Read before looking at image

Canvas is actually for SwiftUI, Also you can use the benefits of it by porting your UIKit elements into the SwiftUI preview. But sometimes for some un/known reason, canvas can not reload the view in time and it's not working.

For example:

1. When you work on a catalyst project and select mac as a target.
2. When you use a custom run script that cause a change on the xcworkspace during the build process (like auto build number increment script or etc.)
3. When you have a long task that triggers the watch dog event. (like recursion or massive loop)
4. Syntax error
5. Complex swift code
6. etc.

These are just a few of them that I faced and conformed. For more information about the issue, most of the time the canvas itself reports the issue and you can read about it with clicking on Diagnostics button above the canvas window:

But some times you can't even see this pan. Sometimes disable and enable the canvas works for that situation and sometimes restarting Xcode (or even mac) works, but some times not!

Hope it helps you save some time.