Re: CoreData headaches
Re: CoreData headaches
- Subject: Re: CoreData headaches
- From: Steve Mills <email@hidden>
- Date: Thu, 27 Oct 2016 23:02:22 -0500
On Oct 27, 2016, at 22:49:15, Dave Fernandes <email@hidden> wrote:
>
> But what managed object are you dealing with? Is the Asset a managed object? If so, you can only access its properties from the queue of its managed object context. So if it is a main queue context, you can only access the MO’s properties from the main queue. If it is a private queue context, you use performBlock on the MO’s context to execute a block of code in the context’s queue and you must access the MO’s properties from inside that block. If I’m stating something you already know, forgive me. It is not clear from your post. Example code would help.
Yes, the Asset is an NSManagedObject. In this call chain, there is no NSManagedObjectContext in sight. The NSCollectionView is simply creating item views to display for each Asset. It asks the Asset to create an image thumbnail. The Asset does so in via the steps I outlined before.
Now way back in the document's init method, I set up an NSManagedObjectContext with type NSMainQueueConcurrencyType. So yes, there is a main queue type context, but the code that is requesting the thumbnails knows nothing about it or even cares.
>> On Oct 27, 2016, at 11:24 PM, Steve Mills <email@hidden> wrote:
>>
>> I'll explain what's going on. Each NSCollectionViewItem in the NSCollectionView is being assigned its representedObject (the Asset, which is a reference to an image file). At that point, I ask the Asset to requestPreviewImageAtSize:, which creates an NSBlockOperation (and adds it to an NSOperationQueue), whose block creates a CGImageSourceRef. Before it can create the CGImageSourceRef, it needs to get the url. It does this by asking for its .folder and .name properties. Those are nonatomic properties of the Asset. Once it has the url and the CGImageSourceRef, it creates the CGImageRef, converts that to an NSImage, and finally assigns that to the Asset's atomic .thumb property.
>>
>> The folder and name properties were generated by mogenerator, a tool for creating document model classes for CoreData .xcdatamodeld files. The thumb property is something I added manually to cache the thumbnail, and the AssetItemView's NSImageView's Value binds to it as well.
>>
>> So I'm not directly dealing with any NSManagedObjectContext here. That's all taken care of when the items are added to it or when a document is opened. All I'm doing is generating a preview for each managed object.
--
Steve Mills
Drummer, Mac geek
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