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.

Good read : Click here

Help from : 

https://iphonecodecenter.wordpress.com 

UITableviewDelegate and UITableViewDataSource Methods

Ans : 

UITableview's Delegate Methods :

The UIViewController in which UITableView you use must adopt the UITableViewDelegate protocol. Optional methods of the protocol allow the delegate to manage selections, configure section headings and footers, help to delete and reorder cells, and perform other actions.

Configuring Rows for the Table View

- tableView:heightForRowAtIndexPath:

- tableView:indentationLevelForRowAtIndexPath:

- tableView:willDisplayCell:forRowAtIndexPath:

Managing Accessory Views

- tableView:accessoryButtonTappedForRowWithIndexPath:

- tableView:accessoryTypeForRowWithIndexPath: Deprecated in iOS 3.0

Managing Selections

- tableView:willSelectRowAtIndexPath:

- tableView:didSelectRowAtIndexPath:

- tableView:willDeselectRowAtIndexPath:

- tableView:didDeselectRowAtIndexPath:

Modifying the Header and Footer of Sections

- tableView:viewForHeaderInSection:

- tableView:viewForFooterInSection:

- tableView:heightForHeaderInSection:

- tableView:heightForFooterInSection:

Editing Table Rows

- tableView:willBeginEditingRowAtIndexPath:

- tableView:didEndEditingRowAtIndexPath:

- tableView:editingStyleForRowAtIndexPath:

- tableView:titleForDeleteConfirmationButtonForRowAtIndexPath:

- tableView:shouldIndentWhileEditingRowAtIndexPath:

Reordering Table Rows

- tableView:targetIndexPathForMoveFromRowAtIndexPath:toProposedIndexPath:

Copying and Pasting Row Content

- tableView:shouldShowMenuForRowAtIndexPath:

- tableView:canPerformAction:forRowAtIndexPath:withSender:

- tableView:performAction:forRowAtIndexPath:withSender: 

UITableview's Data Source Methods : The NSTableViewDataSource protocol declares the methods that an instance of NSTableView that provides the data to a table view and allows editing of the contents of its data source object.

All the methods are described in the following.

Getting Values

- numberOfRowsInTableView: (Required)

- numberOfSectionInTableView:

- cellForRowAtIndexPath : (Required)

- tableView:objectValueForTableColumn:row:

Setting Values

- tableView:setObjectValue:forTableColumn:row:

Implementing Pasteboard Support

- tableView:pasteboardWriterForRow:

Drag and Drop

- tableView:acceptDrop:row:dropOperation:

- tableView:namesOfPromisedFilesDroppedAtDestination:forDraggedRowsWithIndexes:

- tableView:validateDrop:proposedRow:proposedDropOperation:

- tableView:writeRowsWithIndexes:toPasteboard:

- tableView:draggingSession:willBeginAtPoint:forRowIndexes:

- tableView:updateDraggingItemsForDrag:

- tableView:draggingSession:endedAtPoint:operation:

Sorting

- tableView:sortDescriptorsDidChange:

Tips : UITableViewDelegate has no any required methods.

Latest version of iOS, Swift, XCode

Ans : 
As per today (18-Oct-2018),

Latest version of

XCode : 10.0
Swift : 4.2
iOS : 12.0
Mac OS : mojave 10.14
Objective C : 2.0

I am using XCode 9.3, Swift 3.0, iOS 11 and mac os as High sierra.

Swift Version History :

Date	Version
2014-09-09	Swift 1.0
2014-10-22	Swift 1.1
2015-04-08	Swift 1.2
2015-09-21	Swift 2.0
2016-09-13	Swift 3.0
2017-09-19	Swift 4.0
2018-03-29	Swift 4.1
2018-09-17	Swift 4.2

ARC - Automatic Reference Counting

Ans : Automatic Reference Counting is memory management feature in iOS that provides automatic referencing counting system. According to attribute type of property like retain and release, it increment and decrements reference count at runtime.

ARC is does not handle reference cycle automatically. 

Unlike garbage collection, ARC does not handle reference cycles automatically.


Default property attributes : 

i> Memory management : strong  weak  copy  assign 
ii> Thread Safety : atomic nonatomic
iii> Mutability : readwrite readonly

@property (strong, atomic, readwrite) NSArray *name;

For IBOutlet,

@property (nonatomic, retain) IBOutlet UILabel *label;


@property (weak) IBOutlet UILabel *instructions;
In 2015, apple recommend to use Strong.


To stop retain cycle, user should mention weak reference when needed.

Q : What is retain?
A.Retain works same as Strong according to apple document. If we assign retain, it will convert to strong or consider as Strong. 

Read : Difference between Strong and Weak attribute

How to find current location?

Ans : 

Framework : MapKit
Class : CLLocationManager
Delegate : CLLocationManagerDelegate

Write NSLocationAlwaysUsageDescription and also its description in info.plist file.

Code :

@IBAction func setCurrentLocation(sender: AnyObject) {
        if CLLocationManager.locationServicesEnabled() {
            locationManager.delegate = self
            locationManager.desiredAccuracy = kCLLocationAccuracyBest

            if self.locationManager.respondsToSelector(#selector(CLLocationManager.requestAlwaysAuthorization)) {
                locationManager.requestAlwaysAuthorization() // request for authorisation for first time when app open
            } else {
                 locationManager.startUpdatingLocation()
            }
        }
    }


 //Updated location

    func locationManager(manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {

//Get last updated location(current)

        let location = locations.last! as CLLocation

        let center = CLLocationCoordinate2D(latitude: location.coordinate.latitude, longitude: location.coordinate.longitude)

        let region = MKCoordinateRegion(center: center, span: MKCoordinateSpan(latitudeDelta: 0.20, longitudeDelta: 0.20))

//Set region of current location in map view with zooming

        self.mpView.setRegion(region, animated: true)

//Show current location (blue dot) on map
        self.mpView.showsUserLocation = true

    }

What is protocol? Why it is used?

Ans : Protocols are way to specify a set of methods that class has to implement if class want to work with this protocol. Protocol has 2 types of methods like Required type and Optional type methods.

1. If @required are tagged above methods, then these methods must be implemented on class which confirm this protocol.

2. If @optional are tagged above methods, then these methods is not necessary to implement on it.

Why it is used? 

Protocol provide blueprint methods for classes. It has certain methods and parameters that classes have to implement in its body. Its like that classes should have certain characteristic if it confirm protocol. Like if you want to become human(class) you must be sleep, eat, breathe(characteristics).

For more detail,
I have created Delegate and Protocol. See example.

What is category?

Ans : It is used to extend the functionality of the class. Implementation of category is in different file with name like classname + categoryname.h and .m file.

Read : Differnce between Category and Extension

What is delegate?

Ans : Delegate is means of communication between objects of iOS Applications. Delegate allows one object to send message to another object when an event occurs.

i.e

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@”title” message:@”message” delegate:self cancelButtonTitle:@”Ok” otherButtonTitle:nil];

Here delegate is self. So now self is responsible for handling  all event fired by this instance of UIAlertView class.

Which button of UIAlertView is clicked, for that, event is clickedButtonAtIndex is called in or by Self or currentViewController.

Create PreDefined delegate :

1. There are two ViewController NameVC and SurNameVC.
2.  In NameVC, there are 2 textfield named as Name and FullName and 1 button as Submit.
3. If I write in name and click on submit, it went to SurNameVC to take Surname parameter.
4. On SurNameVC, after write Surname, on clicking of Submit, It call delegate method and went back to NameVC and Print Full Name in FullName textfield.

Implement above delegate and protocol in Objective-C : 

I have made protocol on SurNameVC like

    @protocol SurNameVCDelegate

      -(void)setSurName:(NSString *) strSurName;

    @end

    

    @property (nonatomic, retain) id delegate;

Now on NameVC submit button click, choose delegate of  SurNameVC object as self.

   objSurNameVC.delegate = self

and create method -(void)setSurName:(NSString *) strSurName;

on NameVC and it is called from surNameVC submit button. So setSurName is delegate method. We can print fullname by concatenating Name and Surname in FullName textfield.

So we delegate just pass message from one view controller to another view controller by delegate method.

Implement delegate and protocol in Swift : 

I have made custom UISlider. I want to send some value from custom UISlider value changed to view controller in which it is used. So for that, I have used delegate - protocol method.

customSlider.swift  Custom Slider file

import UIKit

protocol SliderDelegate: class {

    func sliderValueChanged(_ sender : UISlider)

}

class mpgpsSlider: UIView {

     

      weak var delegate : SliderDelegate?

      required init?(coder aDecoder: NSCoder) {

        super.init(coder: aDecoder)

        

        let bundle = Bundle.init(for: type(of: self))

        let nib = UINib(nibName: "Slider", bundle: bundle)

        let view = nib.instantiate(withOwner: self, options: nil)[0] as! UIView

        view.frame = bounds

        view.autoresizingMask = [.flexibleWidth,.flexibleHeight]

        addSubview(view)

        

        slider.addTarget(self, action: #selector(sliderValueChanged(_:)), for:                  .valueChanged)

    }

     @objc func sliderValueChanged(_ sender : UISlider)  {

        delegate?.sliderValueChanged(sender)

      }

}

ViewController.swift ViewController in which custom slider is used.

import UIKit

class VehicleProfileVC: BaseViewController,SliderDelegate{

    

    override func viewDidLoad() {

        super.viewDidLoad()

        slider.delegate = self

    }

    

    func sliderValueChanged(_ sender: UISlider) {

        label.text = String(sender.value)

    }

}

Difference between Objective-C Category and Extension

Ans : 

1.Category is a way to add methods to a class whether or not source code is available implies you can add category to foundation classes like NSString and also to your own custom classes.

2.We can add extra instance variable and property in class extension but not in Category.

3.Any variable and method inside the extension is not even accessible to inherited class.

4.Category and Extension both are basically made to handle large code base but category is a way to extend class API in multiple source file while extension is a way to add required methods out side the main interface file.

5.Use category when you have to break your same class code into different source file according to different functionality and Extension when you just need to add some required methods to existing class outside the main interface file. also when you need to modify a publicly declared instance variable in a class. for ex: readonly to readwrite you can re declare it in extension.

Read : What is Category?