Where do I create global variables for an iOS app?

Question

Here is my code:

I want to be able to create a global NSMutableArray that can store Budget* objects that can then be written to a .pList file... I'm only learning what pLists are, and I am a bit hazy about how to implement them...

Where am I going wrong here?

- (IBAction)btnCreateBudget:(id)sender 
{
    Budget *budget = [[Budget alloc] init];

    budget.name = self.txtFldBudgetName.text;
    budget.amount = [self.txtFldBudgetAmount.text intValue];    

    // Write the data to the pList
    NSMutableArray *anArray = [[NSMutableArray alloc] init]; // I want this to be a global variable for the entire app. Where do I put this?

    [anArray addObject:budget];

    [anArray writeToFile:[self dataFilePath] atomically:YES];

    /* As you can see, below is where I test the code. Unfortunately, 
    every time I run this, I get only 1 element in the array. I'm assuming 
    that this is because everytime the button is pressed, I create a brand new 
    NSMutableArray *anArray. I want that to be global for the entire app. */

    int i = 0;
    for (Budget * b in anArray)
    {
        i++;
    }
    NSLog(@"There are %d items in anArray",i);

}

-(NSString *) dataFilePath
{ 
    NSArray *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); 
    NSString *documentDirectory = [path objectAtIndex:0]; 
    return [documentDirectory stringByAppendingPathComponent:@"BudgetData.plist"];
}

edit: I'd like to add that I am creating the anArray array so that it can be accessible by other views. I understand that this can be done with NSNotification? or Should I do this the appDelegate classes? The end goal is to have the anArray object populate a UITableView that is in a separate View.

Answer 1

Just put the declaration outside the method instead of inside it.

NSMutableArray *anArray = nil;

- (IBAction)btnCreateBudget:(id)sender 
{
    ...
    if ( anArray == nil )
      anArray = [[NSMutableArray alloc] init]; 
    ...
}

If it's only used inside the one file, make it "static" instead to prevent name collisions with other files:

    static NSMutableArray *anArray = nil;

If it's only used inside the one method, make it "static" and put it inside that method:

- (IBAction)btnCreateBudget:(id)sender 
{
    static NSMutableArray *anArray = nil;
    ...
    if ( anArray == nil )
      anArray = [[NSMutableArray alloc] init]; 
    ...
}

Note that people usually use some kind of naming convention for global variables, like "gArray", to easily differentiate them from local variables, instance variables, or method parameters.

Answer 2

Global variable is not necessary in this case. You can do something like this:

1. Read old data to mutable array (initWithContentsOfFile:).
2. Add new record to the array.
3. Save the array to same file.

But the second problem in your code is that if your Budget class is not a property list type (NSString, NSData, NSArray, or NSDictionary objects) writeToFile: will not save it sucessfully.

Answer 3

You need to make sure that your Budget class invokes NSCoder and then the NSCoder initWithCoder: and NSCoder decodeWithCoder: methods. Otherwise, writeToFile: will not work for you NSObject class.

But I digress. The answer to the original question should be the following.

In your .h file you need to do the following.

@interface WhateverClassName : UIViewController 
{
    NSMutableArray *anArray;

}

@property(nonatomic, retain) NSMutableArray *anArray;
@end

Then, you need to make sure you @synthesize the NSMutableArray so that you don't get any freaky warnings. This is done just after the @implementation line in your .m file.

Then, within the function that you want it to be allocated into memory, simply do the following.

anArray = [[NSMutableArray alloc] initWithObjects:nil];

This is now a global variable. It is global in the sense that it can be used from any function and is not limited to use in one function.

Answer 4

If you would like to have data accessible to the entire application or context ("global"), you can use a singleton. However, do this with care and make sure it is actually necessary and appropriate. I would suggest doing plenty of reading up on it prior to any implementation of a singleton. Carter Allen has a good basic implementation here.

According to "The end goal is to have the anArray object populate a UITableView that is in a separate View" you wouldn't need to write anything to a file, database or singleton. Just set the object. Like stated by Sebastien Peek.

If you wish for offline data storage, look into sqlite, json, plist , etc