Static NSArray of strings - how/where to initialize in a View Controller

Question

In a Master-Detail app I'd like to display a TableView with 5 sections titled:

1. Your Move
2. Their Move
3. Won Games
4. Lost Games
5. Options

So I create a blank Master-Detail app in Xcode 5.0.2 and then in its MasterViewController.m (which is a UITableViewController) I'm trying to implement the method:

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section {
    return _titles[section];
}

My question is however how to init the NSArray _titles?

I'm trying in the MasterViewController.m:

#import "MasterViewController.h"
#import "DetailViewController.h"

static NSArray *_titles_1 = @[
    @"Your Move",
    @"Their Move",
    @"Won Games",
    @"Lost Games",
    @"Options"
];

@interface MasterViewController () {
    NSMutableArray *_games;

    NSArray *_titles_2 = @[
                         @"Your Move",
                         @"Their Move",
                         @"Won Games",
                         @"Lost Games",
                         @"Options"
    ];
}
@end

@implementation MasterViewController

- (void)awakeFromNib
{
    if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad) {
        self.clearsSelectionOnViewWillAppear = NO;
        self.preferredContentSize = CGSizeMake(320.0, 600.0);
    }
    [super awakeFromNib];
}

- (void)viewDidLoad
{
     ....
}

but both tries above give me syntax errors:

UPDATE:

To my surprise there are many suggestions for this simple question, but as an iOS/Objective-C newbie I'm not sure, which solution is most appropriate.

dispatch_once - isn't it a runtime operation to execute something once in a multi-threaded app? Isn't it overkill here? I was expecting a compile-time solution for initiating a const array...

viewDidLoad - when my app changes between background and foreground, wouldn't it unnecessary initiate my const array again and again?

Shouldn't I better set the NSArray in awakeFromNib (since I use stroyboard scenes for all my ViewControllers)? Or maybe in initSomething (is the correct method initWithStyle?)

Solution 1

Write a class method that returns the array.

+ (NSArray *)titles
{
    static NSArray *_titles;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _titles = @[@"Your Move",
                    @"Their Move",
                    @"Won Games",
                    @"Lost Games",
                    @"Options"];
    });
    return _titles;
}

Then you can access it wherever needed like so:

NSArray *titles = [[self class] titles];

Solution 2

You can init it in class method +initialize

static NSArray *_titles_1;

@implementation MasterViewController
+ (void)initialize {
    _titles_1 = @[
        @"Your Move",
        @"Their Move",
        @"Won Games",
        @"Lost Games",
        @"Options"
    ];
}
@end

Solution 3

You should declare your static array above @implementation.

static NSArray *titles_1 = nil;

@implementation ...

And define it in init or awakeFromNib or any other method like viewDidLoad, applicationDidFinishLaunching wherever you want.

- (void)initMethod{ //change the method name accordingly

    if (titles_1 == nil){
        [self setTitles_1:@[ @"Your Move", @"Their Move", @"Won Games", @"Lost Games", @"Options" ]];
    }
}

Solution 4

You can also do something like this:

static NSString * const strings[] = {
        [0] = @"string_1",
        [1] = @"string_2",
        [2] = @"string_3",
        [3] = @"string_4",
        // ...
    };

It's not an NSArray but you can access NSStrings like this strings[n]

Solution 5

I wonder, if the following would be a good way (answering my own question):

#import "MasterViewController.h"
#import "DetailViewController.h"

static const NSArray *_titles;

@interface MasterViewController () {
    NSMutableArray *_objects;
    NSMutableArray *_yourMove;
    NSMutableArray *_theirMove;
    NSMutableArray *_wonGames;
    NSMutableArray *_lostGames;
    NSMutableArray *_options;
}
@end

@implementation MasterViewController

+ (void)initialize
{
    // do not run for derived classes
    if (self != [MasterViewController class])
        return;

    _titles = @[
        @"Your Move",
        @"Their Move",
        @"Won Games",
        @"Lost Games",
        @"Options"
    ];
}

This way the const NSArray is initalized once and right before I need it (in the MasterViewController class). And the self check prevents this method from running again - when some inheriting class does not implement its own +initialize method.

View more solutions

Comments

[jlehr]

That would mean every instance would have its own copy of the array. I don't think that's what the OP was looking for.

[Simon McLoughlin]

The user is asking WHERE to create this, not how. You haven't provided any help, the users answer shows him trying to instantiate an object in an interface

[jlehr]

@SimonMcLoughlin Quoting the OP, "My question is however how to init the NSArray _titles?" [emphasis added]

[Simon McLoughlin]

fair enough, the point still stands that he's trying to instantiate the object in the interface, this would remove his xcode complier warnings, ill edit with a static array

[jlehr]

If you look carefully, you'll see that the OP posted two different failed attempts, the first of which was an attempt to initialize a global constant with an NSArray.

[Simon McLoughlin]

@jlehr and if you read the rest of the question you will see he is getting syntax warnings due to the above mentioned issue. The user is asking for a basic understanding of how to instantiate an object in the flow of a viewController object. Fair point your solution might be the best way to create it, but the user is quite clearly looking for info as to WHERE to create it

[Hot Licks]

@jlehr - How many instances of this view controller do you think there are going to be?

[Simon McLoughlin]

@HotLicks no that was a valid point, the user did clearly define a static array. The user more than likely had a reason for going out of there way to do that

[Hot Licks]

@SimonMcLoughlin - Can you say "premature optimization"?

[Alexander Farber]

Thanks! This seems to be a runtime solution (instead of the compile-time that I was expecting) - is it the best choice for iOS?

[Alexander Farber]

If I use viewDidLoad, then the _titles will be assigned again and again, when I press Home button to send the app to background and then bring it to foreground?

[jlehr]

Objects can't be allocated at compile time, so this is typically what we do. Note that dispatch_once is a Grand Central Dispatch function that ensures the code in the provided block is only run once in the lifetime of the app.

[Alexander Farber]

Thanks, but I've search and it seems (surprisingly!) that this method can be called more than once for a class - when an inheriting class does not implement own +initialize method.

[Emmanuel]

@AlexanderFarber Then check that _titles_1 is nil or for the current class in +initialize

[Alexander Farber]

If I check for nil - can't it happen that the +initialize method runs once, but for a wrong class - for the inheriting class?

[gnasher729]

+initialize runs once for the class itself, and once for every subclass that doesn't override it. That's why you compare "if (self == [MyIntendedClass class])" first. The check for nil makes it necessary to use synchronisation because two subclasses could be initialised from different threads.

[Emmanuel]

@gnasher729 +initialize is thread safe, according Apple documentation : The runtime sends the initialize message to classes in a thread-safe manner.

[Brody Robertson]

You can use the fully qualified class name to call the static method. Instead of [[self class] titles] you can just call [MyClassName titles];

[jlehr]

@BrodyRobertson Targeting a specific instead of obtaining a reference to the class dynamically class breaks inheritance. Also, not sure what you mean by 'fully qualified' class name -- Objective-C doesn't provide namespaces for classes. Did you have something else in mind?

[Brody Robertson]

@jlehr a fully qualified name is an unambiguous name that specifies which object, function, or variable a call refers to without regard to the context of the call. I don't understand what you mean when you say calling a static method via it's qualified classname breaks inheritance.

[Admin]

Sometimes, most always the simplest solution is not only the most effective, but the best... Being relatively new to Obj-C (7 years now) I'm still put off by the syntax, but I was pretty sure, a simple static array didn't need to involve 25 lines of code.. My C/C++ background told me this would probably work... NSString *dayNamesArray[7] = {@"blah",@"blah",@"blah",@"blah",@"blah",@"blah",@"and blah"}; I give you credit for the most common sense solution... Thanks!

[jlehr]

Objective-C class methods are not static -- they're dynamically dispatched exactly the same way instance methods are, are inherited, and can be overridden in subclasses.

[Tiago Mendes]

with that code you get the error: "Initializer element is not a compile-time constant"