Testing projects with SWT at different zoom levels

[ptziegler]

This is a problem I ran into while investigating eclipse-gef/gef-classic#945. The height of a font (obviously) depends on the native monitor zoom. But the GitHub tests all run at 100% monitor zoom, so it's difficult to write regression tests for such behavior.

Is there any way to tell SWT to execute tests as if at e.g. 125% monitor zoom? I tried a similar approach to #2967 and am currently stuck setting DPIUtil.setDeviceZoom at the start of the test suite. I'm curious whether there is a cleaner approach to this.

[HeikoKlare]

Unfortunately, there is no good way to test with different monitor zoom yet. We have also discussed this quite often as we desperately need it to avoid regressions, but it's not that simple.
There is a test utility which does something in the direction you want, but it is Windows-only and requries access to package-protected methods:

eclipse.platform.swt/bundles/org.eclipse.swt/Eclipse SWT Tests/win32/org/eclipse/swt/widgets/DPITestUtil.java

Lines 29 to 35 in e640660

	public static void changeDPIZoom (Shell shell, int nativeZoom) {
	DPIUtil.setDeviceZoom(nativeZoom);
	Event event = shell.createZoomChangedEvent(nativeZoom, true);
	shell.sendZoomChangedEvent(event, shell);
	DPIChangeExecution data = (DPIChangeExecution) event.data;
	waitForDPIChange(shell, TIMEOUT_MILLIS, data.taskCount);
	}

So that is only possible in Win32-specific tests with a utility class in the org.eclipse.swt.widgets package that makes a (discouraged) call of those methods. I.e., nothing one would really recommend.

And even with such a utility, what you can test with it is really limited. The scaling capabilities are rather tighly coupled to what the OS is doing underneath (DPI information and events, font scaling etc.), which is almost impossible to adapt or mock. So as long as we rely on the adoption of the Win32 library/controls, I currently don't see any way of properly testing with other zooms than actually changing the environment settings (i.e., change the zoom of your primary monitor, the Jenkins agent or whatever the test is running on).

[HeikoKlare]

Unfortunately, there is no good way to test with different monitor zoom yet. We have also discussed this quite often as we desperately need it to avoid regressions, but it's not that simple.
There is a test utility which does something in the direction you want, but it is Windows-only and requries access to package-protected methods:

eclipse.platform.swt/bundles/org.eclipse.swt/Eclipse SWT Tests/win32/org/eclipse/swt/widgets/DPITestUtil.java

Lines 29 to 35 in e640660

	public static void changeDPIZoom (Shell shell, int nativeZoom) {
	DPIUtil.setDeviceZoom(nativeZoom);
	Event event = shell.createZoomChangedEvent(nativeZoom, true);
	shell.sendZoomChangedEvent(event, shell);
	DPIChangeExecution data = (DPIChangeExecution) event.data;
	waitForDPIChange(shell, TIMEOUT_MILLIS, data.taskCount);
	}

So that is only possible in Win32-specific tests with a utility class in the org.eclipse.swt.widgets package that makes a (discouraged) call of those methods. I.e., nothing one would really recommend.

I tried a similar approach to #2967 and am currently eclipse-gef/gef-classic#959 setting DPIUtil.setDeviceZoom at the start of the test suite. I'm curious whether there is a cleaner approach to this.

Using DPIUtil.setDeviceZoom won't even work, as the value will be overwritten whenever a shell is created (at least on Windows). So you would have to reset it after a shell was created (as done by the utility linked above), but you also need to make the shell aware of the modified zoom (which is done by changing the zoom via an according event, as done by the linked utility).

And even with such a utility as mentioned above, what you can test with it is really limited. The scaling capabilities are rather tighly coupled to what the OS is doing underneath (DPI information and events, font scaling etc.), which is almost impossible to adapt or mock. So as long as we rely on the adoption of the Win32 library/controls, I currently don't see any way of properly testing with other zooms than actually changing the environment settings (i.e., change the zoom of your primary monitor, the Jenkins agent or whatever the test is running on).

[ptziegler]

Thanks for the explanation. I already had a gut feeling that this isn't easy to test.

Using DPIUtil.setDeviceZoom won't even work, as the value will be overwritten whenever a shell is created (at least on Windows). So you would have to reset it after a shell was created (as done by the utility linked above)

For the test itself, it was sufficient. Otherwise I wouldn't have gotten the initial test failures. But I see your point. I was already expecting this to be nothing but a dirty hack, so this is the confirmation.

I think in that case I'll keep things as is and just keep an eye out for any changes that might make this possible.