How to test MatDialog is opened using mock in angular unit test

I am currently using a component, in which on button click a pop-up will be opened to greet user. I am using angular Material Dialog for opening pop-up and the component code block which will call on click is

public openMatDialog(): void {
this.isDialogOpen = true;

this.dialogReference = this.dialog.open(GreetComponent, {
autoFocus: false,
disableClose: false,
});

this.dialogReference.afterClosed().subscribe(
() => {
this.isDialogOpen = false;
}
);


}

I also need to check boolean property to indicate dialog opened and closed.

In the component spec, I am providing mock for dialog to prevent actual dependency like below,

export class MatDialogMock {
open() {
return {
afterClosed: () => of(true)
};
}
}


in spec providers,

{ provide: MatDialog, useValue: matDialog },


in beforeEach,

const matDialog = new MatDialogMock();


When I testing Dialog open like below,

it('Greet User', () => {
spyOn(matDialog, 'open');
component.openMatDialog();
expect(matDialog.open).toHaveBeenCalled();
});


it is getting failed with error,

Error: Expected spy open to have been called.


Kindly guide how to check dialog open using mock in spec.

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