NSManagedObjectContext - FetchRequest 死锁答案

【问题标题】：NSManagedObjectContext - FetchRequest DeadlockNSManagedObjectContext - FetchRequest 死锁
【发布时间】：2016-06-16 06:55:06
【问题描述】：

更新说明：如果你们想知道，我正在使用Big Nerd Ranch CoreDataStack。

我已经为这个特定问题苦苦挣扎了一段时间。基本上我正在尝试从 CNContactStore 获取联系人并在自定义 NSOperation 中获取 ContactDetails (NSManagedObject)。

现在我正在尝试在单元测试中运行整个过程。到目前为止，这就是我的代码的样子。

单元测试

func testThatLoaderOperationWorks()
{   
    var coreDataStack: CoreDataStack?   

    let semaphore = dispatch_semaphore_create(0);

    CoreDataStack.constructSQLiteStack(withModelName: "DataModel") { result in
        switch result
        {
        case .Success(let stack):
            coreDataStack = stack
        case .Failure(let error):
            coreDataStack = nil
            print (error)
        }


        dispatch_semaphore_signal(semaphore);
    }

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)


    let contactStore = CNContactStore()

    let loaderOperation = LoaderOperation.init(withWorkerContext: (coreDataStack?.newChildContext())!, andContactStore: contactStore)
    loaderOperation.completionBlock = {
        XCTAssert(true)
    }

    let operationQueue = NSOperationQueue()
    operationQueue.maxConcurrentOperationCount = 1
    operationQueue.addOperation(loaderOperation)
    loaderOperation.waitUntilFinished()
}

操作子类

override func main()
{       
    let keysToFetch = [
        CNContactFormatter.descriptorForRequiredKeysForStyle(.FullName),
        CNContactEmailAddressesKey,
        CNContactPhoneNumbersKey,
        CNContactImageDataAvailableKey,
        CNContactThumbnailImageDataKey]


    var allContainers: [CNContainer] = []
    do
    {
        allContainers = try contactStore.containersMatchingPredicate(nil)
    }
    catch
    {
        print("Error fetching containers")
    }

    var contactList: [CNContact] = []


    for container in allContainers
    {
        let fetchPredicate = CNContact.predicateForContactsInContainerWithIdentifier(container.identifier)

        do
        {
            let containerResults = try contactStore.unifiedContactsMatchingPredicate(fetchPredicate, keysToFetch: keysToFetch)
            contactList.appendContentsOf(containerResults)
        }
        catch
        {
            print("Error fetching results for container")
        }
    }

    self.workerContext.performBlockAndWait
    {           
        let fetchRequest = NSFetchRequest(entityName: "ContactDetails")
        do
        {
            let list = try self.workerContext.executeFetchRequest(fetchRequest)
            print("The List: \(list)")
        }
        catch
        {
            print(error)
        }
    }
}

从技术上讲，我想要实现的是能够获取联系人并将它们与我从 CoreData 获取的数据进行交叉引用。但是当我运行 executeFetchRequest 时就会发生死锁。我在某处做错了吗？

【问题讨论】：

标签： ios swift core-data concurrency deadlock

【解决方案1】：

我找到了答案。真的很简单。这个主要原因是我试图让测试方法等待waitUntilFinished()，直到操作结束，而在操作本身中，我试图让它强制等待performBlockAndWait()，从而导致死锁。

删除waitUntilFinished() 并将performBlockAndWait() 替换为performBlock()。

另外，为了使用异步代码进行单元测试，您必须expectationWithDescription("description")。

基本上你的单元测试方法应该是这样的

    let asyncExpectation = expectationWithDescription("longRunningFunction")

    .... some stuff here ....


    let loaderOperation = LoaderOperation.init(withWorkerContext: (coreDataStack?.newChildContext())!, andContactStore: contactStore)
    loaderOperation.completionBlock = {
        asyncExpectation.fulfill()
    }

    let operationQueue = NSOperationQueue()
    operationQueue.addOperation(loaderOperation)


    self.waitForExpectationsWithTimeout(10) { (error) in
        XCTAssertNil(error, "Something went wrong")
        XCTAssert(true)
    }

【讨论】：