import Foundation

/// An abstract class that makes building simple asynchronous operations easy.
/// Subclasses must implement `execute()` to perform any work and call
/// `finish()` when they are done. All `NSOperation` work will be handled
/// automatically.
open class AsynchronousOperation: Operation {

    // MARK: - Properties

    private let stateQueue = DispatchQueue(
        label: "com.calebd.operation.state",
        attributes: .concurrent)

    private var rawState = OperationState.ready

    @objc private dynamic var state: OperationState {
        get {
            return stateQueue.sync(execute: { rawState })
        }
        set {
            willChangeValue(forKey: "state")
            stateQueue.sync(
                flags: .barrier,
                execute: { rawState = newValue })
            didChangeValue(forKey: "state")
        }
    }

    public final override var isReady: Bool {
        return state == .ready && super.isReady
    }

    public final override var isExecuting: Bool {
        return state == .executing
    }

    public final override var isFinished: Bool {
        return state == .finished
    }
    
    public final override var isAsynchronous: Bool {
        return true
    }


    // MARK: - NSObject

    @objc private dynamic class func keyPathsForValuesAffectingIsReady() -> Set<String> {
        return ["state"]
    }

    @objc private dynamic class func keyPathsForValuesAffectingIsExecuting() -> Set<String> {
        return ["state"]
    }

    @objc private dynamic class func keyPathsForValuesAffectingIsFinished() -> Set<String> {
        return ["state"]
    }


    // MARK: - Foundation.Operation

    public override final func start() {
        super.start()

        if isCancelled {
            finish()
            return
        }

        state = .executing
        execute()
    }


    // MARK: - Public

    /// Subclasses must implement this to perform their work and they must not
    /// call `super`. The default implementation of this function throws an
    /// exception.
    open func execute() {
        fatalError("Subclasses must implement `execute`.")
    }

    /// Call this function after any work is done or after a call to `cancel()`
    /// to move the operation into a completed state.
    public final func finish() {
        state = .finished
    }
}

@objc private enum OperationState: Int {
    case ready
    case executing
    case finished
}