A wrapper around iOS Entity/loadAsync(named:in:) and visionOS Entity(named:in:) async that works on both platforms

Entity+LoadFileAsync.swift

//
//  Entity+LoadFileAsync.swift
//
//  Created by Drew Olbrich on 8/4/23.
//  Copyright © 2023 Lunar Skydiving LLC. All rights reserved.
//
//  MIT License
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in all
//  copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//  SOFTWARE.
//

import Foundation
import RealityKit
import Combine

extension Entity {
    
    /// Loads an entity from a file in a bundle asynchronously.
    ///
    /// Unlike `Entity(named:in:) async` or `Entity.loadAsync(named:in:)`, this method
    /// works on both iOS and visionOS.
    ///
    /// If `flattened` is true, the entity hierarchy in the file will be flattened into
    /// a single `ModelEntity`.
    @MainActor static func loadFileAsync(named name: String, in bundle: Bundle? = nil, flattened: Bool) async throws -> Entity {
        if #available(iOS 18.0, visionOS 1.0, *) {
            if flattened {
                return try await ModelEntity(named: name, in: bundle)
            } else {
                return try await Entity(named: name, in: bundle)
            }
        } else {
            // The RealityKit `Entity(named:in:) async` initializer isn't available in iOS 17,
            // so we implement our own replacement and wrap both implementations in
            // `Entity/loadFileAsync(named:in:)`.
            let entity: Entity = try await withCheckedThrowingContinuation { continuation in
                func loadAsync<T: Entity>(with loadRequest: LoadRequest<T>) {
                    var cancellable: AnyCancellable?
                    cancellable = loadRequest.sink(receiveCompletion: { completion in
                        switch completion {
                        case .finished:
                            break
                        case .failure(let error):
                            continuation.resume(throwing: error)
                        }
                        cancellable?.cancel()
                    }, receiveValue: { entity in
                        continuation.resume(returning: entity)
                        cancellable?.cancel()
                    })
                }
                if flattened {
                    loadAsync(with: Entity.loadModelAsync(named: name, in: bundle))
                } else {
                    loadAsync(with: Entity.loadAsync(named: name, in: bundle))
                }
            }
            
#if DEBUG
            await entity.apply(recursive: true) { entity async in
                if let modelEntity = entity as? ModelEntity {
                    // On iOS 17, RealityKit doesn't appear to be able to read USD files that contain
                    // primitive types like boxes. USD files must contain polygons.
                    assert(modelEntity.model != nil)
                }
            }
#endif
            
#if !os(visionOS)
            // On iOS 17 and earlier, `Entity.loadModelAsync` doesn't assign a name to
            // flattened entities. For consistency with visionOS, we assign the name.
            if flattened {
                assert(entity.name == "")
                entity.name = name
            }
#endif
            return entity
        }
    }
    
}