setDelegate method
Set a delegate to receive billing lifecycle callbacks (purchases,
failures, entitlement changes, restores, billing-unavailable).
Pass null to clear the current delegate and stop listening.
Implementation
void setDelegate(AppDNABillingDelegate? delegate) {
_delegate = delegate;
_delegateSub?.cancel();
_delegateSub = null;
if (delegate == null) return;
_delegateSub = _delegateChannel.receiveBroadcastStream().listen(_dispatch);
}