import AVFoundation

/// The `PlayerLoader` class provides a callback mechanism for consumers of `AVPlayer` when loading a resource.
/// AVFoundation works by an inconvenient series of APIs that rely on KVO for asynchronous operations. This class
/// eliminates the boilerplate that KVO imposes and makes the callsite much more clean.
public final class PlayerLoader: NSObject {
    public typealias Callback = (AVPlayer) -> ()

    /// The callback will initialize to a non-nil value. The callback is set to nil once it's invoked, at which time the
    /// KVO observation is removed. This is necessary to avoid double-removing the KVO observation.
    private var callback: Callback?

    private let player: AVPlayer

    // This is declared `var` because we have to take its pointer value
    private static var observationContext = NSObject()
    private let observationKeypath = #keyPath(AVPlayer.currentItem.status)

    init(player: AVPlayer, callback: @escaping Callback) {
        self.callback = callback
        self.player = player

        super.init()

        if self.player.currentItem?.status == AVPlayerItemStatus.readyToPlay {
            self.callback?(self.player)
        } else {
            self.player.addObserver(
                self,
                forKeyPath: self.observationKeypath,
                options: [NSKeyValueObservingOptions.new],
                context: &PlayerLoader.observationContext
            )
        }
    }

    public convenience init(url: URL, callback: @escaping Callback) {
        self.init(player: AVPlayer(url: url), callback: callback)
    }

    override public func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        guard let context = context, context == &PlayerLoader.observationContext else {
            return
        }

        guard let keyPath = keyPath, keyPath == self.observationKeypath else {
            return
        }

        guard let currentItem = self.player.currentItem else {
            return
        }

        guard !CMTimeGetSeconds(currentItem.duration).isNaN else {
            return
        }

        guard currentItem.status == AVPlayerItemStatus.readyToPlay else {
            return
        }

        self.player.removeObserver(self, forKeyPath: self.observationKeypath)

        self.player.preroll(atRate: 1.0) { _ in
            self.callback?(self.player)
            self.callback = nil
        }
    }

    deinit {
        // This is really awkward but is one of the reasons why this class exists in the first place.
        if self.callback != nil {
            self.player.removeObserver(self, forKeyPath: self.observationKeypath)
        }
    }
}