NSMutableArray Returns NULL

Question

I try adding objects to NSMutableArray from another class (secondViewController) and then add it to my UITableView in my FirstViewController, but it returns null when I print it using NSLog. Here is my set up.

FirstViewController.h:

@interface FirstViewController : UIViewController <UITableViewDelegate,UITableViewDataSource>{
    IBOutlet UITableView *mytableview;
    NSMutableArray *mytableinfo;
}
@property (nonatomic,retain) IBOutlet UITableView *mytableview;
@property (retain) IBOutlet NSMutableArray *mytableinfo;

FirstViewController.m

#import "FirstViewController.h"
#import "SecondViewController.h"
@synthesize mytableinfo,mytableview;
-(IBAction)addShift:(id)sender{
SecondViewController *secondViewController = [[SecondViewController alloc]init];
[self presentModalViewController:secondViewController animated:YES];
}
- (void)viewDidLoad
{
    mytableinfo = [[NSMutableArray alloc]init];
    [super viewDidLoad];
}

SecondViewController.m

#import "SecondViewController.h"
#import "FirstViewController.h"

@implementation SecondViewController
@synthesize dateformatter,mydatepicker,startingTime;

-(IBAction)saveShift:(id)sender{
    FirstViewController *firstViewController = [[FirstViewController alloc]init];
    [firstViewController.mytableinfo addObject:@"Hello world"];
    NSLog(@"%@",firstViewController.mytableinfo);
    [self dismissModalViewControllerAnimated:YES];
}

My goal is to ultimately feed a mytableviewfrom mytableinfo. I'm not even sure if this is the best way to go about it. Any advice would be appreciated.

Answer 1

In SecondViewController, you are creating a FirstViewController with alloc init. At that point, mytableinfo on FirstViewController is nil because you don't allocate until viewDidLoad.

What loads SecondViewController? Because you're dismissing it modally. If it's FirstViewController, then when you alloc init first view controller, you're not calling the instance that presented it modally.

It's also not very MVC to have one view poke at another like that. It creates code that's couple at the view layer and modifying data at the view layer. It is better to create a model and have both views modifying that model.

How to create a NSMutable Array which can access from different view controllers

Another way to communicate between views is for one view to pass a delegate (a callback) to the other view. That allows the other view to not be coupled to the other view - it only knows about the protocol for the delegate.

What exactly does delegate do in xcode ios project?

Answer 2

There is a point that look strange to me in your "SecondViewController" you dissmiss it like it's a modal. My Question is then... who started the modal presentation? A "FirstViewController"? If it's the case, why are you creating a new one, on dismissing the second, the First that launched it will resume it's activity.
An other thing that I don't understand is that the designated initializer for a UIViewController is

- (id)initWithNibName:(NSString *)nibName bundle:(NSBundle *)nibBundle

You can pass nil to both argument, if no nib need to be associated with.

And finaly, if you need to get back a NSMutableArray to a 1st ViewController (VC) from a 2nd VC that was modally presented by the 1st you can do this in the 2nd VC:

- (id)initWithMutableArray:(NSMutableArray *)theArray    {
//... put standard init code here   }

And make that the default initializer of your second VC. But this make sense only if 2nd VC absolutely need a mutable array.

---

And now for my curiosity because I don't understand this line

@property (retain) IBOutlet NSMutableArray *mytableinfo;

Why is this an IBOutlet? That look like a potential source of problem. IBOutlet are usually pointers to UI elements in a xib file.

Answer 3

When populating a UITableView with an array that can be modified by multiple modal views during the course of your app, I find one of the best ways to do this is with NSUserDefaults. You can create an NSUserDefaults object for reference like this:

NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];

Then you can assign objects to each key in defaults, which is really just a plist (which is just a list of keys with objects associated with them.

So then, when you want to store the array in defaults, you can say:

[defaults setObject:mytableinfo forKey:@"tableInformationKey"];

Then, whenever you want to access that data, you can say:

NSMutableArray* tableInfoCopy = [defaults mutableArrayValueForKey:@"tableInformationKey"];

That will make you a copy of the array you have stored in NSUserDefaults (NSUserDefaults can be accessed from anywhere in your app), so then you can make changes to that mutable array you just made. Once you are done making changes, you can reassign it to NSUserDefaults like this:

[defaults setObject:tableInfoCopy forKey@"tableInformationKey"];

So when you populate your UITableView, in

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 

put something like:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 
{
    NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"Foobar"];
    if (cell == nil) {
        // No cell to reuse => create a new one
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"Foobar"] autorelease];

        // Initialize cell with some customization
        cell.selectionStyle = UITableViewCellSelectionStyleBlue;
        cell.textLabel.textColor = [UIColor blackColor];
    }

    NSArray* arrayOne = [defaults objectForKey:@"tableInformationKey"];

    NSString* title = [arrayTwo objectAtIndex:indexPath.row]; 
    //this goes to the index in the array of whatever cell you are
    // at, which will populate your table view with the contents of this array (assuming the array contains strings)

    // Customize cell
    cell.textLabel.font = [UIFont fontWithName:@"Helvetica" size:25];

    return cell;
}

Answer 4

Use them the easiest way is to put the array in your AppDelegate

// populate appDelegate's array
AppDelegate *appDelegate = (myAppDelegate *) [[UIApplication sharedApplication] delegate];  
[appDelegate.arrayMyTableInfo addObject:@"HelloWorld"];

Answer 5

Here is how you can do what I've suggested : (This is part of your code updated)
This is in your secondView, this is one of many way to pass the array to your second view.

@synthesize array4Test;
- (id)initWithMutableArray:aMutableArray
{
self = [self initWithNibName:nil bundle:nil];
if (self)
{
    self.array4Test = aMutableArray;
}
return  self;
}
- (void)dealloc 
{    
//  HERE clean up the property is set to retain.
self.array4Test = nil;
[super dealloc];
}

Here is the code for the firstView

-(IBAction)addShift:(id)sender{
SecondViewController *secondViewController = [[SecondViewController alloc] initWithMutableArray:self.mytableinfo];
[self presentModalViewController:secondViewController animated:YES];
}
-(void)viewWillAppear:(BOOL)animated{
[super viewWillAppear:animated];
[self.mytableview reloadData];
NSString *aString = [mytableinfo lastObject];
if (aString)
{
    NSLog(@"This just came back from the second View\n%@", aString);
}
}