Initialising a static variable in Objective-C cate

问题:

I was trying to create a static variable to store a dictionary of images. Unfortunately, the best way I could find to initialise it was to check in each function that used the variable. Since I am creating this variable inside a category, I can't just initialise it inside the initialiser. Is there a neater way of initialising navigationBarImages?

static NSMutableDictionary *navigationBarImages = NULL;

@implementation UINavigationBar(CustomImage)
//Overrider to draw a custom image
- (void)drawRect:(CGRect)rect
{
    if(navigationBarImages==NULL){
        navigationBarImages=[[NSMutableDictionary alloc] init];
    }
    NSString *imageName=[navigationBarImages objectForKey:self];
    if (imageName==nil) {
        imageName=@"header_bg.png";
    }
    UIImage *image = [UIImage imageNamed: imageName];
    [image drawInRect:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)];
}

//Allow the setting of an image for the navigation bar
- (void)setImage:(UIImage*)image
{
    if(navigationBarImages==NULL){
        navigationBarImages=[[NSMutableDictionary alloc] init];
    }
    [navigationBarImages setObject:image forKey:self];
}
@end

回答1:

__attribute__((constructor))
static void initialize_navigationBarImages() {
  navigationBarImages = [[NSMutableDictionary alloc] init];
}

__attribute__((destructor))
static void destroy_navigationBarImages() {
  [navigationBarImages release];
}

These function will be called automatically when the program starts and ends.

回答2:

Consider this approach,

static NSMutableDictionary *navigationBarImages()
{
    static NSMutableDictionary *dict = NULL;
    if(dict == NULL)
    {
        dict = [[NSMutableDictionary alloc] init];
    }
    return [[dict retain] autorelease];
}

then whenever you woulde use navigationBarImages, replace it with navigationBarImages(), like this:

change

NSString *imageName=[navigationBarImages objectForKey:self];

to

NSString *imageName=[navigationBarImages() objectForKey:self];

If the function call overhead bothers you, maybe use a temporary variable to catch the return of navigationBarImages(),

NSMutableDictionary *dict = navigationBarImages();
[dict doSomething];
[dict doSomething];

The drawback is once you called navigationBarImages(), the instance of NSMutableDictionary got created, then it'll never get chance to dealloc until the end of the program.

回答3:

All you need is to set your static once at a known point before it is used. For example, you can set an NSApplication delegate and have it do the work in -applicationDidFinishLaunching:

回答4:

One option is to use C++. Change the file's extension to .mm and replace = NULL with [[NSMutableDictionary alloc] init].

回答5:

You could add +initialize in the .m file of your category — you'll just need to make sure you're not smashing an existing implementation or you'll get general wonkiness. (Obviously, you can be sure of this if you wrote the code, but with third-party code, this is probably not the best approach.)