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objective-c interview questions

Top objective-c frequently asked interview questions

Passing Data between View Controllers

I'm new to iOS and, Objective-C and the whole MVC paradigm and I'm stuck with the following:

I have a view that acts as a data entry form and I want to give the user the option to select multiple products. The products are listed on another view with a UITableView controller and I have enabled multiple selections.

My question is, how do I transfer the data from one view to another? I will be holding the selections on the UITableView in an array, but how do I then pass that back to the previous data entry form view so it can be saved along with the other data to core data on submission of the form?

I have surfed around and seen some people declare an array in the app delegate. I read something about Singletons but don't understand what these are and I read something about creating a data model.

What would be the correct way of performing this and how would I go about it?


Source: (StackOverflow)

How to check for an active Internet connection on iPhone SDK?

I would like to check to see if I have an Internet connection on the iPhone using the Cocoa Touch libraries.

I came up with a way to do this using an NSURL. The way I did it seems a bit unreliable (because even Google could one day be down and relying on a third party seems bad), and while I could check to see for a response from some other websites if Google didn't respond, it does seem wasteful and an unnecessary overhead on my application.

- (BOOL) connectedToInternet
{
    NSString *URLString = [NSString stringWithContentsOfURL:[NSURL URLWithString:@"http://www.google.com"]];
    return ( URLString != NULL ) ? YES : NO;
}

Is what I have done bad? (Not to mention stringWithContentsOfURL is deprecated in 3.0) And if so, what is a better way to accomplish this?


Source: (StackOverflow)

What is a typedef enum in Objective-C?

I don't think I fundamentally understand what an enum is, and when to use it.

For example:

typedef enum {
    kCircle,
    kRectangle,
    kOblateSpheroid
} ShapeType;

What is really being declared here?


Source: (StackOverflow)

performSelector may cause a leak because its selector is unknown

I'm getting the following warning by the ARC compiler:

"performSelector may cause a leak because its selector is unknown".

Here's what I'm doing:

[_controller performSelector:NSSelectorFromString(@"someMethod")];

Why do I get this warning? I understand the compiler can't check if the selector exists or not, but why would that cause a leak? And how can I change my code so that I don't get this warning anymore?


Source: (StackOverflow)

How to make a UITextField move up when keyboard is present

With the iPhone SDK:

I have a UIView with UITextFields that brings up a keyboard. I need it to be able to:

  1. Allow scrolling of the contents of the UIScrollView to see the other text fields once the keyboard is brought up

  2. Automatically "jump" (by scrolling up) or shortening

I know that I need a UIScrollView. I've tried changing the class of my UIView to a UIScrollView but I'm still unable to scroll the textboxes up or down.

Do I need both a UIView and a UIScrollView? Does one go inside the other? [EDIT: I now know that you want a UIView inside of a UIScrollView, and the trick is to programatically set the content size of the UIScrollView to the frame size of the UIView.]

Then what needs to be implemented in order to automatically scroll to the active text field?

Ideally as much of the setup of the components as possible will be done in Interface Builder. I'd like to only write code for what needs it.

Note: the UIView (or UIScrollView) that I'm working with is brought up by a tabbar (UITabBar), which needs to function as normal.


Edit: I am adding the scroll bar just for when the keyboard comes up. Even though it's not needed, I feel like it provides a better interface because then the user can scroll and change textboxes, for example.

I've got it working where I change the frame size of the UIScrollView when the keyboard goes up and down. I'm simply using:

-(void)textFieldDidBeginEditing:(UITextField *)textField { 
    //Keyboard becomes visible
    scrollView.frame = CGRectMake(scrollView.frame.origin.x, 
                     scrollView.frame.origin.y, 
scrollView.frame.size.width,
scrollView.frame.size.height - 215 + 50);   //resize
}

-(void)textFieldDidEndEditing:(UITextField *)textField {
   //keyboard will hide
    scrollView.frame = CGRectMake(scrollView.frame.origin.x, 
       scrollView.frame.origin.y, 
     scrollView.frame.size.width,
      scrollView.frame.size.height + 215 - 50); //resize
}

However this doesn't automatically "move up" or center the lower text fields in the visible area, which is what I would really like.


Source: (StackOverflow)

How can I disable ARC for a single file in a project?

I am using ARC successfully in my project. However, I have encountered a few files (e.g., in unit tests and mock objects) where the rules of ARC are a little more fragile right now. I recall hearing that there was a way to disable ARC on a per-file basis, though I have been unable to find this option.

Is this possible? How do I disable ARC on a per-file basis?


Source: (StackOverflow)

How do I set up NSZombieEnabled in Xcode 4?

How do I set up NSZombieEnabled and CFZombieLevel for my executable in Xcode 4?


Source: (StackOverflow)

Send and receive messages through NSNotificationCenter in Objective-C? [closed]

I need a simple example program to send and receive a message through NSNotificationCenter in Objective-C ?


Source: (StackOverflow)

Creating an abstract class in Objective C

I'm originally a Java programmer who now works with Objective-C. I'd like to create an abstract class but that doesn't appear to be possible in Objective-C. Is this possible?

If not, how close to an abstract class can I get in Objective-C?


Source: (StackOverflow)

Constants in Objective-C

I'm developing a Cocoa application, and I'm using constant NSStrings as ways to store key names for my preferences.

I understand this is a good idea, because it allows easy changing of keys if necessary. Plus, it's the whole 'separate your data from your logic' notion.

Anyway, is there a good way to make these constants defined once for the whole application? I'm sure that there's an easy and intelligent way, but right now my classes just redefine the ones they use.


Source: (StackOverflow)

Codesign error: Provisioning profile cannot be found after deleting expired profile

Tried to rebuild an app that was just working yesterday. Got a message that a profile had expired, so I removed it from the iPod and from iTunes. When I chose a new profile (one with an * in the identifier), I now get an error:

Code Sign Error: Provisioning Profile (long string) can't be found.

What am I missing? I looked through related questions and didn't see this scenario already. Thanks


Source: (StackOverflow)

Shortcuts in Objective-C to concatenate NSStrings

Are there any shortcuts to (stringByAppendingString:) string concatenation in Objective-C, or shortcuts for working with NSString in general?

For example, I'd like to make

NSString *myString = @"This";
NSString *test = [myString stringByAppendingString:@" is just a test"];

something more like

string myString = "This";
string test = myString + " is just a test";

Source: (StackOverflow)

How to sort an NSMutableArray with custom objects in it?

What I want to do seems pretty simple, but I can't find any answers on the web. I have an NSMutableArray of objects, let's say they are 'Person' objects. I want to sort the NSMutableArray by Person.birthDate which is an NSDate.

I think it has something to do with this method:

NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(???)];

In Java I would make my object implement Comparable, or use Collections.sort with an inline custom comparator...how on earth do you do this in Objective-C?


Source: (StackOverflow)

How to call Objective C code from Swift

In the new Swift language from Apple, how does one call Objective-C code?

Apple mentioned that they could co-exist in one application, but does this mean that one could technically re-use old classes made in Objective-C whilst building new classes in swift?

The Reasoning

Objective-C is a platform-independent language, whereas Swift is platform-dependent. Writing non-platform-dependent code (business logic libraries) in swift would thus not be wise. However writing platform-dependent code in it (interface related for example) would be perfectly fine. Not to say it would be a good idea, however it is definitely an interest


Source: (StackOverflow)

How to convert an NSString into an NSNumber

How can I convert a NSString containing a number of any primitive data type (e.g. int, float, char, unsigned int, etc.)? The problem is, I don't know which number type the string will contain at runtime.

I have an idea how to do it, but I'm not sure if this works with any type, also unsigned and floating point values:

long long scannedNumber;
NSScanner *scanner = [NSScanner scannerWithString:aString];
[scanner scanLongLong:&scannedNumber]; 
NSNumber *number = [NSNumber numberWithLongLong: scannedNumber];

Thanks for the help.


Source: (StackOverflow)