Introduction

In iOS applications, performing long tasks or heavy computations on the main thread can lead to an unresponsive user interface, resulting in a poor user experience. To mitigate this, developers often display a loading indicator or overlay, allowing users to understand that a task is in progress and the application has not frozen. This article provides a technical dive into efficiently implementing an overlay when executing long-running tasks in iOS, using both UIKit and SwiftUI.

Why Use Overlays?

Loading overlays serve several purposes:

• User Experience: They inform users that a task is ongoing, which prevents frustrations associated with perceived app freezes.
• Feedback: Users receive visual feedback that their input has been acknowledged.
• Prevent Interaction: Overlays can disable user interactions to prevent additional actions that could disrupt the current process.

Technical Considerations

When implementing a loading overlay, key considerations include:

• Thread Management: Ensure tasks are offloaded from the main thread to prevent UI blocking.
• Customizable Overlays: Ability to easily customize the appearance based on the app's design.

Implementing Overlays in UIKit

Basic Implementation

• Thread Offloading: The long task is executed on a background thread using `DispatchQueue.global`.
• UI Updates: Post-task UI updates are dispatched back to the main thread with `DispatchQueue.main.async`.
• Activity Indicator: An `UIActivityIndicatorView` provides visual feedback, added to a semi-transparent black view.
• ZStack: The overlay is layered on top of the main content.
• State Management: The `@State` property `isOverlayVisible` controls the overlay's visibility.
• SwiftUI's `ProgressView`: Provides a native loading indicator.
• Offload to Background Threads: Always perform long tasks on background threads to keep the main thread responsive.
• Use of Swift Concurrency: Consider using Swift's async/await for tasks in apps targeting iOS 15 and later.
• Avoid Complex UIs in Overlays: Keep overlays simple to minimize rendering performance issues.