Java 具有可以在新线程中运行任务的 Future 或 FutureTask。然后,将执行结果返回给原线程。Swift 中是否有任何功能可以实现这一点?
6 回答
语言不提供(即标准库),但您当然可以自己动手或简单地使用诸如https://github.com/Thomvis/BrightFutures之类的库
您正在研究某种称为Futures 和 promises的语言结构。您可以找到一些示例,例如:
- https://bitbucket.org/al45tair/async(在 Swift 中类似于 C# 的 async/await 原语)
- https://github.com/mxcl/PromiseKit(承诺工具包http://promisekit.org/)
- 前面提到过https://github.com/Thomvis/BrightFutures
然而,语言本身缺少这样的特性。
如果 Apple 确实在 Swift 中实现了 Futures 或 Promises,他们会这么说吗?毕竟,他们总是避免谈论未来的产品。;)
无论如何,最初的问题似乎通常是关于异步工作的方法,而不一定是专门用 Futures/Promises 样式模型来做这件事。因此,虽然其他答案中提到的第三方库非常适合该模型,但您也可以在没有该模型的情况下使用与 ObjC:dispatch
或NSOperation
. 例如:
NSOperationQueue().addOperationWithBlock {
// do background work
NSOperationQueue.mainQueue().addOperationWithBlock {
// back to main thread for follow up work
}
}
现在也有FutureKit 类似于 BrightFuture,但组合更像 BFTask
我应该提到Bolts BFTask,虽然它是用 Objective-C 编写的,但它也是一个很好的候选者。(现在在 Facebook iOS SDK 中使用)
我最终得到以下基于Operation
和OperationQueue
类的解决方案(仅限 iOS SDK,Swift 3):
简而言之:将代码包装成同步或异步操作。使用实用程序类链接操作。将操作添加到串行队列中。
如果出现错误,无需取消当前操作,只需跳过实际代码即可。此外,异步执行块必须调用finalize
回调来通知操作队列完成。可以选择DispatchQueue
作为参数提供。代码块将在该队列上异步执行。
fileprivate func publishProductOnWebsite(listing: Listing) {
var resultSKU: String?
var resultError: Swift.Error?
let chain = OperationsChain{ isExecuting, finalize in
let task = ServerAPI.create(publishInfo: listing.publishInfo) { sku, error in
guard isExecuting() else {
return // We are canceled. Nothing to do.
}
if let error = error {
resultError = error
} else if let sku = sku {
resultSKU = sku // Arbitrary thread. But OK as this example for serial operation queue.
}
finalize() // This will finish asynchronous operation
}
task.resume()
}
chain.thenAsync(blockExecutionQueue: DispatchQueue.main) { _, finalize in
if let sku = resultSKU {
listing.sku = sku
DBStack.mainContext.saveIfHasChanges(savingParent: true) { error in
resultError = error
finalize()
}
} else {
finalize()
}
}
chain.thenSync(blockExecutionQueue: DispatchQueue.main) { [weak self] in
if let error = resultError {
self?.handleError(error) // Executed on Main thread.
} else {
self?.trackPublish()
self?.eventHandler?(.publishCompleted)
}
}
operationQueue.cancelAllOperations()
operationQueue.addOperations(chain.operations, waitUntilFinished: false)
}
OperationsChain类:将代码块包装Operation
到数组中,并将操作保存到operations
维护依赖项的数组中。
public class OperationsChain {
public private(set) var operations = [Operation]()
public init(blockExecutionQueue: DispatchQueue? = nil,
executionBlock: @escaping AsynchronousBlockOperation.WorkItemBlock) {
let op = AsynchronousBlockOperation(blockExecutionQueue: blockExecutionQueue, executionBlock: executionBlock)
operations.append(op)
}
public init(blockExecutionQueue: DispatchQueue? = nil,
executionBlock: @escaping SynchronousBlockOperation.WorkItemBlock) {
let op = SynchronousBlockOperation(blockExecutionQueue: blockExecutionQueue, executionBlock: executionBlock)
operations.append(op)
}
@discardableResult
public func thenAsync(blockExecutionQueue: DispatchQueue? = nil,
executionBlock: @escaping AsynchronousBlockOperation.WorkItemBlock) -> AsynchronousBlockOperation {
let op = AsynchronousBlockOperation(blockExecutionQueue: blockExecutionQueue, executionBlock: executionBlock)
if let lastOperation = operations.last {
op.addDependency(lastOperation)
} else {
assertionFailure()
}
operations.append(op)
return op
}
@discardableResult
public func thenSync(blockExecutionQueue: DispatchQueue? = nil,
executionBlock: @escaping SynchronousBlockOperation.WorkItemBlock) -> SynchronousBlockOperation {
let op = SynchronousBlockOperation(blockExecutionQueue: blockExecutionQueue, executionBlock: executionBlock)
if let lastOperation = operations.last {
op.addDependency(lastOperation)
} else {
assertionFailure()
}
operations.append(op)
return op
}
}
SynchronousBlockOperation和AsynchronousBlockOperation类。
public final class SynchronousBlockOperation: Operation {
public typealias WorkItemBlock = (Void) -> Void
fileprivate var executionBlock: WorkItemBlock?
fileprivate var blockExecutionQueue: DispatchQueue?
public init(blockExecutionQueue: DispatchQueue? = nil, executionBlock: @escaping SynchronousBlockOperation.WorkItemBlock) {
self.blockExecutionQueue = blockExecutionQueue
self.executionBlock = executionBlock
super.init()
}
public override func main() {
if let queue = blockExecutionQueue {
queue.async { [weak self] in
self?.executionBlock?()
}
} else {
executionBlock?()
}
}
}
open class AsynchronousBlockOperation: AsynchronousOperation {
public typealias FinaliseBlock = (Void) -> Void
public typealias StatusBlock = (Void) -> Bool
public typealias WorkItemBlock = (@escaping StatusBlock, @escaping FinaliseBlock) -> Void
fileprivate var executionBlock: WorkItemBlock?
fileprivate var blockExecutionQueue: DispatchQueue?
public init(blockExecutionQueue: DispatchQueue? = nil, executionBlock: @escaping AsynchronousBlockOperation.WorkItemBlock) {
self.blockExecutionQueue = blockExecutionQueue
self.executionBlock = executionBlock
super.init()
}
open override func onStart() {
if let queue = blockExecutionQueue {
queue.async { [weak self] in
self?.executionBlock?({ return self?.isExecuting ?? false }) {
self?.finish()
}
}
} else {
executionBlock?({ [weak self] in return self?.isExecuting ?? false }) { [weak self] in
self?.finish()
}
}
}
}
AsynchronousOperation类: 的可重用子类Operation
。
open class AsynchronousOperation: Operation {
fileprivate var lockOfProperties = NonRecursiveLock.makeDefaultLock()
fileprivate var lockOfHandlers = NonRecursiveLock.makeDefaultLock()
fileprivate var mFinished = false
fileprivate var mExecuting = false
}
extension AsynchronousOperation {
public final override var isAsynchronous: Bool {
return true
}
public final override var isExecuting: Bool {
return lockOfProperties.synchronized { mExecuting }
}
public final override var isFinished: Bool {
return lockOfProperties.synchronized { mFinished }
}
}
extension AsynchronousOperation {
public final override func start() {
if isCancelled || isFinished || isExecuting {
return
}
willChangeValue(forKey: "isExecuting")
lockOfProperties.synchronized { mExecuting = true }
onStart()
didChangeValue(forKey: "isExecuting")
}
public final override func cancel() {
super.cancel()
if isExecuting {
onCancel()
finish()
} else {
onCancel()
lockOfProperties.synchronized {
mExecuting = false
mFinished = true
}
}
}
public final func finish() {
willChangeValue(forKey: "isExecuting")
willChangeValue(forKey: "isFinished")
lockOfProperties.synchronized {
mExecuting = false
mFinished = true
}
onFinish()
didChangeValue(forKey: "isExecuting")
didChangeValue(forKey: "isFinished")
}
}
extension AsynchronousOperation {
/// Subclasses must launch job here.
///
/// **Note** called between willChangeValueForKey and didChangeValueForKey calls, but after property mExecuting is set.
open func onStart() {
}
/// Subclasses must cancel job here.
///
/// **Note** called immediately after calling super.cancel().
open func onCancel() {
}
/// Subclasses must release job here.
///
/// **Note** called between willChangeValueForKey and didChangeValueForKey calls,
/// but after properties mExecuting and mFinished are set.
open func onFinish() {
}
}
Swift 的Combine 框架使用了这些结构