在多线程应用程序中NSManagedObjectContext的通用方法 [英] Generic approach to NSManagedObjectContext in multi-threaded application

问题描述

我在这里读了一些关于NSManagedObjectContext和多线程应用程序的帖子。我还讨论了CoreDataBooks示例，以了解单独的线程如何需要自己的NSManagedObjectContext，以及保存操作如何与主NSManagedObjectContext合并。我发现示例是好的，但也太具体应用。我试图推广这个，并想知道我的方法是否是声音。

I've read a number of posts here about NSManagedObjectContext and multi-threaded applications. I've also gone over the CoreDataBooks example to understand how separate threads require their own NSManagedObjectContext, and how a save operation gets merged with the main NSManagedObjectContext. I found the example to be good, but also too application specific. I'm trying to generalize this, and wonder if my approach is sound.

我的方法是有一个通用的函数获取当前线程的NSManagedObjectContext。该函数返回主线程的NSManagedObjectContext，但如果从不同的线程调用，则会创建一个新的（或从缓存中获取）。如下：

My approach is to have a generic function for fetching the NSManagedObjectContext for the current thread. The function returns the NSManagedObjectContext for the main thread, but will create a new one (or fetch it from a cache) if called from within a different thread. That goes as follows:

+(NSManagedObjectContext *)managedObjectContext {
    MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *moc = delegate.managedObjectContext;

    NSThread *thread = [NSThread currentThread];

    if ([thread isMainThread]) {
        return moc;
    }

    // a key to cache the context for the given thread
    NSString *threadKey = [NSString stringWithFormat:@"%p", thread];

    // delegate.managedObjectContexts is a mutable dictionary in the app delegate
    NSMutableDictionary *managedObjectContexts = delegate.managedObjectContexts;

    if ( [managedObjectContexts objectForKey:threadKey] == nil ) {
        // create a context for this thread
        NSManagedObjectContext *threadContext = [[[NSManagedObjectContext alloc] init] autorelease];
        [threadContext setPersistentStoreCoordinator:[moc persistentStoreCoordinator]];
        // cache the context for this thread
        [managedObjectContexts setObject:threadContext forKey:threadKey];
    }

    return [managedObjectContexts objectForKey:threadKey];
}

如果从主线程调用，保存操作很简单。从其他线程调用的保存操作需要在主线程中合并。为此，我有一个通用的提交函数：

Save operations are simple if called from the main thread. Save operations called from other threads require merging within the main thread. For that I have a generic commit function:

+(void)commit {
    // get the moc for this thread
    NSManagedObjectContext *moc = [self managedObjectContext];

    NSThread *thread = [NSThread currentThread];

    if ([thread isMainThread] == NO) {
        // only observe notifications other than the main thread
        [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(contextDidSave:)
                                                 name:NSManagedObjectContextDidSaveNotification
                                               object:moc];
    }

    NSError *error;
    if (![moc save:&error]) {
        // fail
    }

    if ([thread isMainThread] == NO) {
        [[NSNotificationCenter defaultCenter] removeObserver:self 
                                                    name:NSManagedObjectContextDidSaveNotification 
                                                  object:moc];
    }
}

在 contextDidSave：函数我们执行合并，如果通过 commit 中的通知调用。

In the contextDidSave: function we perform the merge, if called by the notification in commit.

+(void)contextDidSave:(NSNotification*)saveNotification {
    MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *moc = delegate.managedObjectContext;

    [moc performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)
                      withObject:saveNotification
                   waitUntilDone:YES];
}

$ b < >

Finally, we clean-up the cache of NSManagedObjectContext with this:

+(void)initialize {
    [[NSNotificationCenter defaultCenter] addObserver:self 
                                             selector:@selector(threadExit) 
                                                 name:NSThreadWillExitNotification 
                                               object:nil]; 
}

+(void)threadExit {
    MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSString *threadKey = [NSString stringWithFormat:@"%p", [NSThread currentThread]];  
    NSMutableDictionary *managedObjectContexts = delegate.managedObjectContexts;    

    [managedObjectContexts removeObjectForKey:threadKey];
}

这个编译似乎工作，但我知道线程问题可能是棘手的种族条件。是否有人看到这种方法的问题？

This compiles and seems to work, but I know threading problems can be tricky due to race conditions. Does anybody see a problem with this approach?

此外，我使用这个从异步请求（使用ASIHTTPRequest）的上下文中，它从一个服务器并更新并将商店插入iPhone。看起来NSThreadWillExitNotification不会在请求完成后触发，然后相同的线程用于后续请求。这意味着相同的NSManagedObjectContext用于同一线程上的单独请求。这是一个问题吗？

Also, I'm using this from within the context of an asynchronous request (using ASIHTTPRequest), which fetches some data from a server and updates and inserts the store on the iPhone. It seems NSThreadWillExitNotification doesn't fire after the request completes, and the same thread is then used for subsequent requests. This means the same NSManagedObjectContext is used for separate requests on the same thread. Is this a problem?

推荐答案

发布这个问题一年后，我终于构建了一个框架来概括和简化我的核心数据。它超出了原来的问题，并添加了许多功能，使Core Data交互更容易。详情如下： https://github.com/chriscdn/RHManagedObject

A year after posting this question I finally built a framework to generalize and simplify my working with Core Data. It goes beyond the original question, and adds a number of features to make Core Data interactions much easier. Details here: https://github.com/chriscdn/RHManagedObject

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