NSBlockOperation不执行

Question

由于我目前正在处理web服务调用和数据库检索，我偶然发现了NSBlockOperation未启动的问题。

不过，一开始要解释一下。主要目标是有一个由以下操作组成的操作队列：

1. 操作1-向web服务发送请求以开始创建数据库.zip文件
2. 操作2-在完成#1后，重复调用web服务以了解.zip文件压缩过程的进展情况。
3. 操作3-在完成#2之后，使用NSURLSessionDownloadTask下载数据库。

为此，我定义了以下类：

class DatabaseServerDownload {
typealias Callback = (storedLocation : NSURL) -> ()
typealias Failure = ((error : NSError) -> Void)

let callback : Callback
let failure : Failure

let operationQueue : NSOperationQueue

required init(callback : Callback, failure : Failure) {
    operationQueue = NSOperationQueue()
    operationQueue.maxConcurrentOperationCount = 1

    self.callback = callback
    self.failure = failure
}

func start() {
    let databaseCreationOperation = createDatabaseOnServer()
    let databaseCheckOperation = repeatedlyCheckForDatabaseReadiness()
    let databaseDownloadOperation = downloadDatabaseFromServer()

    databaseCheckOperation.addDependency(databaseCreationOperation)
    databaseDownloadOperation.addDependency(databaseCheckOperation)

    operationQueue.addOperations([databaseCreationOperation, databaseCheckOperation, databaseDownloadOperation], waitUntilFinished: true)
}

func createDatabaseOnServer() -> BlockOperation {
    let operation : BlockOperation = BlockOperation()
    operation.addExecutionBlock({SVMRSendTask(method: BeginCreatingDatabase(), onSuccess: {operation.finish()}, onFailure: {e in}).apply()})
    return operation
}

func repeatedlyCheckForDatabaseReadiness() -> NSBlockOperation {
    return NSBlockOperation(block: {SVMRRequestTask(method: DatabaseZipReady()).apply(5, c: {s in s == "OK"}, f: {s in String(s)}, onSuccess: {r in}, onFailure: {e in})})
}

func downloadDatabaseFromServer() -> NSBlockOperation {
    let sourceURL : NSURL = RequestDatabaseIdentitiesForDownload.sharedInstance.databaseURL
    return NSBlockOperation(block: {DownloadTask(url: sourceURL, progressCallback: {p in}, successCallback: {l in}, failureCallback: {e in }).start()})
}
}

当SVMRRequestTask将web服务方法作为参数(方法是抽象类SVMR的实例)时，它使用NSURLSessionTasks执行web服务请求。

而BlockOperation如下所示：

class BlockOperation : NSBlockOperation {

private var _isFinished : Bool = false
private var _isExecuting : Bool = false

override var finished : Bool {
    return _isFinished
}

override var executing : Bool {
    return _isExecuting
}

func finish() {
    if cancelled {
        return
    }

    self.willChangeValueForKey("ready")
    _isReady = false
    self.didChangeValueForKey("ready")

    self.willChangeValueForKey("executing")
    _isExecuting = false
    self.didChangeValueForKey("executing")

    self.willChangeValueForKey("finished")
    _isFinished = true
    self.didChangeValueForKey("finished")
}
}

有人能解释我到底做错了什么吗?因为我是第一次使用NSOperation类。在此之前，我当然翻阅了整个苹果公司的文档和其他一些参考资料，但不幸的是，我找不到任何东西。

Answer 1

您可以通过NSBlockOperation来解决这个问题，这是通过添加依赖关系来序列化操作队列的最佳方法，

@dsafa我在gist中为您创建了一个示例：https://gist.github.com/prajwalabove/66eecbf6b6aa096d5e18b04bd0cc18e8