Disposable.swift

//
//  Disposable.swift
//  ReactiveSwift
//
//  Created by Justin Spahr-Summers on 2014-06-02.
//  Copyright (c) 2014 GitHub. All rights reserved.
//

/// Represents something that can be “disposed”, usually associated with freeing
/// resources or canceling work.
public protocol Disposable: AnyObject {
	/// Whether this disposable has been disposed already.
	var isDisposed: Bool { get }

	/// Disposing of the resources represented by `self`. If `self` has already
	/// been disposed of, it does nothing.
	///
	/// - note: Implementations must issue a memory barrier.
	func dispose()
}

/// Represents the state of a disposable.
private enum DisposableState: Int32 {
	/// The disposable is active.
	case active

	/// The disposable has been disposed.
	case disposed
}

extension UnsafeAtomicState where State == DisposableState {
	/// Try to transition from `active` to `disposed`.
	///
	/// - returns: `true` if the transition succeeds. `false` otherwise.
	@inline(__always)
	fileprivate func tryDispose() -> Bool {
		return tryTransition(from: .active, to: .disposed)
	}
}

/// A disposable that does not have side effect upon disposal.
internal final class _SimpleDisposable: Disposable {
	private let state = UnsafeAtomicState<DisposableState>(.active)

	var isDisposed: Bool {
		return state.is(.disposed)
	}

	func dispose() {
		_ = state.tryDispose()
	}

	deinit {
		state.deinitialize()
	}
}

/// A disposable that has already been disposed.
internal final class NopDisposable: Disposable {
	static let shared = NopDisposable()
	var isDisposed = true
	func dispose() {}
	private init() {}
}

/// A type-erased disposable that forwards operations to an underlying disposable.
public final class AnyDisposable: Disposable {
	private final class ActionDisposable: Disposable {
		let state: UnsafeAtomicState<DisposableState>
		var action: (() -> Void)?

		var isDisposed: Bool {
			return state.is(.disposed)
		}

		init(_ action: (() -> Void)?) {
			self.state = UnsafeAtomicState(.active)
			self.action = action
		}

		deinit {
			state.deinitialize()
		}

		func dispose() {
			if state.tryDispose() {
				action?()
				action = nil
			}
		}
	}

	private let base: Disposable

	public var isDisposed: Bool {
		return base.isDisposed
	}

	/// Create a disposable which runs the given action upon disposal.
	///
	/// - parameters:
	///   - action: A closure to run when calling `dispose()`.
	public init(_ action: @escaping () -> Void) {
		base = ActionDisposable(action)
	}

	/// Create a disposable.
	public init() {
		base = _SimpleDisposable()
	}

	/// Create a disposable which wraps the given disposable.
	///
	/// - parameters:
	///   - disposable: The disposable to be wrapped.
	public init(_ disposable: Disposable) {
		base = disposable
	}

	public func dispose() {
		base.dispose()
	}
}

/// A disposable that will dispose of any number of other disposables.
public final class CompositeDisposable: Disposable {
	private let disposables: Atomic<Bag<Disposable>?>
	private var state: UnsafeAtomicState<DisposableState>

	public var isDisposed: Bool {
		return state.is(.disposed)
	}

	/// Initialize a `CompositeDisposable` containing the given sequence of
	/// disposables.
	///
	/// - parameters:
	///   - disposables: A collection of objects conforming to the `Disposable`
	///                  protocol
	public init<S: Sequence>(_ disposables: S) where S.Iterator.Element == Disposable {
		let bag = Bag(disposables)
		self.disposables = Atomic(bag)
		self.state = UnsafeAtomicState(.active)
	}

	/// Initialize a `CompositeDisposable` containing the given sequence of
	/// disposables.
	///
	/// - parameters:
	///   - disposables: A collection of objects conforming to the `Disposable`
	///                  protocol
	public convenience init<S: Sequence>(_ disposables: S)
		where S.Iterator.Element == Disposable?
	{
		self.init(disposables.compactMap { $0 })
	}

	/// Initializes an empty `CompositeDisposable`.
	public convenience init() {
		self.init([Disposable]())
	}

	public func dispose() {
		if state.tryDispose(), let disposables = disposables.swap(nil) {
			for disposable in disposables {
				disposable.dispose()
			}
		}
	}

	/// Add the given disposable to the composite.
	///
	/// - parameters:
	///   - disposable: A disposable.
	///
	/// - returns: A disposable to remove `disposable` from the composite. `nil` if the
	///            composite has been disposed of, `disposable` has been disposed of, or
	///            `disposable` is `nil`.
	@discardableResult
	public func add(_ disposable: Disposable?) -> Disposable? {
		guard let d = disposable, !d.isDisposed, !isDisposed else {
			disposable?.dispose()
			return nil
		}

		return disposables.modify { disposables in
			guard let token = disposables?.insert(d) else { return nil }

			return AnyDisposable { [weak self] in
				self?.disposables.modify {
					$0?.remove(using: token)
				}
			}
		}
	}

	/// Add the given action to the composite.
	///
	/// - parameters:
	///   - action: A closure to be invoked when the composite is disposed of.
	///
	/// - returns: A disposable to remove `disposable` from the composite. `nil` if the
	///            composite has been disposed of, `disposable` has been disposed of, or
	///            `disposable` is `nil`.
	@discardableResult
	public func add(_ action: @escaping () -> Void) -> Disposable? {
		return add(AnyDisposable(action))
	}

	deinit {
		state.deinitialize()
	}

	/// Adds the right-hand-side disposable to the left-hand-side
	/// `CompositeDisposable`.
	///
	/// ````
	///  disposable += producer
	///      .filter { ... }
	///      .map    { ... }
	///      .start(observer)
	/// ````
	///
	/// - parameters:
	///   - lhs: Disposable to add to.
	///   - rhs: Disposable to add.
	///
	/// - returns: An instance of `DisposableHandle` that can be used to opaquely
	///            remove the disposable later (if desired).
	@discardableResult
	public static func += (lhs: CompositeDisposable, rhs: Disposable?) -> Disposable? {
		return lhs.add(rhs)
	}

	/// Adds the right-hand-side `ActionDisposable` to the left-hand-side
	/// `CompositeDisposable`.
	///
	/// ````
	/// disposable += { ... }
	/// ````
	///
	/// - parameters:
	///   - lhs: Disposable to add to.
	///   - rhs: Closure to add as a disposable.
	///
	/// - returns: An instance of `DisposableHandle` that can be used to opaquely
	///            remove the disposable later (if desired).
	@discardableResult
	public static func += (lhs: CompositeDisposable, rhs: @escaping () -> Void) -> Disposable? {
		return lhs.add(rhs)
	}
}

/// A disposable that, upon deinitialization, will automatically dispose of
/// its inner disposable.
public final class ScopedDisposable<Inner: Disposable>: Disposable {
	/// The disposable which will be disposed when the ScopedDisposable
	/// deinitializes.
	public let inner: Inner

	public var isDisposed: Bool {
		return inner.isDisposed
	}

	/// Initialize the receiver to dispose of the argument upon
	/// deinitialization.
	///
	/// - parameters:
	///   - disposable: A disposable to dispose of when deinitializing.
	public init(_ disposable: Inner) {
		inner = disposable
	}

	deinit {
		dispose()
	}

	public func dispose() {
		return inner.dispose()
	}
}

extension ScopedDisposable where Inner == AnyDisposable {
	/// Initialize the receiver to dispose of the argument upon
	/// deinitialization.
	///
	/// - parameters:
	///   - disposable: A disposable to dispose of when deinitializing, which
	///                 will be wrapped in an `AnyDisposable`.
	public convenience init(_ disposable: Disposable) {
		self.init(Inner(disposable))
	}
}

extension ScopedDisposable where Inner == CompositeDisposable {
	/// Adds the right-hand-side disposable to the left-hand-side
	/// `ScopedDisposable<CompositeDisposable>`.
	///
	/// ````
	/// disposable += { ... }
	/// ````
	///
	/// - parameters:
	///   - lhs: Disposable to add to.
	///   - rhs: Disposable to add.
	///
	/// - returns: An instance of `DisposableHandle` that can be used to opaquely
	///            remove the disposable later (if desired).
	@discardableResult
	public static func += (lhs: ScopedDisposable<CompositeDisposable>, rhs: Disposable?) -> Disposable? {
		return lhs.inner.add(rhs)
	}

	/// Adds the right-hand-side disposable to the left-hand-side
	/// `ScopedDisposable<CompositeDisposable>`.
	///
	/// ````
	/// disposable += { ... }
	/// ````
	///
	/// - parameters:
	///   - lhs: Disposable to add to.
	///   - rhs: Closure to add as a disposable.
	///
	/// - returns: An instance of `DisposableHandle` that can be used to opaquely
	///            remove the disposable later (if desired).
	@discardableResult
	public static func += (lhs: ScopedDisposable<CompositeDisposable>, rhs: @escaping () -> Void) -> Disposable? {
		return lhs.inner.add(rhs)
	}
}

/// A disposable that disposes of its wrapped disposable, and allows its
/// wrapped disposable to be replaced.
public final class SerialDisposable: Disposable {
	private let _inner: Atomic<Disposable?>
	private var state: UnsafeAtomicState<DisposableState>

	public var isDisposed: Bool {
		return state.is(.disposed)
	}

	/// The current inner disposable to dispose of.
	///
	/// Whenever this property is set (even to the same value!), the previous
	/// disposable is automatically disposed.
	public var inner: Disposable? {
		get {
			return _inner.value
		}

		set(disposable) {
			_inner.swap(disposable)?.dispose()

			if let disposable = disposable, isDisposed {
				disposable.dispose()
			}
		}
	}

	/// Initializes the receiver to dispose of the argument when the
	/// SerialDisposable is disposed.
	///
	/// - parameters:
	///   - disposable: Optional disposable.
	public init(_ disposable: Disposable? = nil) {
		self._inner = Atomic(disposable)
		self.state = UnsafeAtomicState(DisposableState.active)
	}

	public func dispose() {
		if state.tryDispose() {
			_inner.swap(nil)?.dispose()
		}
	}

	deinit {
		state.deinitialize()
	}
}