On Feb 15, 2006, at 6:00 PM, Joe Huber wrote:
NEVER use Refresh to force the canvas to refresh itself. Refresh
will first erase the canvas and a short time later the Paint event
will fire. And while your app is working on doing the redrawing,
the canvas is sitting there EMPTY. This is what causes the flicker.
On OS9 and windows systems it's best to do all your drawing to a
picture and then draw the finished picture to the canvas in one
call to DrawPicture. This way the screen is never erased, it's
never blank, it just goes instantly from the old image to the new
image.
If you hang onto this picture, you can use it in the Paint event to
flickerlessly update the canvas if RB should ever ask you to.
In summary, think of the Paint event as simply REDRAWING what was
already showing, and use your own graphics.Draw calls to CHANGE
what should be shown.
And don't ever call Refresh...it's almost as evil as DoEvents. :-)
Just kidding, but it's close.
I guess if you do not know how to do it, then Refresh could be
considered evil (or at least have unexpected results).
For one thing, if you call Refresh(False) then the background will
*not* be erased thereby eliminating the flickering. But you can only
use Refresh(False) when you are not dealing with transparency effects
underneath the canvas.
The second point is that both Brad and I have overridden the Refresh
and RefreshRect methods, changing the order in which it processes the
request. Here is my method:
Sub Refresh(EraseBackground As Boolean = True)
//
========================================================================
============
'
' Refresh() deletes the existing buffer and calls Paint() to
redraw the entire
' buffer. EraseBackground determines whether the background is
erased before
' the new buffer image is drawn to the screen (required for
transparency effects).
'
' This class overrides the Super.Refresh() method but retains
almost the exact
' behavior including EraseBackground default to True. What is
different is that
' the order of method calls is designed to bring screen
flickering to an absolute
' minimum. What this means is that the new buffer picture is
always built before
' the background is erased and so the only thing left to do is
to draw the buffer
' to the screen. Again like the regular Refresh() version, if
you call Refresh(False)
' then the buffer will draw directly to the screen without
first erasing the
' background.
'
//
========================================================================
============
If isBuffered Then // False for any OS X version, True
for all others
NewBuffer // method to create new Picture buffer
object "mBuffer"
Paint(mBuffer.Graphics) // new Paint() Event defined
BufferedCanvas class
Super.Refresh(EraseBackground) // finally calls the
Super.Refresh()
Else
Super.Refresh(EraseBackground)
End If
End Sub
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