Description:
Xojo incorrectly renders a Canvas Subclass unless the user steps through a breakpoint. If the user does not break execution, the Canvas Subclass renders the image twice. If the user steps through a breakbpoint, Xojo correctly renders one image.
This behavior is consistent. Stepping through the code should not alter behavior.
Repro steps:
Create a project with a Subclass for Canvas. Add a property - RealImage to the Subclass
Add an instance of the Subclass to the Window, with a Backdrop image
Add a Paint event, using the code below
Observe behavior by running the program
Add a breakpoint in the Paint event and restart, stepping through it
Observe behavior
Here is the Paint event:
// In the Paint event of the Canvas
if Me.RealImage = Nil then
Me.RealImage = Me.Backdrop // Get the backdrop
Me.Backdrop = nil // And then remove it
end if
If Me.RealImage <> Nil Then
// Draw the image scaled to fit the Canvas
g.DrawPicture(Me.RealImage, 0, 0, Me.Width, Me.Height, 0, 0, Me.RealImage.Width, Me.RealImage.Height)
End If
I would assume the backdrop removal would require a repaint of the control, which you are currently in. So it would render it twice.
Try moving your RealImage extraction code to an opening event and call refresh after updating the backdrop, may or may not actually need it but cannot hurt.
Only have actually drawing logic in the paint event.
Opening Event
if Me.RealImage = Nil then
Me.RealImage = Me.Backdrop // Get the backdrop
Me.Backdrop = nil // And then remove it
end if
RaiseEvent opening //Create opening event and raise it so sub classes / window can handle it
Refresh() //might not need this
Painting Event
If Me.RealImage <> Nil Then
// Draw the image scaled to fit the Canvas
dim ar as double = me.Realimage.Width / me.RealImage.height //Get aspect ratio of image
dim w as integer = g.width //try fitting to width
dim h as integer = round(w / ar) //get height
if h > g.height then //check height fits
h = g.height //Nope fit to height
w = round(h * ar) //get width
end if
//Render in the center of g
g.DrawPicture(Me.RealImage, g.width / 2 - w / 2, g.height / 2 - h / 2, w,h, 0, 0, Me.RealImage.Width, Me.RealImage.Height)
End If
Stepping through the code would be doing an app.doevents type thing that allows the app to update. This does not happen until end of methods / events during normal running.
Moving the code to opening event is better way to handle it as you only want to check it once. Not every paint event.
Right. My example is not the way to do this but I think there is a underlying flaw in how Xojo is processing it. Otherwise, the interrupt by the debugger wouldn’t matter.
It’s not about using the code I have, it’s about Xojo’s debugger changing the output. My guess is that breaking execution is causing another Paint event.
I’m not saying you’re wrong, but personally, I never think of any IDE debugger being a promise of catching your code executing as it would when your app is a stand-alone built app.
For example, sometimes when debugging inside a Focus event, debugger stepping will inadvertently cause focus events to fire repeatedly (losing & receiving) making it near impossible to replicate what should be happening.
Sometimes System.DebugLog is a better choice than breakpoints.
I appreciate your viewpoint but that would severely weaken the purpose of a debugger - to be able to step through a program and predictably observe the results. I don’t think I’ve ever seen such behavior on GDB, Visual Studio, VS Code, Code::Blocks, etc. Perhaps the quirks are typical for Xojo. I have many years of experience in other environments and languages but I am still new to Xojo. I’ve used it very little but I am hopeful that it will work for a medium-sized project.
@Greg_O makes an important point and it certainly looks like a timing issue. However, I would still contend that this is a matter Xojo should look into and see if they can improve the behavior.
And I’ve seen the VS debugger tell me lies several times, but thankfully that’s all in the past. I’m unintentionally retired now, and only play at Xojo to help keep me occupied, when I’m well enough.
All development platforms have their quirks, but I find Xojo the least problematic and somewhat calming, for an all-in-one tool.
I use it to set a picture at design time. And then grab it on open. Helps if you are doing custom buttons and want to see an icon or mask image etc at design time.
Unless you were referring to someone else, that’s what I clearly say I am doing, and show in all my sample code on this thread. I was just giving Beatrix an reason of why you would use the Backdrop Image in a canvas with custom rendering.
Also the reply button, does not seem to add the quote or who you are replying to anymore.
