您的位置：首页 > 移动开发 > IOS开发

CoreData 深入理解2 (iOS5 以后线程安全与同步)

2014-03-24 14:28 281 查看

up
vote6down
votefavorite

9

I've seen a few videos / threads that say it's possible to create 'children' MOCs -- MOCs that use other MOCs as their persistant stores. Useful, for example, in a context where you're threading your application, and want to have a single master MOC that can
save / rollback the changes that the child threads create. (From what I understand, a MOC and it's managedObjects MUST all be used on the same thread)

The question is, how do I create a child MOC? I can't track down the WWDC videos I was watching that introduced them, and everything I"ve seen has been talking about how to use them ONCE they're made. I can easily alloc a new MOC, but how do I set it's persistent
store to be another MOC? The reference doesn't show any functions that do that!

objective-c ios xcode core-data nsmanagedobjectcontext

share|improve
this question

edited Dec
31 '12 at 12:07

flexaddicted

14.3k94897

asked Sep 4 '12 at 21:15

RonLugge

1,7011924

add
comment

2 Answers

active oldest votes

up
vote16down
voteaccepted

Create a new MOC for which you are in total control of the synchronization. This is the same as calling

init

and
the same behavior as pre-parent/child relationships.

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSConfinementConcurrencyType];

You parent a MOC to another MOC by setting its property:

moc.parentContext = someOtherMocThatIsNowMyParent;

Here, the child chooses the parent. I'm sure my kids wish they were NSManagedObjectContexts. A parent context must be of either

NSPrivateQueueConcurrencyType

NSMainQueueConcurrencyType

.

You can create a MOC that is "bound" to a private queue:

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];

which means you should only access it via the

performBlock

performBlockAndWait

API.
You can call those methods from any thread as they will ensure proper serialization of the code in the block. For example...

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];
moc.parentContext = someOtherMocThatIsNowMyParent;
[moc performBlock:^{
// All code running in this block will be automatically serialized
// with respect to all other performBlock or performBlockAndWait
// calls for this same MOC.
// Access "moc" to your heart's content inside these blocks of code.
}];

The difference between

performBlock

and

performBlockAndWait

is
that

performBlock

will
create a block of code, and schedule it with Grand Central Dispatch to be executed asynchronously at some time in the future, on some unknown thread. The method call will return immediately.

performBlockAndWait

will
do some magic synchronization with all the other

performBlock

calls,
and when all the blocks that have been presented prior to this one are done, this block will execute. The calling thread will pend until this call has completed.

You can also create a MOC that is "bound" to the main thread, just like private concurrency.

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSMainQueueConcurrencyType];
moc.parentContext = someOtherMocThatIsNowMyParent;
[moc performBlock:^{
// All code running in this block will be automatically serialized
// with respect to all other performBlock or performBlockAndWait
// calls for this same MOC.  Furthermore, it will be serialized with
// respect to the main thread as well, so this code will run in the
// main thread -- which is important if you want to do UI work or other
// stuff that requires the main thread.
}];

which means you should only access it directly if you know you are on the main thread, or via the

performBlock

performBlockAndWait

API.

Now, you can use the "main concurrency" MOC either via the

performBlock

methods,
or directly if you know you are already running in the main thread.

share|improve
this answer

answered Sep 4 '12 at 22:10

Jody Hagins

13.4k11734

Thank you for such an (incredibly) thorough answer. – RonLugge Sep
4 '12 at 22:15

Hello, I know it's really old but my question isn't big enough to create a new one, I was wondering where I should call save on the
context, inside the performBlock or outside ? Thank you ! – ItsASecret Oct
12 '13 at 16:15

Inside performBlock – Jody
Hagins Oct
16 '13 at 1:57

add
comment

up
vote1down
vote

Initialize child MOC then:

[_childMOC performBlockAndWait:^{
[_childMOC setParentContext:parentMOC];
}];

share|improve
this answer

edited Sep
4 '12 at 22:04

answered Sep 4 '12 at 21:38

mkral

2,0101928

So... I can ACCESS MOCs accross threads, I'm just not allowed to operate (insert, delete, save, rollback) accross them? – RonLugge Sep
4 '12 at 21:46

You can call performBlock and performBlockAndWait from other threads. Not much else. – Jody
Hagins Sep
4 '12 at 22:11

内容来自用户分享和网络整理，不保证内容的准确性，如有侵权内容，可联系管理员处理

标签：

相关文章推荐

新的分享

章节导航