Refresh app's XCUIElementQuery after screen updates

I'm writing UI tests for my application. How can I refresh list of app.buttons/app.otherElements, after screen has been updated?

Example: I'm pushing login button on login screen, waiting for server response. After response arrived I see another screen and need to push button on it, but button.exists == NO.

Should I make new instance of app([[XCUIApplication alloc] init]) each time when screen updates?