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What is AutoReleasePool?

Ans : 

In simple word, it is pool contained objects that will be released in some time.

Object's retain and release is constant action on object. For that Retain() and Release() methods called in objective c to control memory flow. As iOS objects work on retain count concept, it tells retain counts and it will be released if it reaches to 0.

Sometimes, we can not continuously use Release() method to release object.

Code for example :

 -(NSString *)getCoolLabel {
    NSString *label = [[NSString alloc] initWithString:@"SwiftRocks"];
    [label release];
    return label;


Here label has 2 retain count and after return it has 1 retain count. It will be in memory as we can not execute release after return label; statement.

So solution :
return [label autorelease];
It will not release label instantly, but it inserts label in pool, and in some time, when autorelease pool thread execute, it

Note : Instead of instantly reducing the retain count of an object, autorelease() adds the object to a pool of objects that need to be released sometime in the future, but not now. By default, the pool will release these objects at the end of the run loop of the thread being executed, which is more than enough time to cover all usages of getCoolLabel() without causing memory leaks.

Whatever code that takes much memory we can add in autoreleasepool block.

@autoreleasepool {
           NSString *contents = [self getFileContents:files[i]];
           NSString *emojified = [contents emojified];
           [self writeContents:contents toFile:files[i]];


Above all code written in objective - c, but in swift AutoReleasePool is required?

Yes, it depends on code.  It’s a different story if your code is dealing with legacy Obj-C code, specially old Foundation classes in iOS.

To put it short, autoreleasepool is still useful in iOS/Swift development as there are still legacy Obj-C classes in UIKit and Foundation that call autorelease, but we not have to worry where there is pure swift code.

What is Generics in Swift? Write a program to make and use generics.

Ans :

Apple doc says: Generic code enables you to write flexible, reusable functions and types that can work with any type. You can write code that avoids duplication by using generics in most cases.

Code for Example :

 let intArray = [1,2,3,4,5] // type int
 let stringArray = [“abhi”, “iOS”] // type string

If we want to print all values, we have to make 2 functions accepting integer array or string array.
But using Generic, we can make 1 method accepting generic type.

 func printAnyArray<T>(arr:[T]) { { print($0) }


 <T> after the function name represents a generic function. A generic function can work with whatever type. We can put whatever instead of T. T is placeholder type. We can use for dictionary like Dictionary<key,value> and for array like Array<element> .

Generic function with parameter type : 

func anotherMethod<T, U> (first: T, second: U) { print("first one called")}
func anotherMethod<T> (first: T, second: T) { print("second one called")}
anotherMethod(first: 123, second: "456"// T - Int, U - String
anotherMethod(first: 123, second: 456// T - Int

In above methods, we use different placeholder type in method1 and same placeholder type in method2. If out passing parameter types are different then 2nd method will be called, and if same then 1st method will be called.

Generic function with type constraints :

func middleVal<T>(array: [T]) -> T? {
  guard !array.isEmpty else { return nil }
  return array.sorted()[(array.count - 1) / 2]


Sometimes, we require specific types only i.e type which confirm comparable protocol

func middleVal<T: Comparable>(array: [T]) -> T? {
  guard !array.isEmpty else { return nil }
  return array.sorted()[(array.count - 1) / 2]


Above method will only accepts. int, float, double.

If you have any comment, question, or recommendation, feel free to post them in the comment section below!  

What is callback function?

Ans : A callback function is a function that is passed as an argument to another function, to be “called back” at a later time.

Code for Example :

func printValue(index: Int, element: Int) {
    print("index = \(index), Element = \(element)")
func each(array: [Int], callback: (Int, Int) -> Void) {
    for i in 0..<array.count {
        callback(i, array[i])

each(array: [1, 2, 3, 4, 5],callback: printValue)

Here each function type is ( [Int], (Int, Int) -> Void ) .
Here callback function type is ( (Int, Int) -> Void ) .

Callback function mostly used when we want to pass block of code that should be executed in another function execution complete.

What are best approach to implement if large data is coming from core data and show data on tableview?

Ans : 

Use NSFetchResultsController : A controller that you use to manage the results of a Core Data fetch request and to display data to the user.

If we have very large data in database and we want all data to show in tableview, then we have to write some extra code on core data fetch request as fetching data will stuck UI.
Then we can use NSFetchResultsController as it has ability to fetch data from memory in batches. So data coming in batch when we scroll tableview and saved in cache.

We have to make property of type NSFetchResultsController :

- (NSFetchedResultsController *)fetchedResultsController {
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
        entityForName:@"FailedBankInfo" inManagedObjectContext:managedObjectContext];
    [fetchRequest setEntity:entity];
    NSSortDescriptor *sort = [[NSSortDescriptor alloc]
        initWithKey:@"details.closeDate" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
    [fetchRequest setFetchBatchSize:20];
    NSFetchedResultsController *theFetchedResultsController =
        [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
            managedObjectContext:managedObjectContext sectionNameKeyPath:nil
    self.fetchedResultsController = theFetchedResultsController;
    _fetchedResultsController.delegate = self;
    return _fetchedResultsController;

and in ViewDidLoad :

- (void)viewDidLoad {
    [super viewDidLoad];
    NSError *error;
 if (![[self fetchedResultsController] performFetch:&error]) {
  // Update to handle the error appropriately.
  NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
  exit(-1);  // Fail
    self.title = @"Failed Banks";

We get data in numberOfRowInSection :
id  sectionInfo =
        [[_fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];

We get data in cellForRowAtIndexPath :
FailedBankInfo *info = [_fetchedResultsController objectAtIndexPath:indexPath];

We also have to write FetchResultsControllerDelegate methods as it is as at apple provide at end of ViewController's code.

Enjoy lazy loading data...

Tutorial : Ray Wanderlich

How you can add column in entity in core data that can be also added in next release?

Ans : When you initialise a Core Data stack, one of the steps involved is adding a store to the persistent store coordinator. When you encounter this step, Core Data does a few things prior to adding the store to the coordinator. First, Core Data analyzes the store’s model version. Next, it compares this version to the coordinator’s configured data model. If the store’s model version and the coordinator’s model version don’t match, Core Data will perform a migration, when enabled.

Note: If migrations aren’t enabled, and the store is incompatible with the model, Core Data will simply not attach the store to the coordinator and specify an error with an appropriate reason code.

Migrations happen in three steps:
  1. Core Data copies over all the objects from one data store to the next. 
  2. Core Data connects and relates all the objects according to the relationship mapping. 
  3. Enforce any data validations in the destination model. Core Data disables destination model validations during the data copy.
Database Migration : 

Lightweight Migration
Manual Migration
Custom Manual Migration
Fully Manual Migration

Setting for Lightweight Migration: 

NSMigratePersistentStoresAutomaticallyOption: true
NSInferMappingModelAutomaticallyOption: true

Coredata Migration Tutorial : RayWanderLich