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Daniel Pertu describes adding tap-to-focus to expo-camera by patching its Android Kotlin and iOS Swift implementations, along with its TypeScript declarations and compiled JavaScript. His patch targets expo-camera@57.0.5; it is an implementation account, not evidence that the patch works with later releases or on every device. Expo’s documented autoFocus setting controls focus mode, which is not the same as choosing a focus point by tapping.
What the patch changes
Pertu says continuous autofocus was not enough for his capture flow: users needed to nominate a small point of interest on the preview. His solution adds a native focus-point method and a tap interaction that requests focus at the selected location.
The patch changes four parts of the package:
- Android: Kotlin camera code implements the native focus behavior.
- iOS: Swift camera code implements the corresponding behavior.
- TypeScript: the API declaration exposes the new method to app code.
- Compiled package output: JavaScript and declaration output are patched as well as source files.
Pertu’s explanation for changing compiled output is that the installed package’s runtime uses that output. Editing source alone could therefore leave the new method unavailable when the app runs. This is specific to the package files and publishing arrangement he describes; check the exact installed release before assuming its layout is the same.
How the tap interaction is intended to work
Map the tap to the camera preview
A touch gives coordinates in the view, but Pertu warns that the preview uses aspect-fill. Because the camera image is cropped to fill the view, simply dividing the touch’s horizontal and vertical positions by the view’s width and height does not necessarily identify the same point in the camera’s coordinate space. The mapping must account for the preview layout and crop.
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The article does not establish a generally validated coordinate transform. Treat its warning as a design requirement: work out how the preview is laid out in your app, then verify that taps land on the intended subject on the physical devices and orientations you support. A transform that appears correct in one layout may not hold when the preview’s dimensions or cropping change.
Give users visible confirmation
Pertu draws a focus ring at the selected point so the user can see which location was requested. He keys the feedback to a timestamp so that tapping the same location again can still produce a fresh visual response. The ring confirms the requested point; it should not be presented as proof that the camera has successfully focused there.
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Restore continuous focus safely
The described behavior temporarily sets the requested focus point and later restores continuous focus. Pertu also checks that the active capture device is still the same before applying delayed state changes: if the app changes lenses in the meantime, a delayed update could otherwise target the wrong device. These are safeguards in his implementation, not guarantees supplied by expo-camera.
Why patch compiled JavaScript as well as native code
Changing Kotlin and Swift supplies platform behavior, but app code also needs a declared and callable method. Pertu therefore updates the TypeScript API and compiled package output alongside the native files. When adapting the idea, trace how the precise installed package resolves its runtime entry point and declarations; do not assume that editing a source file automatically changes what Metro or the native build consumes.
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Choose between a package patch and a local Expo module
The account uses a package patch to make the new control available alongside the existing camera component. Expo also documents local Expo modules as a way to add native Swift and Kotlin code. These approaches put code in different places and entail different maintenance responsibilities.
| Consideration | Patch to expo-camera |
Local Expo module |
|---|---|---|
| Where the behavior lives | Inside the installed camera package, including its native and JavaScript layers, as in Pertu’s account. | In a separate local module containing native code, an approach documented by Expo. |
Relationship to CameraView |
Can integrate with the package’s camera implementation; that integration is the route Pertu chose. | Must be designed to work with the app’s camera flow; the cited guidance does not specify a tap-to-focus integration. |
| Upgrade work | Package changes can make the patch stop applying or require manual reconciliation. | The app team owns the separate module and must keep its integration aligned with the camera lifecycle. |
| Build and Expo Go | Pertu says his custom native patch requires real native builds. | Custom Swift and Kotlin code likewise requires a native build; Expo Go is limited to native libraries already in its SDK or libraries that need no custom native code. |
Pertu’s view that patching avoided maintaining a fork or a separate module applies to his app, not every project. Prefer the route that fits how closely the feature must live inside CameraView, who will maintain the native code, and how the app builds and tests native changes.
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Expo Go and web limitations
Do not expect this custom native patch to become available in Expo Go. Expo Go can use native libraries included in its SDK or libraries that do not require custom native code; a change to camera code in Kotlin and Swift needs a build that contains that change. Expo’s local-module guidance is another way to add native code, but it does not remove the need for a suitable native build.
Pertu describes his web path as a browser media preview without the native focus control. In that app, the web experience keeps a plain shutter instead of offering a tap-to-focus interaction. That is his implementation choice, not a claim that every browser or camera is incapable of focusing.
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Pin the patch and recheck it on upgrades
Pertu pins his patch to expo-camera@57.0.5. He intentionally treats a patch-application failure after an upgrade as a prompt to inspect the package changes, rather than allowing the feature to disappear unnoticed. The retrieved Expo camera documentation recommended ~57.0.6, and the changelog included releases through 58.0.7; those version references do not establish that Pertu’s patch works on either later version.
Before adopting or upgrading this implementation:
- Check the exact installed
expo-cameraversion and review the patch against that release’s native source, declarations, and compiled output. - Inspect how that release manages camera devices and focus state, especially when switching lenses or applying delayed updates.
- Build the app with the custom native changes rather than relying on Expo Go.
- Test on physical Android and iOS devices, checking tap placement across the preview’s actual aspect-fill layout, repeated taps, focus feedback, and lens changes.
- When a package update makes the patch fail, review the changed camera lifecycle and runtime files before updating or removing the patch.
The available documentation describes autoFocus as a focus-mode setting, not a tap-selected focus point. It does not settle whether a newer first-party API has since been added, so check the documentation and API for the exact version you install before maintaining a custom patch.
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