# init(blockTip:headerTip:progress:warningSet:warningUnset:flushError:fatalError:) Creates notification callbacks. ``` init(blockTip: ((SynchronizationState, BlockTreeEntrySnapshot, Double) -> Void)? = nil, headerTip: ((SynchronizationState, Int64, Int64, Bool) -> Void)? = nil, progress: ((String, Int32, Bool) -> Void)? = nil, warningSet: ((Warning, String) -> Void)? = nil, warningUnset: ((Warning) -> Void)? = nil, flushError: ((String) -> Void)? = nil, fatalError: ((String) -> Void)? = nil) ``` ## Parameters `blockTip` Handler for chain tip updates. See [`blockTip`](/documentation/BitcoinKernel/NotificationCallbacks/blockTip). `headerTip` Handler for new best headers. See [`headerTip`](/documentation/BitcoinKernel/NotificationCallbacks/headerTip). `progress` Handler for block sync progress. See [`progress`](/documentation/BitcoinKernel/NotificationCallbacks/progress). `warningSet` Handler for new validation warnings. See [`warningSet`](/documentation/BitcoinKernel/NotificationCallbacks/warningSet). `warningUnset` Handler for cleared validation warnings. See [`warningUnset`](/documentation/BitcoinKernel/NotificationCallbacks/warningUnset). `flushError` Handler for errors flushing data to disk. See [`flushError`](/documentation/BitcoinKernel/NotificationCallbacks/flushError). `fatalError` Handler for unrecoverable library errors. See [`fatalError`](/documentation/BitcoinKernel/NotificationCallbacks/fatalError). ## Discussion Set only the callbacks you need; unset callbacks are ignored. All closures are captured at init time and cannot be changed later.