问题描述
我已阅读以下。具体在上，代码是某物，如下所示：
The following code is picked up from an RayWenderlich video. Specifically at 10:05 the code is something like this:
class CoreDataStack {
var coordinator : NSPersistentStoreCoordinator
init(coordinator: NSPersistentStoreCoordinator){
self.coordinator = coordinator
}
// private, parent, in background used for saving
private lazy var savingContext : NSManagedObjectContext = {
let moc = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
moc.persistentStoreCoordinator = coordinator
return moc
}()
lazy var mainManagedObjectedContext : NSManagedObjectContext = {
let moc = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
moc.parent = self.savingContext
return moc
}()
func saveMainContext() {
guard savingContext.hasChanges || mainManagedObjectedContext.hasChanges else {
return
}
mainManagedObjectedContext.performAndWait {
do {
try mainManagedObjectedContext.save()
}catch let error{
fatalError(error.localizedDescription)
}
}
savingContext.perform {
do {
try self.savingContext.save()
}catch let error{
fatalError(error.localizedDescription)
}
}
}
}
据我了解，会发生什么情况是主上下文只是将更改传递给其父上下文（即私有背景上下文）。它同步执行此操作。
From what I understand what happens is that the main context just passes the changes to its parent context which is a private, background context. It does this synchronously.
然后，父级（私有上下文）在异步后台线程中对sqlite进行实际保存。长话短说，这对我们的表现有很大帮助。但是数据完整性呢？！
Then the parent, private context, does the actual saving against sqlite in a background thread asynchronously. Long story short this helps us a lot with performance. But what about data integrity?!
想象一下我是否要这样做：
Imagine if I was to do this:
let coredataManager = CoreDataStack()
coredataManager.saveMainContext() // save is done asynchronously in background queue
coredataManager.mainManagedObjectedContext.fetch(fetchrequest)
如何保证我的抓取正在读取最新和更新的结果？
How can I guarantee that my fetch is reading the most recent and updated results?
如果我们异步进行写入，那么是否不可能同时进行另一次读取会导致意外结果，即保存更改的结果可能存在或不存在？
If we do our writes asynchronously then isn't there a chance that another read at the same time could end up with unexpected results ie results of the save changes could or could not be there?
编辑：
我对以下代码进行了改进。我可以将保存内容输入completenessHandler参数中。但这并不能解决整个问题。如果我是从mainQueue发出fetchRequest的，而其他地方却不知道同时进行保存？
I've made an improvement with the code below. I can make my save take in a completionHandler parameter. But that doesn't resolve the entire problem. What if I'm making a fetchRequest from a mainQueue somewhere else that isn't aware that a save is happening at the same time?
enum SaveStatus{
case noChanges
case failure
case success
}
func saveMainContext(completionHandler: (SaveStatus -> ())) {
guard savingContext.hasChanges || mainManagedObjectedContext.hasChanges else {
completionHandler(.noChanges)
return
}
mainManagedObjectedContext.performAndWait {
do {
try mainManagedObjectedContext.save()
}catch let error{
completionHandler(.failure)
fatalError(error.localizedDescription)
}
}
savingContext.perform {
do {
try self.savingContext.save()
completionHandler(.succes)
}catch let error{
completionHandler(.failure)
fatalError(error.localizedDescription)
}
}
}
推荐答案
该问题并非特定于核心数据。
The question isn't specific to core-data.
这是经典的读写问题。
保护数据源的常用方法是访问数据源使用序列队列。否则，如果没有串行队列，您将遇到读写问题。
The common approach with protecting a datasource is to access your datasource using a serial queue. Otherwise yeah without the serial queue you will have a read-write problem.
在以下示例中：
let coredataManager = CoreDataStack() // 1
coredataManager.saveMainContext() // 2 save is done asynchronously in background queue
coredataManager.mainManagedObjectedContext.fetch(fetchrequest) // 3
coredataManager 要从串行队列访问。因此，即使第二行的写入操作是异步完成的，第3行的读取操作也必须等到 serial 队列被解除阻塞。
coredataManager is to be accessed from a serial queue. So even if the write in the 2nd line is done asynchronously, the read at line 3, will have to wait until the serial queue is unblocked.
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