Difference between XIB and Storyboard

Ans : 


XIB :

1) Xib files are used with a single UIView.

3)It's utilizes more memory as compared to storyboard and quiet slow.

4) It is compatible from iOS5 and onwards

5) You can do localizations for different languages and countries using different XIBs .

6) It's difficult to use same Xib to support multiple devices.

Storyboard :

1)You can layout all your Scenes like View Controllers, Nav Controllers, TabBar Controllers, etc in a single storyboard.

3)Usually fast and allocates less memory.

4)It's not compatible prior to iOS 5 .

5)"Dynamic" and "Prototype" cells can be used easily.

6)Storyboards best to use for the apps with a small to medium amount of screens.

Difference between synchronous and asynchronous calls in Objective-C

Ans : 

Synchronous : This call means task will be executed in order.
Asynchronous : This call means task may or may not be executed in order.

When call is called synchronously, then thread that initiated that operation will be wait to current task to be finished.
When call is called asynchronously, then it will not wait.

If we want to do some task without harassing UI, we can do those tasks in background thread. This goal is to keep free main thread, so it continuously respond UI event. So we can dispatch our task in background state asynchronously.

So for do task in background thread, we will divide in 2 parts.

1. GCD - Grand Central Dispatch. By using GCD, you have to grab one of global background queue or create your own background queue.

// one of the global concurrent background queues
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// or you could create your own serial background queue:
// dispatch_queue_t queue = dispatch_queue_create("com.iosiqa.app.queuename", 0);

2. Dispatch your task to that queue asynchronously

dispatch_async(queue, ^{
    // task that to be done in background and it may be slow
});

The pattern for operation queues is very similar. Create an operation queue and add operations to that queue.

Lets see example :

Asynchronous call with Multithreading :

// Methods gets called in different thread and does not block the current thread.
[NSURLConnection sendAsynchronousRequest:request
                                   queue:queue
                       completionHandler:
    ^(NSURLResponse *response, NSData *data, NSError *error) {
}];

Synchronous call with Multithreading (not so useful):

//Do something
dispatch_sync(queue, ^{
    //Do something else // work in another queue or thread
});

//Do some more task

Difference between thread-safe and non-thread-safe in iOS

Ans : 

Thread-Unsafe -> If any object allow to modify by more than one thread at the same time.  (non-atomic property is thread-unsafe. Comments are welcomed)

Thread-safe -> If any object not allow to modify by more than one thread at the same time.Immutable objects are generally thread-safe. (atomic property attribute type. Comments are  welcomed)

In general, immutable classes like NSArray, let are thread-safe, while their mutable variants like NSMutableArray,var are thread-unsafe.

Which type of encryption you have used in iOS App?

Ans : I have used trippleDES for passing data in request to server.

NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dicPostDict options:kNilOptions error:&error];

 NSString *strJsonData=[[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];

 NSData *encryptedJsonString = [CommonMethods tripleDesEncryptString:strJsonData  key:@"2-055PL&okjnnhkey@inihgr" error:nil];
 NSString *strencryptedJsonString = [encryptedJsonString base64Encoding];

Access controls in Swift

Ans : 

Access controls keyword enables you to hide the implementation details of your code, and to specify a preferred interface through which that code can be accessed and used.

Swift 3, Swift 4

There are 5 access controls.
1. open (most accessible, least restrictive)
2. public
3. internal (default)
4. fileprivate
5. private (least accessible, more restrictive)

1. open : It enable entity to be used in and outside of defining module and also other module. UIButton, UITableView is in UIKit. We import UIKit and make subclass of UITableView and use in our module in which we have imported UIKit. So tableview subclass of UITableView defined in UIKit is used in our module. Sot it is accessible in our module.

open class UITableView : UIScrollView, NSCoding { }

2. public : open allows us to subclass from another module. public allows us to subclass or override from within module in which it defined.

//module X
public func A(){}
open func B(){}

//module Y
override func A(){} // error
override func B(){} // success

So open class and class members can be accessible and overridden in which it is defined and also in which module it is imported.
public class and class members can be accessible and overridden only in which it is defined.


3. internal : Internal classes and members can be accessed anywhere within the same module(target) they are defined. You typically use internal-access when defining an app’s or a framework’s internal structure.

4. fileprivate : Restricts the use of an entity to its defining file. It is used to hide implementation details when details are used in entire file. fileprivate method is only accessible from that swift file in which it is defined.

5. private : Restricts the use of an entity to the enclosing declaration and to extension of that swift file or class. It is used to hide single block implementation. private entity can be accessible in swift 4 but it gives error in swift 3.

MVC in iOS

Ans : The main goal of MVC pattern is separate data/logic, view and controller. There are 3 layers.

1. Model : Models are representation of your app's data. There is user class or struct. So it has fields like name, birthdate, etc. It is data reside in Model layer.

2. View : It is object which user can see and interact with. UILabel showing text is one kind of view.

3. Controller : Controller mediates between Model and View. It takes data from model and show on views and also update model when user interacts with view.

File Structure :

Understand MVC using UITableView

Explain MVC through implementation of UITableView

Ans :

Read first about MVC

Let's understand to implement UITableview in MVC

1. We write fetching data query or request web-service and parse json,xml and save data to any global variable like dictionary, array, etc. So all inserting, fetching methods and data variable are comes under M.....Model

2. We make custom tableview cell, and put various types of views like UILabel, UITextField, UIButton for displaying in UITableView on screen. So this comes under V....View.

3. For displaying TableView, we will take UIVIewController or UITableviewController. This are responsible for various activity like telling how many section, rows are there, on delete button deleting row, how much height of cell, etc... So this comes under C....Controller.

Which delegate method called when I click on push notifications?

Ans : 
Different app delegate method called depends on following scenarios

For silent notification : 

App is in Foreground
No system alert shown
application:didReceiveRemoteNotification:fetchCompletionHandler: is called

App is in Background
System alert is shown
application:didReceiveRemoteNotification:fetchCompletionHandler: is called

App is in Suspended
App state changes to Background
System alert is shown
application:didReceiveRemoteNotification:fetchCompletionHandler: is called

App is Not Running because killed by user
System alert is shown
No callback is called

Normal Push Notification (no content-available) :

App is in Foreground
No system alert shown
application:didReceiveRemoteNotification:fetchCompletionHandler: is called

App is in Background or Suspended
System alert is shown
No method is called, but when user tap on the push and the app is opened
application:didReceiveRemoteNotification:fetchCompletionHandler: is called

App is in Not Running
System alert is shown
No method is called, but when user tap on the push and the app is opened
application:didFinishLaunchingWithOptions: then application:didReceiveRemoteNotification:fetchCompletionHandler: are both called

Add column in SQLite

Ans : 
If we have uploaded our app ver 1.0 and we want to add columns in sqlite database, then following thing can done.

If we have sqlite database which table structure can be changed after release. Then we need to maintain database version. We can update database version by excecuting query like PRAGMA user_version = version_num; (For swift : PRAGMA table_info(tblTest))

In second release If we need to add columns, then we can check database version, If database version is old version then we can execute following query :

ALTER TABLE {tableName} ADD COLUMN COLNew {type};

Query for last inserted row in SQL database

Ans. 

If column is primary key and integer, then ,

Sqlite : SELECT * FROM table ORDER BY column DESC LIMIT 1;

or

SELECT * FROM table WHERE ID = SELECT LAST_INSERT_ROWID()

FMDB : 

Last inserted id is : [fmdb lastInsertRowId];

Sqlite3 all functions : 

sqlite3_open: This function is used to create and open a database file. It accepts two parameters, where the first one is the database file name, and the second a handler to the database. If the file does not exist, then it creates it first and then it opens it, otherwise it just opens it.
sqlite3_prepare_v2: The purpose of this function is to get a SQL statement (a query) in string format, and convert it to an executable format recognisable by SQLite3.
sqlite3_step: This function actually executes a SQL statement (query) prepared with the previous function. It can be called just once for executable queries (insert, update, delete), or multiple times when retrieving data. It’s important to have in mind that it can’t be called prior to the sqlite3_preprare_v2 function.
sqlite3_column_count: This method’s name it makes it easy to understand what is about. It returns the total number of columns (fields) a contained in a table.
sqlite3_column_text: This method returns the contents of a column in text format, actually a C string (char *) value. It accepts two parameters: The first one is the query converted (compiled) to a SQLite statement, and the second one is the index of the column.
sqlite3_column_name: It returns the name of a column, and its parameters are the same to the previous function’s.
sqlite3_changes: It actually returns the number of the affected rows, after the execution of a query.
sqlite3_last_insert_rowid: It returns the last inserted row’s ID.
sqlite3_errmsg: It returns the description of a SQLite error.
sqlite3_finalize: It deletes a prepared statement from memory.

sqlite3_close: It closes an open database connection. It should be called after having finished any data exchange with the database, as it releases any reserved system resources.