iOS change Integer/NSNumber of parent ViewController

Question

I have a viewController with a property of NSInteger (or NSNumber), What I want to do is pass the integer to the child ViewController, do some change (maybe add by some number).

But when I use NSNumber as property in the parent and child controller and pass it by set property:

//parent and child property.    
@property (nonatomic,strong) NSNumber *num;

//pass to the child
child.num = self.num;

//do some change in child
self.num = [NSNumber numberWithInteger:[self.num integerValue] + 1];

This cannot change the value of the parent , because NSNumber is readonly and in the child they alloc a new one, without change the value of the parent.

So:

How I can pass a NSInteger or NSNumber to the child ViewController and change the value in it, and when child is pop away, the parent controller get the change?

Answer 1

You are defining and assigning a new NSNumber in the following line:

self.num = [NSNumber numberWithInteger:[self.num integerValue] + 1];

You can't use this method because NSNumbers are immutable. So self.num will point to another location different from parent's num . To avoid problems like this you can use delegate pattern or simply passing parent's viewController pointer to the child and changing its num value.

For example:

Parent's Header:

@property (nonatomic,strong) NSNumber *num;

Child's Header:

@property (nonatomic,weak) parentViewControllerClass * parentClass;

Code:

self.parentClass.num = [NSNumber numberWithInteger:[self.parentClass.num integerValue] + 1];

PS You can check out how to define protocols here and here . With that you can implement the delegate pattern.

Answer 2

Maybe in other language you could pass pointer to pointer (I'm not sure if you should ), but in Objective-C you can't request address of property.

What you can do is:

1. Use mentioned delegate pattern - what it means:
   1. Create protocol that specifies method like setNumber:
   2. Implement this protocol in your parent controller (just implement setNumber: function)
   3. Set parent controller as property of child, but as instance of this protocol, not instance of it's class - to keep encapsulation.
   4. Call setNumber: from child controller

2. Create mutable object that is shared between parent and child controller. For example:

    @interface Counter : NSObject @property (nonatomic) NSInteger value; @end 

3. You could post notification about number change from child controller and observe it in parent:

    // post notification in child NSNumber *num = [NSNumber numberWithInteger:1]; [[NSNotificationCenter defaultCenter] postNotificationName:@"numberChangedNotification" object:self userInfo:@{@"number": num}]; // observe notifications in parent // first register observer in viewDidLoad [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(changeNumber:) name:@"numberChangedNotification" object:child]; // and implement updateNumber - changeNumber: (NSNotification *)notification { NSNumber *num = notification.userInfo[@"number"]; // ... }