[英]Operation Queue not executing in order even after setting priority and dependency on Operations
我正在进行三次api调用,并且希望API1首先执行,一旦完成API2应该执行,然后执行API3。 我使用操作队列来增加对操作的依赖性。 我尝试设置优先级,但没有按顺序获取api调用。 帮助我如何正确地做到这一点。
代码是这样的:
let op1 = Operation()
op1.completionBlock = {
self.APICall(urlString: self.url1)
}
op1.queuePriority = .veryHigh
let op2 = Operation()
op2.completionBlock = {
self.APICall(urlString: self.url2)
}
op2.queuePriority = .high
let op3 = Operation()
op3.completionBlock = {
self.APICall(urlString: self.url3)
}
op3.queuePriority = .normal
op2.addDependency(op1)
op3.addDependency(op2)
queue.addOperations([op1, op2, op3], waitUntilFinished: false)
我把API调用方法放在DispatchQueue.main.sync中,如下所示:
func APICall(urlString: String) {
let headers: HTTPHeaders = [
"Accept": "text/html"
]
print(urlString)
DispatchQueue.main.sync {
Alamofire.request(urlString.addingPercentEncoding(withAllowedCharacters: CharacterSet.urlQueryAllowed)!, method: .get, parameters: nil, encoding: JSONEncoding.default, headers: headers).responseJSON {
response in
// self.stopActivityIndicator()
print(response.result.value)
switch response.result {
case .success:
break
case .failure(let error):
break
}
}
}
}
有几个问题:
如果您尝试管理操作之间的依赖关系,则不能将操作的completionBlock
用于依赖项依赖的代码。 直到操作完成后才调用完成块(从而破坏了任何依赖的目的)。
所以以下内容不会按预期工作:
let queue = OperationQueue() let op1 = Operation() op1.completionBlock = { print("starting op1") Thread.sleep(forTimeInterval: 1) print("finishing op1") } let op2 = Operation() op2.completionBlock = { print("starting op2") Thread.sleep(forTimeInterval: 1) print("finishing op2") } op2.addDependency(op1) queue.addOperations([op1, op2], waitUntilFinished: false)
但是如果你定义这样的操作,它将起作用:
let op1 = BlockOperation() { print("starting op1") Thread.sleep(forTimeInterval: 1) print("finishing op1") } let op2 = BlockOperation { print("starting op2") Thread.sleep(forTimeInterval: 1) print("finishing op2") }
(但这只能起作用,因为我重新定义了同步操作。请参见下面的第3点。)
值得注意的是,您通常不会直接使用Operation
。 正如文档所说 :
一个抽象类,表示与单个任务关联的代码和数据。 ...
因为
Operation
类是一个抽象类,所以不要直接使用它,而是使用系统定义的子类(NSInvocationOperation
或BlockOperation
)子类或使用其中一个来执行实际任务。
因此,使用BlockOperation
,上面或BlockOperation
它,如下面第3点所示。
如果必须严格遵守订单,则不应使用优先级来管理操作执行的订单。 正如queuePriority
文档所说 (强调添加):
该值用于影响操作出列和执行的顺序......
您应该仅在需要时使用优先级值来对非依赖操作的相对优先级进行分类。 不应使用优先级值来实现不同操作对象之间的依赖关系管理。 如果需要在操作之间建立依赖关系,请改用
addDependency(_:)
方法。
因此,如果您排队100个高优先级操作和100个默认优先级操作,则无法保证所有高优先级操作都将在优先级较低的操作开始运行之前启动。 它倾向于优先考虑它们,但严格来说并非如此。
第一点是没有实际意义,因为您正在调用异步方法。 所以你不能使用简单的Operation
或BlockOperation
。 如果您不希望后续网络请求在前一个网络请求完成之前启动,则您需要将这些网络请求包装在自定义异步Operation
子类中,并使用所需的所有特殊KVO:
class NetworkOperation: AsynchronousOperation { var request: DataRequest static var sessionManager: SessionManager = { let manager = Alamofire.SessionManager(configuration: .default) manager.startRequestsImmediately = false return manager }() init(urlString: String, parameters: [String: String]? = nil, completion: @escaping (Result<Any>) -> Void) { let headers: HTTPHeaders = [ "Accept": "text/html" ] let string = urlString.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)! let url = URL(string: string)! request = NetworkOperation.sessionManager.request(url, parameters: parameters, headers: headers) super.init() request.responseJSON { [weak self] response in completion(response.result) self?.finish() } } override func main() { request.resume() } override func cancel() { request.cancel() } }
然后你可以这样做:
let queue = OperationQueue() let op1 = NetworkOperation(urlString: ...) { result in ... } let op2 = NetworkOperation(urlString: ...) { result in ... } let op3 = NetworkOperation(urlString: ...) { result in ... } op2.addDependency(op1) op3.addDependency(op2) queue.addOperations([op1, op2, op3], waitUntilFinished: false)
因为那是使用AsynchronousOperation
子类(如下所示),所以在异步请求完成之前,操作不会完成。
/// Asynchronous operation base class /// /// This is abstract to class performs all of the necessary KVN of `isFinished` and /// `isExecuting` for a concurrent `Operation` subclass. You can subclass this and /// implement asynchronous operations. All you must do is: /// /// - override `main()` with the tasks that initiate the asynchronous task; /// /// - call `completeOperation()` function when the asynchronous task is done; /// /// - optionally, periodically check `self.cancelled` status, performing any clean-up /// necessary and then ensuring that `finish()` is called; or /// override `cancel` method, calling `super.cancel()` and then cleaning-up /// and ensuring `finish()` is called. public class AsynchronousOperation: Operation { /// State for this operation. @objc private enum OperationState: Int { case ready case executing case finished } /// Concurrent queue for synchronizing access to `state`. private let stateQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".rw.state", attributes: .concurrent) /// Private backing stored property for `state`. private var _state: OperationState = .ready /// The state of the operation @objc private dynamic var state: OperationState { get { stateQueue.sync { _state } } set { stateQueue.sync(flags: .barrier) { _state = newValue } } } // MARK: - Various `Operation` properties open override var isReady: Bool { return state == .ready && super.isReady } public final override var isAsynchronous: Bool { return true } public final override var isExecuting: Bool { return state == .executing } public final override var isFinished: Bool { return state == .finished } // KVN for dependent properties open override class func keyPathsForValuesAffectingValue(forKey key: String) -> Set<String> { if ["isReady", "isFinished", "isExecuting"].contains(key) { return [#keyPath(state)] } return super.keyPathsForValuesAffectingValue(forKey: key) } // Start public final override func start() { if isCancelled { state = .finished return } state = .executing main() } /// Subclasses must implement this to perform their work and they must not call `super`. The default implementation of this function throws an exception. open override func main() { fatalError("Subclasses must implement `main`.") } /// Call this function to finish an operation that is currently executing public final func finish() { if !isFinished { state = .finished } } }
作为非常小的观察,您的代码使用JSON参数指定了GET请求。 这没有意义。 GET请求没有可以包含JSON的主体。 GET请求仅使用URL编码。 除此之外你没有传递任何参数。
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