Kingfisher/Sources/Views/AnimatedImageView.swift

//
//  AnimatableImageView.swift
//  Kingfisher
//
//  Created by bl4ckra1sond3tre on 4/22/16.
//
//  The AnimatableImageView, AnimatedFrame and Animator is a modified version of 
//  some classes from kaishin's Gifu project (https://github.com/kaishin/Gifu)
//
//  The MIT License (MIT)
//
//  Copyright (c) 2019 Reda Lemeden.
//
//  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.
//
//  The name and characters used in the demo of this software are property of their
//  respective owners.

#if !os(watchOS)
#if canImport(UIKit)
import UIKit
import ImageIO
typealias KFCrossPlatformContentMode = UIView.ContentMode
#elseif canImport(AppKit)
import AppKit
typealias KFCrossPlatformContentMode = NSImageScaling
#endif

/// Delegate of the ``AnimatedImageView``.
///
/// It reports back some events of the animated image view.
public protocol AnimatedImageViewDelegate: AnyObject {

    /// Called after the ``AnimatedImageView`` has finished each animation loop.
    ///
    /// - Parameters:
    ///   - imageView: The ``AnimatedImageView`` that is being animated.
    ///   - count: The loop count.
    func animatedImageView(_ imageView: AnimatedImageView, didPlayAnimationLoops count: UInt)

    /// Called after the ``AnimatedImageView`` has reached the maximum repeat count.
    ///
    /// - Parameter imageView: The ``AnimatedImageView`` that is being animated.
    func animatedImageViewDidFinishAnimating(_ imageView: AnimatedImageView)
}

extension AnimatedImageViewDelegate {
    public func animatedImageView(_ imageView: AnimatedImageView, didPlayAnimationLoops count: UInt) {}
    public func animatedImageViewDidFinishAnimating(_ imageView: AnimatedImageView) {}
}

let KFRunLoopModeCommon = RunLoop.Mode.common

/// Represents a subclass of `UIImageView` for displaying animated images.
///
/// Different from showing an animated image in a normal `UIImageView` (which loads all frames at one time),
/// ``AnimatedImageView`` only tries to load several frames (defined by ``AnimatedImageView/framePreloadCount``) to
/// reduce memory usage. It provides a tradeoff between memory usage and CPU time. If you have a memory issue when
/// using a normal image view to load GIF data, you could give this class a try.
///
/// Kingfisher supports setting GIF animated data to either `UIImageView` or ``AnimatedImageView`` out of the box. So
/// it would be fairly easy to switch between them.
@MainActor
open class AnimatedImageView: KFCrossPlatformImageView {
    /// Proxy object for preventing a reference cycle between the `CADDisplayLink` and `AnimatedImageView`.
    class TargetProxy {
        private weak var target: AnimatedImageView?
        
        init(target: AnimatedImageView) {
            self.target = target
        }
        
        @MainActor @objc func onScreenUpdate() {
            target?.updateFrameIfNeeded()
        }
    }

    /// An enumeration that specifies the repeat count of a GIF.
    public enum RepeatCount: Equatable {
        /// The animated image should be only played once.
        case once
        /// The animated image should be played by a finite times defined in the associated value.
        case finite(count: UInt)
        /// The animated image should be played infinitely.
        case infinite

        public static func ==(lhs: RepeatCount, rhs: RepeatCount) -> Bool {
            switch (lhs, rhs) {
            case let (.finite(l), .finite(r)):
                return l == r
            case (.once, .once),
                 (.infinite, .infinite):
                return true
            case (.once, .finite(let count)),
                 (.finite(let count), .once):
                return count == 1
            case (.once, _),
                 (.infinite, _),
                 (.finite, _):
                return false
            }
        }
    }
    
    // MARK: - Public property
    /// Whether to automatically play the animation when the view becomes visible. 
    ///
    /// The default is `true`.
    public var autoPlayAnimatedImage = true
    
    /// The count of frames that should be preloaded before being shown.
    public var framePreloadCount = 10
    
    /// Specifies whether the GIF frames should be pre-scaled to the image view's size or not.
    ///
    /// If the downloaded image is larger than the image view's size, it will help reduce some memory usage.
    ///
    /// The default is `true`.
    public var needsPrescaling = true

    /// Decode the GIF frames in background thread before using. It will decode frames data and do a off-screen
    /// rendering to extract pixel information in background. This can reduce the main thread CPU usage.
    ///
    @available(*, deprecated, message: """
        This property does not perform as declared and may lead to performance degradation.
        It is currently obsolete and scheduled for removal in a future version.
    """)
    public var backgroundDecode = true

    /// The animation timer's run loop mode. The default is `RunLoop.Mode.common`.
    ///
    /// Setting this property to `RunLoop.Mode.default` will make the animation pause during UIScrollView scrolling.
    public var runLoopMode = KFRunLoopModeCommon {
        willSet {
            guard runLoopMode != newValue else { return }
            stopAnimating()
            displayLink.remove(from: .main, forMode: runLoopMode)
            displayLink.add(to: .main, forMode: newValue)
            startAnimating()
        }
    }
    
    /// The repeat count. The animated image will keep animating until the loop count reaches this value.
    ///
    /// Setting this value to another one will reset the current animation.
    ///
    /// The default is ``RepeatCount/infinite``, which means the animation will last forever.
    public var repeatCount = RepeatCount.infinite {
        didSet {
            if oldValue != repeatCount {
                reset()
                #if os(macOS)
                needsDisplay = true
                layer?.setNeedsDisplay()
                #else
                setNeedsDisplay()
                layer.setNeedsDisplay()
                #endif
            }
        }
    }

    /// The delegate of this `AnimatedImageView` object. 
    ///
    /// See the ``AnimatedImageViewDelegate`` protocol for more information.
    public weak var delegate: AnimatedImageViewDelegate?

    /// The ``Animator`` instance that holds the frames of a specific image in memory.
    public private(set) var animator: Animator?

    // MARK: - Private property
    // Dispatch queue used for preloading images.
    private lazy var preloadQueue: DispatchQueue = {
        return DispatchQueue(label: "com.onevcat.Kingfisher.Animator.preloadQueue")
    }()
    
    // A flag to avoid invalidating the displayLink on deinit if it was never created, because displayLink is so lazy.
    private var isDisplayLinkInitialized: Bool = false
    
    // A display link that keeps calling the `updateFrame` method on every screen refresh.
    private lazy var displayLink: DisplayLinkCompatible = {
        isDisplayLinkInitialized = true
        let displayLink = self.compatibleDisplayLink(target: TargetProxy(target: self), selector: #selector(TargetProxy.onScreenUpdate))
        displayLink.add(to: .main, forMode: runLoopMode)
        displayLink.isPaused = true
        return displayLink
    }()
    
    // MARK: - Override
    override open var image: KFCrossPlatformImage? {
        didSet {
            if image != oldValue {
                reset()
            }
            #if os(macOS)
            needsDisplay = true
            layer?.setNeedsDisplay()
            #else
            setNeedsDisplay()
            layer.setNeedsDisplay()
            #endif
        }
    }
    
    open override var isHighlighted: Bool {
        get {
            super.isHighlighted
        }
        set {
            // Highlighted image is unsupported for animated images.
            // See https://github.com/onevcat/Kingfisher/issues/1679
            if displayLink.isPaused {
                super.isHighlighted = newValue
            }
        }
    }

// Workaround for Apple xcframework creating issue on Apple TV in Swift 5.8.
// https://github.com/apple/swift/issues/66015
#if os(tvOS)
    public override init(image: UIImage?, highlightedImage: UIImage?) {
        super.init(image: image, highlightedImage: highlightedImage)
    }
    
    required public init?(coder: NSCoder) {
        super.init(coder: coder)
    }
    
    init() {
        super.init(frame: .zero)
    }
#endif
    
    deinit {
        if isDisplayLinkInitialized {
            Task { @MainActor in
                self.displayLink.invalidate()
            }
        }
    }
    
#if os(macOS)
    public override init(frame frameRect: NSRect) {
        super.init(frame: frameRect)
        commonInit()
    }
    
    public required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    
    private func commonInit() {
        super.animates = false
        wantsLayer = true
    }
    
    open override var animates: Bool {
        get {
            if isDisplayLinkInitialized {
                return !displayLink.isPaused
            } else {
                return super.animates
            }
        }
        set {
            if newValue {
                startAnimating()
            } else {
                stopAnimating()
            }
        }
    }
    
    open func startAnimating() {
        guard let animator = animator else { return }
        guard !animator.isReachMaxRepeatCount else { return }

        displayLink.isPaused = false
    }
    
    open func stopAnimating() {
        if isDisplayLinkInitialized {
            displayLink.isPaused = true
        }
    }
    
    open override var wantsUpdateLayer: Bool {
        return true
    }
    
    open override func updateLayer() {
        if let frame = animator?.currentFrameImage ?? currentFrame, let layer = layer {
            layer.contents = frame.kf.cgImage
            layer.contentsScale = frame.kf.scale
            layer.contentsGravity = determineContentsGravity(for: frame)
            currentFrame = frame
        }
    }
    
    private func determineContentsGravity(for image: NSImage) -> CALayerContentsGravity {
        switch imageScaling {
            case .scaleProportionallyDown:
                if image.size.width > bounds.width || image.size.height > bounds.height {
                    return .resizeAspect
                } else {
                    return .center
                }
            case .scaleProportionallyUpOrDown:
                return .resizeAspect
            case .scaleAxesIndependently:
                return .resize
            case .scaleNone:
                return .center
            default:
                return .resizeAspect
        }
    }
    
    open override func viewDidMoveToWindow() {
        super.viewDidMoveToWindow()
        didMove()
    }
    
    open override func viewDidMoveToSuperview() {
        super.viewDidMoveToSuperview()
        didMove()
    }
#else
    override open var isAnimating: Bool {
        if isDisplayLinkInitialized {
            return !displayLink.isPaused
        } else {
            return super.isAnimating
        }
    }
    
    override open func startAnimating() {
        guard !isAnimating else { return }
        guard let animator = animator else { return }
        guard !animator.isReachMaxRepeatCount else { return }

        displayLink.isPaused = false
    }
    
    override open func stopAnimating() {
        super.stopAnimating()
        if isDisplayLinkInitialized {
            displayLink.isPaused = true
        }
    }
    
    override open func display(_ layer: CALayer) {
        layer.contents = animator?.currentFrameImage?.cgImage ?? image?.cgImage
    }
    
    override open func didMoveToWindow() {
        super.didMoveToWindow()
        didMove()
    }
    
    override open func didMoveToSuperview() {
        super.didMoveToSuperview()
        didMove()
    }
#endif

    // This is for back compatibility that using regular `UIImageView` to show animated image.
    override func shouldPreloadAllAnimation() -> Bool {
        return false
    }

    // Reset the animator.
    private func reset() {
        animator = nil
        currentFrame = nil
        if let image = image, let frameSource = image.kf.frameSource {
            #if os(visionOS)
            let scale = UITraitCollection.current.displayScale
            #elseif os(macOS)
            let scale = image.recommendedLayerContentsScale(window?.backingScaleFactor ?? 0.0)
            let contentMode = imageScaling
            #else
            var scale: CGFloat = 0
            if #available(iOS 13.0, tvOS 13.0, *) {
                scale = UITraitCollection.current.displayScale
            } else {
                scale = UIScreen.main.scale
            }
            #endif
            currentFrame = image
            let targetSize = bounds.scaled(scale).size
            let animator = Animator(
                frameSource: frameSource,
                contentMode: contentMode,
                size: targetSize,
                imageSize: image.kf.size,
                imageScale: image.kf.scale,
                framePreloadCount: framePreloadCount,
                repeatCount: repeatCount,
                preloadQueue: preloadQueue)
            animator.delegate = self
            animator.needsPrescaling = needsPrescaling
            animator.prepareFramesAsynchronously()
            self.animator = animator
        }
        didMove()
    }
    
    private func didMove() {
        if autoPlayAnimatedImage && animator != nil {
            if let _ = superview, let _ = window {
                startAnimating()
            } else {
                stopAnimating()
            }
        }
    }
    
    /// If the Animator cannot prepare the next frame in time, `animator.currentFrameImage` will return nil.
    /// To prevent unexpected blinking in the ImageView, we maintain a cache of the currently displayed frame
    /// to use as a fallback in such scenarios.
    private var currentFrame: KFCrossPlatformImage?
    
    /// Update the current frame with the displayLink duration.
    private func updateFrameIfNeeded() {
        guard let animator = animator else {
            return
        }

        guard !animator.isFinished else {
            stopAnimating()
            delegate?.animatedImageViewDidFinishAnimating(self)
            return
        }

        let duration: CFTimeInterval

        // CA based display link is opt-out from ProMotion by default.
        // So the duration and its FPS might not match. 
        // See [#718](https://github.com/onevcat/Kingfisher/issues/718)
        // By setting CADisableMinimumFrameDuration to YES in Info.plist may
        // cause the preferredFramesPerSecond being 0
        let preferredFramesPerSecond = displayLink.preferredFramesPerSecond
        if preferredFramesPerSecond == 0 {
            duration = displayLink.duration
        } else {
            // Some devices (like iPad Pro 10.5) will have a different FPS.
            duration = 1.0 / TimeInterval(preferredFramesPerSecond)
        }
        
        if animator.shouldChangeFrame(with: duration) {
            #if os(macOS)
            layer?.setNeedsDisplay()
            #else
            layer.setNeedsDisplay()
            #endif
        }
    }
}

protocol AnimatorDelegate: AnyObject {
    func animator(_ animator: AnimatedImageView.Animator, didPlayAnimationLoops count: UInt)
}

extension AnimatedImageView: AnimatorDelegate {
    func animator(_ animator: Animator, didPlayAnimationLoops count: UInt) {
        delegate?.animatedImageView(self, didPlayAnimationLoops: count)
    }
}

extension AnimatedImageView {

    // Represents a single frame in a GIF.
    struct AnimatedFrame {

        // The image to display for this frame. Its value is nil when the frame is removed from the buffer.
        let image: KFCrossPlatformImage?

        // The duration that this frame should remain active.
        let duration: TimeInterval

        // A placeholder frame with no image assigned.
        // Used to replace frames that are no longer needed in the animation.
        var placeholderFrame: AnimatedFrame {
            return AnimatedFrame(image: nil, duration: duration)
        }

        // Whether this frame instance contains an image or not.
        var isPlaceholder: Bool {
            return image == nil
        }

        // Returns a new instance from an optional image.
        //
        // - parameter image: An optional `UIImage` instance to be assigned to the new frame.
        // - returns: An `AnimatedFrame` instance.
        func makeAnimatedFrame(image: KFCrossPlatformImage?) -> AnimatedFrame {
            return AnimatedFrame(image: image, duration: duration)
        }
    }
}

extension AnimatedImageView {

    // MARK: - Animator

    /// An animator which is used to drive the data behind ``AnimatedImageView``.
    public class Animator {
        private let size: CGSize

        private let imageSize: CGSize
        private let imageScale: CGFloat

        /// The maximum count of image frames that need to be preloaded.
        public let maxFrameCount: Int

        private let frameSource: ImageFrameSource
        private let maxRepeatCount: RepeatCount

        private let maxTimeStep: TimeInterval = 1.0
        private let animatedFrames = SafeArray<AnimatedFrame>()
        private var frameCount = 0
        private var timeSinceLastFrameChange: TimeInterval = 0.0
        private var currentRepeatCount: UInt = 0

        var isFinished: Bool = false

        var needsPrescaling = true

        weak var delegate: AnimatorDelegate?

        // Total duration of one animation loop
        var loopDuration: TimeInterval = 0

        /// The image of the current frame.
        public var currentFrameImage: KFCrossPlatformImage? {
            return frame(at: currentFrameIndex)
        }

        /// The duration of the current active frame.
        public var currentFrameDuration: TimeInterval {
            return duration(at: currentFrameIndex)
        }

        /// The index of the current animation frame.
        public internal(set) var currentFrameIndex = 0 {
            didSet {
                previousFrameIndex = oldValue
            }
        }

        var previousFrameIndex = 0 {
            didSet {
                preloadQueue.async {
                    self.updatePreloadedFrames()
                }
            }
        }

        var isReachMaxRepeatCount: Bool {
            switch maxRepeatCount {
            case .once:
                return currentRepeatCount >= 1
            case .finite(let maxCount):
                return currentRepeatCount >= maxCount
            case .infinite:
                return false
            }
        }

        /// Whether the current frame is the last frame or not in the animation sequence.
        public var isLastFrame: Bool {
            return currentFrameIndex == frameCount - 1
        }

        var preloadingIsNeeded: Bool {
            return maxFrameCount < frameCount - 1
        }

        #if os(macOS)
        var contentMode = NSImageScaling.scaleAxesIndependently
        #else
        var contentMode = UIView.ContentMode.scaleToFill
        #endif

        private lazy var preloadQueue: DispatchQueue = {
            return DispatchQueue(label: "com.onevcat.Kingfisher.Animator.preloadQueue")
        }()

        /// Creates an animator with image source reference.
        ///
        /// - Parameters:
        ///   - source: The reference of animated image.
        ///   - mode: Content mode of the `AnimatedImageView`.
        ///   - size: Size of the `AnimatedImageView`.
        ///   - imageSize: Size of the `KingfisherWrapper`.
        ///   - imageScale: Scale of the `KingfisherWrapper`.
        ///   - count: Count of frames needed to be preloaded.
        ///   - repeatCount: The repeat count should this animator uses.
        ///   - preloadQueue: Dispatch queue used for preloading images.
        convenience init(imageSource source: CGImageSource,
                         contentMode mode: KFCrossPlatformContentMode,
                         size: CGSize,
                         imageSize: CGSize,
                         imageScale: CGFloat,
                         framePreloadCount count: Int,
                         repeatCount: RepeatCount,
                         preloadQueue: DispatchQueue) {
            let frameSource = CGImageFrameSource(data: nil, imageSource: source, options: nil)
            self.init(frameSource: frameSource,
                      contentMode: mode,
                      size: size,
                      imageSize: imageSize,
                      imageScale: imageScale,
                      framePreloadCount: count,
                      repeatCount: repeatCount,
                      preloadQueue: preloadQueue)
        }
        
        /// Creates an animator with a custom image frame source.
        ///
        /// - Parameters:
        ///   - frameSource: The reference of animated image.
        ///   - mode: Content mode of the `AnimatedImageView`.
        ///   - size: Size of the `AnimatedImageView`.
        ///   - imageSize: Size of the `KingfisherWrapper`.
        ///   - imageScale: Scale of the `KingfisherWrapper`.
        ///   - count: Count of frames needed to be preloaded.
        ///   - repeatCount: The repeat count should this animator uses.
        ///   - preloadQueue: Dispatch queue used for preloading images.
        init(frameSource source: ImageFrameSource,
             contentMode mode: KFCrossPlatformContentMode,
             size: CGSize,
             imageSize: CGSize,
             imageScale: CGFloat,
             framePreloadCount count: Int,
             repeatCount: RepeatCount,
             preloadQueue: DispatchQueue) {
            self.frameSource = source
            self.contentMode = mode
            self.size = size
            self.imageSize = imageSize
            self.imageScale = imageScale
            self.maxFrameCount = count
            self.maxRepeatCount = repeatCount
            self.preloadQueue = preloadQueue
        }
        
        deinit {
            resetAnimatedFrames()
        }

        /// Gets the image frame of a given index.
        /// - Parameter index: The index of the desired image.
        /// - Returns: The decoded image at the frame. `nil` if the index is out of bounds or the image is not yet loaded.
        public func frame(at index: Int) -> KFCrossPlatformImage? {
            return animatedFrames[index]?.image
        }

        /// Gets the duration of an image for the given frame index.
        /// - Parameter index: The index of the desired image.
        /// - Returns: The duration of that frame.
        public func duration(at index: Int) -> TimeInterval {
            return animatedFrames[index]?.duration  ?? .infinity
        }

        func prepareFramesAsynchronously() {
            frameCount = frameSource.frameCount
            animatedFrames.reserveCapacity(frameCount)
            preloadQueue.async { [weak self] in
                self?.setupAnimatedFrames()
            }
        }

        func shouldChangeFrame(with duration: CFTimeInterval) -> Bool {
            incrementTimeSinceLastFrameChange(with: duration)

            if currentFrameDuration > timeSinceLastFrameChange {
                return false
            } else {
                resetTimeSinceLastFrameChange()
                incrementCurrentFrameIndex()
                return true
            }
        }

        private func setupAnimatedFrames() {
            resetAnimatedFrames()

            var duration: TimeInterval = 0

            (0..<frameCount).forEach { index in
                let frameDuration = frameSource.duration(at: index)
                duration += min(frameDuration, maxTimeStep)
                animatedFrames.append(AnimatedFrame(image: nil, duration: frameDuration))

                if index > maxFrameCount { return }
                animatedFrames[index] = animatedFrames[index]?.makeAnimatedFrame(image: loadFrame(at: index))
            }

            self.loopDuration = duration
        }

        private func resetAnimatedFrames() {
            animatedFrames.removeAll()
        }

        private func loadFrame(at index: Int) -> KFCrossPlatformImage? {
            let resize = needsPrescaling && size != .zero
            let maxSize = resize ? size : nil
            guard let cgImage = frameSource.frame(at: index, maxSize: maxSize) else {
                return nil
            }
            
            #if os(macOS)
            return KFCrossPlatformImage(cgImage: cgImage, size: .zero)
            #else
            if #available(iOS 15, tvOS 15, *) {
                // From iOS 15, a plain image loading causes iOS calling `-[_UIImageCGImageContent initWithCGImage:scale:]`
                // in ImageIO, which holds the image ref on the creating thread.
                // To get a workaround, create another image ref and use that to create the final image. This leads to
                // some performance loss, but there is little we can do.
                // https://github.com/onevcat/Kingfisher/issues/1844
                // https://github.com/onevcat/Kingfisher/pulls/2194
                guard let unretainedImage = CGImage.create(ref: cgImage) else {
                    return KFCrossPlatformImage(cgImage: cgImage)
                }
                
                return KFCrossPlatformImage(cgImage: unretainedImage)
            } else {
                return KFCrossPlatformImage(cgImage: cgImage)
            }
            #endif
        }
        
        private func updatePreloadedFrames() {
            guard preloadingIsNeeded else {
                return
            }

            let previousFrame = animatedFrames[previousFrameIndex]
            animatedFrames[previousFrameIndex] = previousFrame?.placeholderFrame
            // ensure the image dealloc in main thread
            defer {
                if let image = previousFrame?.image {
                    DispatchQueue.main.async {
                        _ = image
                    }
                }
            }

            preloadIndexes(start: currentFrameIndex).forEach { index in
                guard let currentAnimatedFrame = animatedFrames[index] else { return }
                if !currentAnimatedFrame.isPlaceholder { return }
                animatedFrames[index] = currentAnimatedFrame.makeAnimatedFrame(image: loadFrame(at: index))
            }
        }

        private func incrementCurrentFrameIndex() {
            let wasLastFrame = isLastFrame
            currentFrameIndex = increment(frameIndex: currentFrameIndex)
            if isLastFrame {
                currentRepeatCount += 1
                if isReachMaxRepeatCount {
                    isFinished = true

                    // Notify the delegate here because the animation is stopping.
                    delegate?.animator(self, didPlayAnimationLoops: currentRepeatCount)
                }
            } else if wasLastFrame {

                // Notify the delegate that the loop completed
                delegate?.animator(self, didPlayAnimationLoops: currentRepeatCount)
            }
        }

        private func incrementTimeSinceLastFrameChange(with duration: TimeInterval) {
            timeSinceLastFrameChange += min(maxTimeStep, duration)
        }

        private func resetTimeSinceLastFrameChange() {
            timeSinceLastFrameChange -= currentFrameDuration
        }

        private func increment(frameIndex: Int, by value: Int = 1) -> Int {
            return (frameIndex + value) % frameCount
        }

        private func preloadIndexes(start index: Int) -> [Int] {
            let nextIndex = increment(frameIndex: index)
            let lastIndex = increment(frameIndex: index, by: maxFrameCount)

            if lastIndex >= nextIndex {
                return [Int](nextIndex...lastIndex)
            } else {
                return [Int](nextIndex..<frameCount) + [Int](0...lastIndex)
            }
        }
    }
}

class SafeArray<Element> {
    private var array: Array<Element> = []
    private let lock = NSLock()
    
    subscript(index: Int) -> Element? {
        get {
            lock.lock()
            defer { lock.unlock() }
            return array.indices ~= index ? array[index] : nil
        }
        
        set {
            lock.lock()
            defer { lock.unlock() }
            if let newValue = newValue, array.indices ~= index {
                array[index] = newValue
            }
        }
    }
    
    var count : Int {
        lock.lock()
        defer { lock.unlock() }
        return array.count
    }
    
    func reserveCapacity(_ count: Int) {
        lock.lock()
        defer { lock.unlock() }
        array.reserveCapacity(count)
    }
    
    func append(_ element: Element) {
        lock.lock()
        defer { lock.unlock() }
        array += [element]
    }
    
    func removeAll() {
        lock.lock()
        defer { lock.unlock() }
        array = []
    }
}
#endif