Responsive capture# PRO

A family of AVCapturePhotoOutput switches that change how the capture pipeline behaves between shots. None of them change the photo quality — they change when you can take the next shot, and how soon you get a usable image back from the current one.

#isZeroShutterLagEnabled

Builds a rolling buffer of recent frames so a tap captures the moment just before the tap, not after. Most useful for action shots — kids, pets, sports.

• Defaults to true on devices where it's supported (iPhone XS / XR and newer).
• Set to false if you want strictly the moment of the tap (rare).

if (photoOutput.isZeroShutterLagSupported) {
  photoOutput.isZeroShutterLagEnabled = true
}

#isResponsiveCaptureEnabled + isFastCapturePrioritizationEnabled

These are a pair. They let the pipeline finish the current photo's processing in the background while the user keeps tapping.

Order matters. Apple gates isFastCapturePrioritizationSupported on isResponsiveCaptureEnabled = true (and maxPhotoQualityPrioritization = "quality"). So turn responsive on first — then fast becomes supported.

photoOutput.maxPhotoQualityPrioritization = "quality"

if (photoOutput.isResponsiveCaptureSupported) {
  photoOutput.isResponsiveCaptureEnabled = true
}
// fastSupp only flips to true after the line above runs.
if (photoOutput.isFastCapturePrioritizationSupported) {
  photoOutput.isFastCapturePrioritizationEnabled = true
}

If you flip them out of order from JS, the bridge fixes it: setting fast = true first will turn responsive on if supported, and turning responsive = false first turns fast off. So this also works:

photoOutput.isFastCapturePrioritizationEnabled = true   // responsive gets auto-enabled

If fastSupp stays false even after responsive is on, the active format / preset doesn't allow it — fast is silently ignored. Common blockers: maxPhotoQualityPrioritization not "quality", isLivePhotoCaptureEnabled = true, or a non-photo session preset.

#isAutoDeferredPhotoDeliveryEnabled (read this carefully)

Deferred Photo Delivery lets you tap the shutter again immediately, without waiting for the previous photo's full processing pipeline (Deep Fusion, Photonic Engine, Smart HDR) to finish. Apple does this by handing your app a "proxy" right away and finalizing the real photo asynchronously.

The catch — and the reason this section is much longer than the others — is that the finalized photo never comes back through capturePhoto().

#What you actually get

When deferred kicks in, capturePhoto() resolves with:

{
  image: UIImage,         // ← this is the PROXY, not the final
  metadata: {...},
  isRawPhoto: false,
  isDeferredProxy: true,
}

The image is an unfinished version of the photo: a quick render the system can produce in milliseconds. Visually it has more noise, less detail, and a weaker dynamic range than the final. It's perfectly fine for "show the user what they just shot" thumbnails, but it is not the photo you want to keep.

The system finalizes the real photo on its own clock — usually within seconds, but it can take longer if the device is busy or you've gone to background — and writes the result directly to the user's Photo Library. There is no callback into your app.

#So how do I get the final photo?

Use PhotoKit. Query the user's Photo Library for the most recent image after the proxy resolves:

photoOutput.isAutoDeferredPhotoDeliveryEnabled = true

const result = await photoOutput.capturePhoto()
if (result.isDeferredProxy) {
  // result.image is the proxy. Show it as a thumbnail.
  showThumbnail(result.image)
  // The final image will appear in the Photo Library on its own.
  // If your script needs it, you'll have to use Photo.getLastestPhotos or Photo.pickPhotos
}

#When not to enable this

If your script wants the photo bytes immediately (for upload, processing, file save) and doesn't have or want Photo Library permission, leave deferred off. The default behavior — wait for the final, then resolve — is the right one for most scripts.

You'd opt in when:

• You're building a tap-many-photos-in-a-row UI and need quick feedback per shot
• You're already saving everything to the Photo Library anyway
• You'd rather show a less-perfect preview now than a spinner

#Detection cheat sheet