# runDeferredStartWhenNeeded() Tells the session to run deferred start when appropriate. ``` func runDeferredStartWhenNeeded() ``` ## Discussion For best perceived startup performance, call this after displaying the first frame, so that deferred start processing doesn’t interfere with other initialization operations. For example, if using a to draw camera frames, add a `presentHandler` (using ) to the first drawable and call [`runDeferredStartWhenNeeded()`](/documentation/AVFoundation/AVCaptureSession/runDeferredStartWhenNeeded()) from there. If one or more outputs need to start to perform a capture operation, and [`runDeferredStartWhenNeeded()`](/documentation/AVFoundation/AVCaptureSession/runDeferredStartWhenNeeded()) has not run yet, the session runs the deferred start on your app’s behalf. Only call this method once for each configuration commit - after the first call, subsequent calls to [`runDeferredStartWhenNeeded()`](/documentation/AVFoundation/AVCaptureSession/runDeferredStartWhenNeeded()) have no effect. The deferred start runs asynchronously, so this method returns immediately. > Note: You can only call this when ``doc://com.apple.avfoundation/documentation/AVFoundation/AVCaptureSession/automaticallyRunsDeferredStart`` is `false`. Otherwise, the session throws an `NSInvalidArgumentException`. > Important: To avoid blocking your app’s UI, don’t call this method from the application’s main actor or queue. --- 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)