CODEWITHSUNDEEP
ioscore-data
an0
an0

Reputation: 17530

How to make child MOC see changes of parent MOC and how to resolve conflicts between them?

It seems the new queue based MOC together with nested MOC introduced in iOS 5 serve as an simpler and cleaner concurrency model, so I'm happily using it now. But there are several things that are not clear to me:

  1. Do child MOCs pulls parent MOC's changes automatically? Or do I still need to coordinate them manually? If the latter, how to manually refresh the children to incorporate parent changes? (I know how things work the other way around -- parent MOC gets changes from a child MOC when the child saves.)
  2. What will happen on saving a child MOC if there are conflicts between changes in the child and changes in parent? Is mergePolicy also responsible for resolving conflicts between child MOC and parent MOC?

Upvotes: 2

Views: 1184

Answers (1)

Jody Hagins
Jody Hagins

Reputation: 28349

  1. Yes and no, depending on your definition of "pulls parentMOCs changes automatically." If you mean, the next time a child fetches, it will get the updated data, then yes. If you mean will objects automatically get changed, then no. The reason is that a MOC is a scratchpad, and it will not change without you changing it.

  2. Yes, you will have to resolve merge conflicts if you are making changes to the same objects/relationships from multiple places. This is a complex issue, and can not be sufficiently answered in a SO answer. You should really read the Core Data Programming Guide, especially the section on Change Management.

EDIT

  1. Say I'm holding object A in child MOC and add a relationship B to A in parent MOC. What do I get if I access A.B? nil? What should I do if I want to see A.B in child MOC? Use refreshObject:mergeChanges: to refresh A, or use objectWithID: to fetch A again? Is there any way to refresh the whole child MOC? I have a very complex object network in child MOC and I don't want to(or just can't) refresh/re-fetch every object one by one.

When looking at object A through the child MOC, you will NOT see the change in the parent MOC until you refetch from the child MOC. If you want to see what's in the parent, you can do so via another fetch on the MOC, or through any of the methods to get a apecific object from the store. However, I must stress that you need to read the documentation about each fetch, because they may have side effects. The exact answer depends on which approach you choose.

Typically, if you are doing continual change, you want to either make changes in a child, and push them to the parent, or handle the DidChange notification to make the changes seem "automatic."

  1. Does mergePolicy work here?

Yes.

Again, the best answer I can give you here on SO is to point you to the "Core Data Programming Guide." Pay special attention to these two sections:

https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdFetching.html#//apple_ref/doc/uid/TP40002484-SW1

https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdChangeManagement.html#//apple_ref/doc/uid/TP30001201-CJBDBHCB

Upvotes: 5

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