Source
public enum SourceDescribes the source view from which the popover is showing.
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                  Specifies that the popover should display from a UIBarButtonIteminstance.DeclarationSwift case barButtonItem(UIBarButtonItem)
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                  Specifies that the popover should display from a UIViewinstance and be anchored on the specificframe. If theframeisnilthen the provided view’s frame will be used as the anchor frame.DeclarationSwift case view(container: UIView, frame: CGRect?)
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