Delphi, Firemonkey - How to draw on component canvas outside of paint

Question

I am writing my first component for Firemonkey. It is very dynamic control and to keep refresh times minimal, when a property changes, I only want to modify the effected attributes rather than repaint the entire control.

The first issue I found was that unless you are in the Paint loop, you need to call Canvas.SetMatrix(AbsoluteMatrix) first otherwise canvas functions are referenced to the parents coordinates. I don't quite understand this.

The second issued is that when use this control on OSX, unless I call the inherited paint procedure (which I override) nothing changes on the canvas gets displayed. This works fine in Win32

Component is based off of TControl

Answer 1

You might not like this, but you're not supposed to paint outside of a paint event. So don't do it. Windows is a bit more forgiving if you break that rule, but you shouldn't do it on Windows either. For example, if your window is (partly) hidden, no updates are needed and the OS will skip the paint event. So instead of trying to work against the OS it is better to work with it. And usually there is a better alternative.

You can keep an internal "cache" bitmap and update that as needed. Then when the paint event comes, you can draw this entire bitmap. If possible, update this cached bitmap in the paint event if it needs changing.

If you want to temporarily highlight items, you can have a transparent window on top and paint on that window. Let the OS window manager do the heavy work for you.