How to Unit Test App.addListener in Jasmine

Hi,

I am trying to write Jasmine unit tests for events passed from App.
Here is my code .ts file:

` public _disableBackButton(): void {
App.addListener('backButton', () => {
return;
});
}`

Here is .spec file:

it('should disable back button', fakeAsync(() => {
spyOn(App, 'addListener').and.callFake((type: string, fn: BackButtonListener) => {
return { type, fn };
});

component._disableBackButton();
App.addListener('backButton', () => ({}));
expect(App.addListener).toBeDefined();
}));

But it can't run into callFake.
The docs on testing seem limited, and I cannot see any issues on this. Am I missing a simple thing?

Thanks in advance!