject: [DUG] Updating a property's memory address doesn't cause a redraw

Hi there

I've written a generalised interpolator that gradually changes the
value of a property such as panel1.width from startValue to endValue
over a given period. This gives me a general way of animating
movements and positions of interface components.

I tell the interpolator the address of the property, the start and
finish values:

interpolater.addProperty(addr(panel1.width), panel1.width, 
panel1.width+100);

As you can see, the property is passed as a pointer. The interpolator
goes into a loop for whatever time duration I've specified, updating
the property pro rata on each iteration. However, although the memory
location where the property is stored does genuinely get updated, it
seems that Delphi isn't aware that updating that memory location
should generate a redraw event for the panel. I can force a redraw by
putting a pair of net-null-effect statements like this

             panel1.width := panel1.width - 1;

             panel1.width := panel1.width + 1;

into the loop, and then the interpolation works fine, but the goal
was to avoid having to tell interpolator anything about the component
that needs to be redrawn other than the memory location of its property.

Things that don't work include

             application.processmessages

             panel1.update

             panel1.repaint

             panel1.invalidate

             panel1.width := panel1.width

Optimisation is not turned on.

It seems as though even if you force Delphi to redraw a component, it
won't check the property values unless it knows that at least one
property has been updated. So the problem is, having updated a memory
location, how do I tell the component that its property has been updated?

Thanks
Paul

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