# CFRunLoopObserverCreateWithHandler(_:_:_:_:_:) Creates a CFRunLoopObserver object with a block-based handler. ``` func CFRunLoopObserverCreateWithHandler(_ allocator: CFAllocator!, _ activities: CFOptionFlags, _ repeats: Bool, _ order: CFIndex, _ block: ((CFRunLoopObserver?, CFRunLoopActivity) -> Void)!) -> CFRunLoopObserver! ``` ## Parameters `allocator` The allocator to use to allocate memory for the new object. Pass `NULL` or [`kCFAllocatorDefault`](/documentation/CoreFoundation/kCFAllocatorDefault) to use the current default allocator. `activities` Set of flags identifying the activity stages of the run loop during which the observer is called. See [`CFRunLoopActivity`](/documentation/CoreFoundation/CFRunLoopActivity)for the list of stages. To have the observer called at multiple stages in the run loop, combine the [`CFRunLoopActivity`](/documentation/CoreFoundation/CFRunLoopActivity) values using the bitwise-OR operator. `repeats` A flag identifying whether the observer is called only once or every time through the run loop. If `repeats` is `false`, the observer is invalidated after it is called once, even if the observer was scheduled to be called at multiple stages within the run loop. `order` A priority index indicating the order in which run loop observers are processed. When multiple run loop observers are scheduled in the same activity stage in a given run loop mode, the observers are processed in increasing order of this parameter. Pass 0 unless there is a reason to do otherwise. `block` The block invoked when the observer runs. The block takes two arguments: - `observer`: The run loop observer that is firing. - `activity`: The current activity stage of the run loop. ## Return Value The new CFRunLoopObserver object. Ownership follows the Create Rule described in [Ownership Policy](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html#//apple_ref/doc/uid/20001148). ## Discussion The run loop observer is not automatically added to a run loop. To add the observer to a run loop, use [`CFRunLoopAddObserver(_:_:_:)`](/documentation/CoreFoundation/CFRunLoopAddObserver(_:_:_:)). An observer can be registered to only one run loop, although it can be added to multiple run loop modes within that run loop. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)