An Entity extension that supports animated changes the opacity of an entity and its descendants

Entity+Opacity.swift

//
//  Entity+Opacity.swift
//
//  Created by Drew Olbrich on 10/25/23.
//  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

public extension Entity {
    
    /// The opacity value applied to the entity and its descendants.
    ///
    /// `OpacityComponent` is assigned to the entity if it doesn't already exist.
    var opacity: Float {
        get {
            return components[OpacityComponent.self]?.opacity ?? 1
        }
        set {
            if newValue >= 1 {
                // If we set the opacity to 1, it's best to remove the `OpacityComponent`, or
                // otherwise RealityKit appears to treat the entity hierarchy as transparent for
                // model sort order purposes, which may introduce unwanted behavior.
                components.remove(OpacityComponent.self)
            } else {
                if !components.has(OpacityComponent.self) {
                    components[OpacityComponent.self] = OpacityComponent(opacity: newValue)
                } else {
                    components[OpacityComponent.self]?.opacity = newValue
                }
            }
        }
    }
    
    /// Sets the opacity value applied to the entity and its descendants with optional animation.
    ///
    /// `OpacityComponent` is assigned to the entity if it doesn't already exist.
    ///
    /// ## Note
    /// When this method executes, RealityKit will log this warning:
    /// ```
    /// Failed to set override status for bind point component member.
    /// ```
    /// This appears to be a RealityKit framework limitation that occurs during the animation
    /// binding process. The warnings do not affect functionality and can be safely ignored.
    func setOpacity(_ opacity: Float, animated: Bool, duration: TimeInterval = 0.2, delay: TimeInterval = 0, completion: (() -> Void)? = nil) {
        guard animated else {
            self.opacity = opacity
            return
        }
        
        guard scene != nil else {
            completion?()
            return
        }
        
        if !components.has(OpacityComponent.self) {
            components[OpacityComponent.self] = OpacityComponent(opacity: 1)
        }

        let animation = FromToByAnimation(name: "Entity.setOpacity", to: opacity, duration: duration, timing: .linear, isAdditive: false, bindTarget: .opacity, delay: delay)
        
        do {
            let animationResource: AnimationResource = try .generate(with: animation)
            let animationPlaybackController = playAnimation(animationResource)
            
            if completion != nil {
                // `cancellable` is captured by the `receiveCompletion` closure below, forming a
                // temporary retain cycle that keeps the subscription alive without any external
                // storage. When the publisher completes (after `.first()` receives the animation
                // event), `receiveCompletion` sets `cancellable` to nil, breaking the cycle and
                // deallocating the subscription.
                var cancellable: AnyCancellable?
                cancellable = scene?.publisher(for: AnimationEvents.PlaybackCompleted.self, on: self)
                    .filter { $0.playbackController == animationPlaybackController }
                    .first()
                    .sink(
                        receiveCompletion: { _ in cancellable = nil },
                        receiveValue: { [weak self] _ in
                            if opacity >= 1 {
                                // If we set the opacity to 1, it's best to remove the `OpacityComponent`, or
                                // otherwise RealityKit appears to treat the entity hierarchy as transparent for
                                // model sort order purposes, which may introduce unwanted behavior.
                                self?.components.remove(OpacityComponent.self)
                            }
                            completion?()
                        }
                    )
                // Suppress the "written to, but never read" warning. The compiler doesn't
                // recognize the closure capture of `cancellable` as a read, but it IS used:
                // `receiveCompletion` sets it to nil to break the retain cycle.
                _ = cancellable
            }
        } catch {
            assertionFailure("Could not generate animation: \(error.localizedDescription)")
        }
    }
    
}