我应该在iOS / OS X应用程序的哪个位置初始化我的单例？

Question

I want to initialize my singleton object which stores and manages the application settings over the entire class within my app. Also, the singleton instance should be initialized by loading the data from NSUserDefaults upon launch. However, I'm not fully sure where I should initialize the singleton upon launch.

In Cocoa app, I first wrote the singleton initialization code within applicationWillFinishLaunching: , taking parameters from NSUserDefaults . However, later I found that this doesn't work properly if I also write the singleton initialization code (taking no parameter!) within my initial view controller, set in storyboard, because the viewWillLoad: , viewDidLoad: etc. of the class of the view controller are called before the applicationWillFinishLaunching: .

So now I'm sure I should write the singleton initalization code within viewWillLoad: earlier than applicationWillFinishLaunching , but still not sure whether it is appropriate. Specifically, I know the NSApplicationMain is the first method to be called upon launch, but it seems that the next method is not anything within AppDelegate , at least if you use storyboard.

To summary, what I want to ask are the following:

• What method from what class will be called after NSApplicationMain , if you use storyboard.

• Where should I write my singleton initialization code within my app? I want to initialize it as soon as possible.

• Does it differ between iOS and OS X app?

Answer 1

You should initialize it when it's first accessed. Something like this, maybe:

+ (instancetype)sharedInstance {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    _instance = [[self alloc] init];
  });
  return _instance;
}

---

As a side note, if you're literally only using this class as an accessor to NSUserDefaults , you might want to consider using static methods instead.

+ (id)mySpecificDataPoint {
  return [[NSUserDefaults standardUserDefaults] objectForKey:@"whatever"];
}
+ (void)setMySpecificDataPoint:(id)data {
  [[NSUserDefaults standardUserDefaults] setObject:data forKey:@"whatever"];
}

---

Or maybe a more well-designed way might be to add a category to NSUserDefaults for this purpose.

@interface NSUserDefaults (MyData)
@property (nonatomic) NSString *someDataPoint;
@property (nonatomic) NSInteger somePrimitiveDataPoint;
@end

@implementation NSUserDefaults (MyData)
- (NSString *)someDataPoint {
  return [self objectForKey:@"someDataPoint"];
}
- (void)setSomeDataPoint:(NSString *)someDataPoint {
  [self setObject:someDataPoint forKey:@"someDataPoint"];
}
- (NSInteger)somePrimitiveDataPoint {
  return [[self objectForKey:@"somePrimitiveDataPoint"] integerValue];
}
- (void)setSomePrimitiveDataPoint:(NSInteger)somePrimitiveDataPoint {
  [self setObject:@(somePrimitiveDataPoint) forKey:@"somePrimitiveDataPoint"];
}
@end

Answer 2

You init the singleton when you have to use it. So as Daji Djan said: lazy wins. Just take attention that, you should not do a long-run process in your applicationWillFinishLaunching , it should return as soon as possible.

If the singleton is not mandatory during applicationWillFinishLaunching, you should call it in viewWillAppear of first view controller if you need to initialize it ASAP.

Answer 3

lazy always wins

if you can get away with it: as late as possible :) AND always do the minimum needed (but do as much as is reasonable to keep your code clean!)