How to Successfully Update @Published Variables in SwiftUI with DispatchQueue.main.async

Learn how to properly update published variables in SwiftUI while displaying video content using `AVCaptureVideoPreviewLayer`. This guide guides you through the implementation and offers effective solutions.
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Developing a SwiftUI app that incorporates real-time video processing can lead to some nuanced challenges, particularly when it comes to updating your UI based on live data inputs. A common question arises when working with AVCaptureVideoPreviewLayer and AVCaptureVideoDataOutputSampleBufferDelegate: How do I ensure my UI properly reflects changes made to @Published variables?

In this guide, we will explore the issue, outline a practical solution, and provide useful code snippets to demonstrate how to handle the issue effectively.

Understanding the Problem

When integrating video capture features in your SwiftUI application, you may find that the UI does not update as expected when changes are made to @Published properties within your view model. This typically happens when updates are being handled on a background thread, which can lead to inconsistencies in your UI rendering.

The Scenario

You are using AVCaptureVideoDataOutputSampleBufferDelegate to receive video frames in real-time. While you have code that successfully updates the view with new data, it seems that after introducing your video preview layer, any changes made to your published variable, such as luminosity readings, are not reflected in the SwiftUI view.

Code Sample Before Fix

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The above statement attempts to update luminosityReading on the main queue, but fails to trigger a UI refresh under certain circumstances.

The Solution

Through exploring various approaches, a workable solution involves creating a custom UIView to manage the video preview layer and encapsulating its functionality within a SwiftUI view. Here’s how to do it step by step.

Step 1: Create a Custom UIView

Start by defining a custom UIView that incorporates your AVCaptureSession:

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Step 2: Use UIViewRepresentable

Next, create a UIViewRepresentable conforming structure that helps bridge the custom view for SwiftUI:

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Step 3: Implement in ContentView

Finally, use the new VideoPreviewHolder in your ContentView. This ensures that the AV session is only active when appropriate, allowing the UI to properly reflect updates.

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Conclusion

Adopting this structure allows you to efficiently manage the video layer while ensuring that your SwiftUI views are updated correctly based on the live data being processed. The key takeaway is to ensure that your UI reflects the state of your underlying data model seamlessly.

Implementing this pattern not only improves performance but also ensures your SwiftUI app can handle real-time updates gracefully.

By taking these steps, you can enhance your SwiftUI applications and avoid the pitfalls associated with updating @Published properties in a multi-threaded environment.