Shyam Shyam - 2 months ago 7
Swift Question

managedObjectContext cannot be used more than once?

I have an app which uses CoreData to save different playlists using

MPMediaPicker
. After saving the first one, while I try to add the second one, the app crashes. Below is the entire debug queue.

Creation of the first record.

musicTemp = <MPMediaItemCollection: 0x13df04440>=========>prepareForSegue
saveValues()
▿ Optional<NSManagedObjectContext>
- Some : <NSManagedObjectContext: 0x13de88a60>

musicTemp = <MPMediaItemCollection: 0x13df04440>=========>saveValues
ModelName.Rule
Optional(<NSManagedObjectContext: 0x13de88a60>)
Number of rows = 1
Number of rows = 1
musicTemp = <MPMediaItemCollection: 0x13df04440>=========>RuleViewCell
mainManagedObjectContext.save


End of first record creation. Start of second record creation,

musicTemp = <MPMediaItemCollection: 0x13dea7620>=========>prepareForSegue
saveValues()
▿ Optional<NSManagedObjectContext>
- Some : <NSManagedObjectContext: 0x13de88a60>

musicTemp = <MPMediaItemCollection: 0x13dea7620>=========>saveValues
ModelName.Rule
Optional(<NSManagedObjectContext: 0x13de88a60>)
2016-09-10 11:39:02.469 AppName[5237:2139350] -[MPMediaItemCollection compare:]: unrecognized selector sent to instance 0x13df04440
All Exceptions
error: use of undeclared identifier 'mainManagedObjectContext'
error: 1 errors parsing expression


App Crashes!!

The related code which does the addition of records,

// Method to save the extracted values to CoreData Objects
private func saveValues() {

// Initialize entity description
let RuleDescription = NSEntityDescription.entityForName("Rule",inManagedObjectContext:mainManagedObjectContext!)
let rule = Rule(entity: RuleDescription!, insertIntoManagedObjectContext: mainManagedObjectContext)
// Set object values from temporary variables
rule.music = musicTemp!
print("musicTemp = \(musicTemp!.count)=========>saveValues")
// Save the object in privateManagedObjectContext
print(NSStringFromClass(rule.classForCoder))
print(mainManagedObjectContext)
if ((mainManagedObjectContext?.hasChanges) != nil) {
do {
try mainManagedObjectContext?.save()
print("mainManagedObjectContext.save")
} catch let saveError as NSError {
print("(AddRuleViewController)(mainManagedObjectContext)saveError: \(saveError), \(saveError.userInfo)")
}
}
}


mainManagedObjectContext
is referenced using dependency injection. I am not able to figure out why it is able to save the first time, but not again. Actually, it complains about
mainManagedObjectContext
.

Answer

There appears to be something strange with your musicTemp variable.

You are hard casting it (which is usually a code smell) and the error is around that variable and the object that it really contains.

The indicator is:

-[MPMediaItemCollection compare:]: unrecognized selector sent to instance 0x14ef90390

Core Data is trying to call compare: on the MPMediaItemCollection object but 0x14ef90390 is not a MPMediaItemCollection.

Start poking at that variable and make sure it really is what you think it is.

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