Multiple UIManagedDocument For Read & Write

问题

I'm building a tab bar application for iPhone and i'm using Core Data with two UIManagedDocuments. In the first tab, i write the data to database and in the second i read them into UITableView with UIFetchedResultsController.

At the start of application, if i write data first, and after then i read results, it works fine. Results appear in the second tab immediately. However, if i read some data first and after then if i write something to database, results appear in second tab with considerable delay (almost 1 minutes). If is there any synchronization problem between two UIManagedObjectContexts or two UIManagedDocuments, how does it works in the first condition? And, is there any solution for this delay?

回答1:

The way that you can ensure that your UIManagedDocument is up to date is to make sure you're saving your changes properly. Given the information you've shown above, I'm not really sure about how you're managing your documents or your managedObjectContexts. There are just too many factors that could be affecting this to be able to give you a 100% concrete answer.

So without knowing what your code looks like and without knowing how you're managing your context, the only thing I can do is give you what I use in my own projects. This may or may not help you, but it helped me - more times than one - when it comes to handing core data by UIManagedDocument.

When it comes to Context:

I use a singleton to manage UIManagedDocument. I do this because I don't want to have to deal with what you're talking about above - having more than one managedObjectContext. When you start dealing with multiple contexts, you have the issue where the data will not be consistent unless you manage all of your contexts properly. If you save on one but don't update the other - then your data can become out of sync. You also have to make sure that each context is working on the property thread - the Apple Docs is a great resource for understanding the whys ad hows this even matters.

The point is, though, that this is one of the biggest problems with working with UIManagedDocument that isn't as bad as when you're working with pure core data and using a SQL persistent store. The main reason that I've found is because of how UIManagedDocument actually saves to its UIDocument store. It is very unpredictable about when it wants to save. This makes knowing when your UIManagedDocument will actually persist and have your data a freaking shot in the dark. You end having to do all kinds of stupid stuff just to make sure that it is always readily available.

Considering I have a belief (that many, maybe rightfully so, believe is an ignorant belief) that working with core data is hard, and UIManagedDocument makes it easier than it would be if you didn't work with it at all. That being said, I don't like it when working with something as simple as UIManagedDocument begins to get complicated - so I use the one thing that has always kept it simple, and that is a singleton, shared-instance of a single UIManagedDocument so that I only have 1 managedObjectContext, ever, to have to work with.

When it comes to saving:

Whenever I make any significant change to a model ( Create, Update, Delete, Edit ), I always make sure to call [document updateChangeCount:UIDocumentChangeDone]; I do this because I do not use the "undo manager" (NSUndoManager) when working with UIManagedDocument. Simply because I haven't needed to yet, plus because I hate all the "workaround" garbage you have to do with it.

Working only on the Main Thread:

Whenever I do anything with my UIManagedDocument or Core Data, I always make sure its on the main thread. I think I've already said it once, but I'll say it again: working in threads is helpful when you need it, but also when you actually understand threading in general. I like working in threads, but it comes at a cost of complexity that makes me not want to work in them when it comes to core data. With that being the case, I tend to stay strictly on the main thread as this keeps things simple and easy (for me).

Saving the Document

When I absolutely need to make sure that the UIManagedDocument is "saved" ( written to disk ), I have 2 methods that I wrote and use that are always readily available for me to call: saveDocument and forceSaveDocument.

The first one ( saveDocument ) merely checks the context for changes. If it has any, it then checks to see if we have any newly inserted objects. When insertedObjects are found, it obtains the perm ids for these items. You can think of this one as a good way to ensure that your core data model is up to date, and that your managed context is in a safe state, so that when your document is actually saved, that you get everything saved in the state that it needs to be in (your ids are realized, your contexts are clean, and what you are about to save represents a state of your model once all work has been complete on it).

Its big brother, forceSaveDocument, actually calls saveDocument first. Again, to make sure that your actual model/context is saved and proper. If it returns successful ( YES ), then it will actually do the real saving and write the document to disk by means of saveToUrl.

Some Code (hopefully it helps):

Here are those 2 methods in case it helps:

-(BOOL)saveDocument {
    NSManagedObjectContext *context = self.document.managedObjectContext;
    if(!context.hasChanges) return YES;

    NSSet *inserts = [context insertedObjects];
    if([inserts count] > 0) {
        NSError *error = nil;

        if(![context obtainPermanentIDsForObjects:[inserts allObjects] error:&error] && error) {
            [self reportError:error];
            return NO;
        }
    }

    return YES;
}

-(void)forceSaveDocument {
    if( [self saveDocument] ) {
        [self.document saveToURL:self.document.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:self.onSaveBlock ? self.onSaveBlock : nil];
    }
}

General Rules/Guidelines

Overall, these are my guidelines that I follow ( and have worked for me for about 3 years now ) when working with UIManagedDocument and Core Data. I'm sure there are better out there from guys/gals much smarter than me, but these have what I use. The benefit I get out of them is that it makes me have to worry less about managing my data and gives me more freedom to work with everything else:

Use a singleton to manage my UIManagedDocument until the need of multiple threads is absolutely necessary - then migrate over to start using multiple contexts ( i've never needed to do this yet - but then again I try to keep things simple )
Always call updateChangeCount:UIDocumentChangeDone when I make any change to a model. It is very light weight and has little impact. If anything, it will help ensure your document stays up to date and never gets too out of sync with your data.
Don't use undo manager unless you actually need it ( I have yet to need it )
Use save/ForceSave sparingly, and only when absolutely necessary (deletes are a good reason to use it. Or if you create a new item on one view controller and need it on the next one, but can't wait for core data and the document to sync up - its kind of like kicking it in the ass and saying "I object to you saving whenever you want - save now lol.. )

Final Thoughts

All of the above is my own belief and understandings. These come from a lot of research, reading, and being a pain the ass when it comes to wanting to do things right, all while keeping it simple. Anyone can write a complex solution - but I think the fundamental question is always: do you really need the complexity, or do you just need it to work so you can focus on more complex issues?

I'm sure the above is way more than you probably wanted, and may even add more questions than you have. If you need some links and resources, let me know and I'll try to throw a few together.

Either way, hope that helps.

来源：https://stackoverflow.com/questions/24218337/multiple-uimanageddocument-for-read-write

标签

ios

core-data

synchronization

uimanageddocument