OOPS Concept in iOS

Ans :  OOPS Concepts are mostly same in all language. Their working are same but conditions are different.

1.      Inheritance : It allows a class to be defined that has a certain set of characteristics(methods and variables) and then other classes to be created which are derived from that class. The derived class inherits all of the features of the parent class and typically then adds some features of its own.

2.      Encapsulation : That binds data and functions together and keeps both safe from outside interference and misuse. Data encapsulation’s benefit of data hiding.

Data encapsulation is a mechanism of bundling the data and the functions that use them.

Data abstraction is a mechanism of exposing only the interface and hiding the implementation details from the user.

3. Polymorphism : Difference process depends on type. The method to be invoked is determined at runtime based on the type of the object. It is like override and overload.

Override : Different class, same method name with signature. It is also called runtime or dynamic polymorphism.

Overload : Same class, method name same with Different parameter. It is also called compile-time or static polymorphism. 

‌Which method is called first when app start?

Ans :

According to apple doc,

• (BOOL)application:(UIApplication *)application willFinishLaunchingWithOptions:(NSDictionary *)launchOptions{}

gets called before

• (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{}

Why array method containObject has id as parameter in ios?

Ans : id is a generic type. This means that the compiler will expect any object type there, and will not enforce restrictions. It can be useful if you're expecting to use more than one class of objects there. So you can add anything of type id to an NSArray.

So array method containObject has id type as parameter.

- (BOOL)containsObject:(ObjectType)anObject;

Usage :

bool bVal = [arr containObject:@5];

Difference between blocks and completion handler in iOS

Ans :

Blocks:

Blocks are a language-level feature added to C, Objective-C and C++, which allow you to create distinct segments of code that can be passed around to methods or functions as if they were values. Blocks are Objective-C objects, which means they can be added to collections like NSArray or NSDictionary.

They can be executed in a later time, and not when the code of the scope they have been implemented is being executed.
Their usage leads eventually to a much cleaner and tidier code writing, as they can be used instead of delegate methods, written just in one place and not spread to many files.
Syntax: ReturnType (^blockName)(Parameters)

see example:

int anInteger = 42;

void (^testBlock)(void) = ^{

    NSLog(@"Integer is: %i", anInteger);   // anInteger outside variables

};

// calling blocks like
testBlock();
Block with argument:

double (^multiplyTwoValues)(double, double) =

                          ^(double firstValue, double secondValue) {

                              return firstValue * secondValue;

                          };
// calling with parameter
double result = multiplyTwoValues(2,4);

NSLog(@"The result is %f", result);

Completion handler:

Whereas completion handler is a way (technique) for implementing callback functionality using blocks.

A completion handler is nothing more than a simple block declaration passed as a parameter to a method that needs to make a callback at a later time.

Note: completion handler should always be the last parameter in a method. A method can have as many arguments as you want, but always have the completion handler as the last argument in the parameters list.

Example:

- (void)beginTaskWithName:(NSString *)name completion:(void(^)(void))callback;

// calling

[self beginTaskWithName:@"MyTask" completion:^{

    NSLog(@"Task completed ..");

}];

More example with UIKit classes methods.

[self presentViewController:viewController animated:YES completion:^{

        NSLog(@"xyz View Controller presented ..");

        // Other code related to view controller presentation...

    }];

[UIView animateWithDuration:0.5

                     animations:^{

                         // Animation-related code here...

                         [self.view setAlpha:0.5];

                     }

                     completion:^(BOOL finished) {

                         // Any completion handler related code here...

                         NSLog(@"Animation over..");

                     }];

Why tableview cell separator leave some space before?

Ans : 

Alternatively, you can also edit this in interface builder (IB):

1. Go to IB.
2. Select Table View.
3. Open "Attribute Inspector" on the right.
4. Change "Separator Insets" from Default to Custom.
5. Change the "Left" attribute from 15 to 0.

15 is default. So Left side space is seen before cell seperator.

Do you know SMTP? is it use SOAP or REST?

Ans : 

SMTP : SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail.

SOAP and REST both allow you to create your own API. API stands for Application Programming Interface. It makes it possible to transfer data from an application to other applications. An API receives requests and sends back responses through internet protocols such as HTTP, SMTP, and others.

It use SOAP web-service. SOAP is protocol and rules are maintained by World wide web consortium(W3C).

Design such button in that text should be displayed below image

Ans : 

1) Select button and go to Attribute Inspector in your storyboard.

2) Assign Image to the button. (Don't use background Image)

3) Set Title text to that button.

4) Now you need to set edge and Inset so first select image from edge and set Inset as you need and then select title from edge and set inset as per your need.

Difference between viewdidload and viewwillappear?

Ans : 

ViewDidLoad - It is excecuted once. So writer settings like set label text in ViewDidLoad.

ViewWillAppear - It is called every time when view appear.

ViewDidLoad - It is called when view is begin constructed.

ViewWillAppear - When view is about ready to appear.

ViewDidLoad :  It is automatically called when view controller completely loaded into memory. Override this method to perform additional initialization on views that were loaded from xib.

I.e instance variable initialization, database access, network request

ViewWillAppear :  It is called when View is about to added on view hierachy. If we want to change some, then we have to override this method.Like change orientation, change screen data.

Suppose your tableview data will be changed periodically. Then you have to write [tableview reloaddata] in ViewWillApper.

Read UIViewController LifeCycle

What are join in sql? Explain Types of Join

Ans :

Join : When we need data from more than 1 table, then we can fetch data from those table using joining those table. It will return only matching data from two tables.

Select * from table1 t1 inner join table2 t2 on t1.column1 = t2.column1

Types of Join :

1. Inner Join : If we use inner join between two table, then only data exists in both table, are returned.

2. Outer Join :
    Left Outer Join : It returns all data from left table(table1) and only matching data from both table.
    Right Outer Join : It returns all data from right table(table2) and only matching data from both table.

3. Cross Join : It is like cartesian join. It returns all records from both table and it returns table1.count * table2.count records in returns. Suppose table1 has 4 records and table2 has 3 records and returns 4*3 records.

4. Self Join : If some column's reference has in same table then self join is used. It is same like inner join but here both left and right table are same. Its like if you table and Columns are such as MainID, Name, ParentID then you can make query like
Select * from table t1 join table t2 on t1.MainID = t2.ParentID

Difference between Strong and Weak in iOS

Ans : 

Strong

A strong property means that you want to “own” the object. Only when you set the property to nil will the object be  destroyed. Unless one or more objects also have a strong reference to the object. This is the one you will use in most cases.

1. Creates ownership between property and assigned value.

1. This is default for object property in ARC so it does not let you worrying about reference count and release the reference automatically.

1. It is replacement for retain. We use if and only if we need to use as retain.

1. Retain count will be incremented.

1. Creates non-ownerships between property and assigned value.

1. Strong is used on parent object and weak is used on child object when parent is released then child object reference is also set to nil

1. It helps to prevents retain cycles.

1. It doesn’t protect the referenced object when collection by garbage collector.

1. Weak is essentially assigned, un-retain property.

1. Retain count will not be incremented.

Weak

Weak property means you don’t want to have control over the objects lifecycle. The object only lives on while another objects has a strong reference to it. If there are no strong references to the object then it will be destroyed. 

Weak reference is useful to avoid situation like retain cycle. Retain cycle occurs when two or more objects have strong reference to each other. This two object will never be freed in memory due to strong reference. So to avoid weak reference, One object has a strong ownership reference to another object, and another object should be have a weak reference to first object.

Strong references should be used when a parent object is referencing a child object and never the other way around. That is, a child class should not have a strong reference to the parent class.

Weak references should be used to avoid retain cycles and an object has the possibility to become nil at any point of it’s lifetime.

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