4. Thread Safety In Core Data
Core Data Optimization
4
• While working with Core Data, it's important to always remember that Core Data isn't
thread safe. Core Data expects to be run on a single thread.
• NSManagedObject, NSManagedObjectContext NSPersistentStoreCoordinator aren't
thread safe.
• This doesn't mean that every Core Data operation needs to be performed on the
main thread, which is used for UI, but it means that we need to take care which
operations are executed on which threads. We have to be careful how changes from
one thread are propagated to other threads.
5. Thread Safety In Core Data
Core Data Optimization
5
1. NSManagedObject: NSManagedObject isn't thread safe but has an
instance objectID that returns an instance of the NSManagedObjectID class which
is thread safe. For getting
do {
let managedObject = try managedObjectContext.existingObjectWithID(objectID)
} catch let fetchError as NSError { print("(fetchError), (fetchError.userInfo)”) }
2. NSManagedObjectContext: Isn't thread safe, we can create a managed object
context for every thread that interacts with Core Data. This strategy is referred
as thread confinement. A common approach is to create thread dictionary to store
MOC.
let currentThread = NSThread.currentThread()
currentThread.threadDictionary.setObject(managedObjectContext, forKey:
"managedObjectContext")
3. NSPersistentStoreCoordinator: We can create a separate persistent store
coordinator for every thread. Which is also possible.
6. Multithreading & Concurrency Strategies
Core Data Optimization
6
The NSPersistentStoreCoordinator class was designed to support multiple managed
object contexts, even if those managed object contexts were created on different
threads. Because the NSManagedObjectContext class locks the persistent store
coordinator while accessing it, it is possible for multiple managed object contexts to use
the same persistent store coordinator even if those managed object contexts live on
different threads. This makes a multithreaded Core Data setup much more manageable
and less complex.
There are two ways for Core Data Concurrency
1.Notifications
2.Parent/Child Managed Object Contexts
Another option is
1.Independent Persistent Store Built with Notification & PrivateQueue
7. Multithreading & Concurrency Strategies
Core Data Optimization
7
There are three types of Concurrency options provided by Apple
1.MainQueueConcurrencyType: The managed object context is only accessible from
the main thread. An exception is thrown if you try to access it from any other thread.
1.PrivateQueueConcurrencyType: When creating a managed object context with a
concurrency type of PrivateQueueConcurrencyType, the managed object context is
associated with a private queue and it can only be accessed from that private queue.
2.ConfinementConcurrencyType: Apple has deprecated this concurrency type as of
iOS 9. This is the concurrency type that corresponds with the thread
confinement concept . If you create a managed object context using init(), the
concurrency type of that managed object context is ConfinementConcurrencyType.
8. Fetching
Core Data Optimization
8
1. Use fetchBatchSize:
• This breaks the result set into batches. The entire request will be evaluated,
and the identities of all matching objects will be recorded, but no more than
batchSize objects' data will be fetched from the persistent store at a time.
• A batch size of 0 is treated as infinite, which disables the batch faulting
behavior.
let fetchRequest = NSFetchRequest(entityName: "DataEntity")
fetchRequest.sortDescriptors = [NSSortDescriptor(key:
UFO_KEY_COREDATA_SIGHTED, ascending: false)]
fetchRequest.fetchBatchSize = 20 //Set number double to visible cells
do
{
let test = try self.managedObjectContext.executeFetchRequest(fetchRequest) as Array
print("test count is: (test.count)")
}
catch let error as NSError
{
print("Error is : (error.localizedDescription)")
}
9. Fetching
Core Data Optimization
9
2. Use resultType: There are four types of result type
• ManagedObjectResultType
• ManagedObjectIDResultType
• DictionaryResultType
• CountResultType
let fetchRequest = NSFetchRequest(entityName: "DataEntity")
fetchRequest.resultType = .DictionaryResultType
do
{
let arrayOfFoundRecords = try
objAppDel.tempManagedObjectContext.executeFetchRequest(fetchRequest
}
catch let error as NSError
{
print("error in fetch is : (error.localizedDescription)")
}
10. Fetching
Core Data Optimization
10
3. Use NSExpressionDescription & NSExpression : Instances of
NSExpressionDescription objects represent a special property description type
intended for use with the NSFetchRequest PropertiesToFetch method. An
NSExpressionDescription describes a column to be returned from a fetch that may
not appear directly as an attribute or relationship on an entity. NSExpression is used
to represent expressions in a predicate. Comparison operations in an NSPredicate
are based on two expressions, as represented by instances of the NSExpression
class. Expressions are created for constant values, key paths etc.
//Create a expression
let expressionDescription = NSExpressionDescription()
expressionDescription.name = "count"
expressionDescription.expression = NSExpression(forFunction: "count:", arguments:
[NSExpression(forKeyPath: "shape")])
//Create a fetch Request
let fetchRequest = NSFetchRequest(entityName: "DataEntity")
11. Fetching
Core Data Optimization
11
3. Use GroupBy: Use groupBy to aggregate for same column name.
let fetchRequest = NSFetchRequest(entityName: "DataEntity")
fetchRequest.propertiesToGroupBy = ["shape"]
4. Use propertiesToFetch: create an array of properties you want to fetch from
CoreData.
let expressionDescription = NSExpressionDescription()
expressionDescription.name = "count”
expressionDescription.expression = NSExpression(forFunction: "count:", arguments:
[NSExpression(forKeyPath: "shape")])
//Create a fetch Request
let fetchRequest = NSFetchRequest(entityName: "DataEntity")
fetchRequest.propertiesToFetch = ["shape", expressionDescription]
12. Fetching
Core Data Optimization
12
5. PreFetch Any Required Relationship : Prefetching allows Core Data to obtain
developer-specified related objects in a single fetch.
let entityDescription = NSEntityDescription.entityForName("Employee",
inManagedObjectContext: self.managedObjectContext)
let fetchRequest = NSFetchRequest()
fetchRequest.entity = entityDescription
fetchRequest.relationshipKeyPathsForPrefetching = ["Department"]
do{
let arrayOfFoundRecords = try
self.managedObjectContext.executeFetchRequest(fetchRequest) as Array
for ( emp in arrayOfFoundRecords )
{
print("emp is: (emp)")
print("dept is: (emp.deptEmp)")
}
}
catch let error as NSError
{
}
13. Fetching
Core Data Optimization
13
Fetch Summary:
1.Don’t’ fetch anything that you don’t need.
2.Always use Batch Fetch.
3.Try to use NSExpressionDescription for Fetch in other word lets allow SQLite to do
calculation.
4.Use Prefeatch (relationshipKeyPathsForPrefetching) for required relationship.
14. Predicates:
Core Data Optimization
14
1. Light Predicate First: Always do String Comparison in last query. For example
fetchRequest.predicate = NSPredicate(format: "shape=%@ AND duration=%d", "circle", 30)
better way it to avoid string as much as possible or use string as a second argument.
So above code will perform better if we write it as:
fetchRequest.predicate = NSPredicate(format: "duration=%d shape=%@", 30, "circle”)
2. Use BeginsWith/EndsWith: Always try to use ‘beginsWith’ in creating a predicate.
15. Predicates:
Core Data Optimization
15
Predicate Summary & Cost Graph
1. Do light comparison first i.e. Numerical comparison.
2. Always try to use BeginsWith/EndsWith instead of Equals/Match.
3. Don’t/avoid to use CD in comparison.
Predicate
Cost
Less More
BeginsWith/
EndsWith
Equality
==
Contains
Matches
16. Search
Core Data Optimization
16
1. Always used Canonical String for search process. For example
String Canonical String
TEST test
åpple apple
Green Åpple green apple
2. Always use BeginsWith/EndsWith for searching.
17. Printing SQL Log
Core Data Optimization
17
Product menu by selecting “Manage Schemes” and then editing the current
scheme you are using. Select the “Run “phase and then select the “Arguments”
tab. Here you will see options to set arguments and environment variables. Add
-com.apple.CoreData.SQLDebug 3 to "Arguments Passed On Launch"
