Instance Variables for Objective C Categories

问题:

I have a situation where it seems like I need to add instance variables to a category, but I know from Apple's docs that I can't do that. So I'm wondering what the best alternative or workaround is.

What I want to do is add a category that adds functionality to UIViewControllers. I would find it useful in all my different UIViewControllers, no matter what specific UIViewController subclass they extend, so I think a category is the best solution. To implement this functionality, I need several different methods, and I need to track data in between them, so that's what led me to wanting to create instance methods.

In case it's helpful, here's what I specifically want to do. I want to make it easier to track when the software keyboard hides and shows, so that I can resize content in my view. I've found that the only way to do it reliably is to put code in four different UIViewController methods, and track extra data in instance variables. So those methods and instance variables are what I'd like to put into a category, so I don't have to copy-paste them each time I need to handle the software keyboard. (If there's a simpler solution for this exact problem, that's fine too--but I would still like to know the answer to category instance variables for future reference!)

回答1:

Yes you can do this, but since you're asking, I have to ask: Are you absolutely sure that you need to? (If you say "yes", then go back, figure out what you want to do, and see if there's a different way to do it)

However, if you really want to inject storage into a class you don't control, use an associative reference.

回答2:

Recently, I needed to do this (add state to a Category). @Dave DeLong has the correct perspective on this. In researching the best approach, I found a great blog post by Tom Harrington. I like @JeremyP's idea of using @property declarations on the Category, but not his particular implementation (not a fan of the global singleton or holding global references). Associative References are the way to go.

Here's code to add (what appear to be) ivars to your Category. I've blogged about this in detail here.

In File.h, the caller only sees the clean, high-level abstraction:

@interface UIViewController (MyCategory)
@property (retain,nonatomic) NSUInteger someObject;
@end

In File.m, we can implement the @property (NOTE: These cannot be @synthesize'd):

@implementation UIViewController (MyCategory)

- (NSUInteger)someObject
{
  return [MyCategoryIVars fetch:self].someObject;
}

- (void)setSomeObject:(NSUInteger)obj
{
  [MyCategoryIVars fetch:self].someObject = obj;
}

We also need to declare and define the class MyCategoryIVars. For ease of understanding, I've explained this out of proper compilation order. The @interface needs to be placed before the Category @implementation.

@interface MyCategoryIVars : NSObject
@property (retain,nonatomic) NSUInteger someObject;
+ (MyCategoryIVars*)fetch:(id)targetInstance;
@end

@implementation MyCategoryIVars

@synthesize someObject;

+ (MyCategoryIVars*)fetch:(id)targetInstance
{
  static void *compactFetchIVarKey = &compactFetchIVarKey;
  MyCategoryIVars *ivars = objc_getAssociatedObject(targetInstance, &compactFetchIVarKey);
  if (ivars == nil) {
    ivars = [[MyCategoryIVars alloc] init];
    objc_setAssociatedObject(targetInstance, &compactFetchIVarKey, ivars, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [ivars release];
  } 
  return ivars;
}

- (id)init
{
  self = [super init];
  return self;
}

- (void)dealloc
{
  self.someObject = nil;
  [super dealloc];
}

@end

The above code declares and implements the class which holds our ivars (someObject). As we cannot really extend UIViewController, this will have to do.

回答3:

I believe it is now possible to add synthesized properties to a category and the instance variables are automagically created, but I've never tried it so I'm not sure if it will work.

A more hacky solution:

Create a singleton NSDictionary which will have the UIViewController as the key (or rather its address wrapped as an NSValue) and the value of your property as its value.

Create getter and setter for the property that actually goes to the dictionary to get/set the property.

@interface UIViewController(MyProperty)

@property (nonatomic, retain) id myProperty;
@property (nonatomic, readonly, retain) NSMutableDcitionary* propertyDictionary;

@end

@implementation  UIViewController(MyProperty)

-(NSMutableDictionary*) propertyDictionary
{
    static NSMutableDictionary* theDictionary = nil;
    if (theDictionary == nil)
    {
        theDictioanry = [[NSMutableDictionary alloc] init];
    }
    return theDictionary;
}


-(id) myProperty
{
    NSValue* key = [NSValue valueWithPointer: self];
    return [[self propertyDictionary] objectForKey: key];
}

-(void) setMyProperty: (id) newValue
{
    NSValue* key = [NSValue valueWithPointer: self];
    [[self propertyDictionary] setObject: newValue forKey: key];    
}

@end

Two potential problems with the above approach:

• there's no way to remove keys of view controllers that have been deallocated. As long as you are only tracking a handful, that shouldn't be a problem. Or you could add a method to delete a key from the dictionary once you know you are done with it.
• I'm not 100% certain that the isEqual: method of NSValue compares content (i.e. the wrapped pointer) to determine equality or if it just compares self to see if the comparison object is the exact same NSValue. If the latter, you'll have to use NSNumber instead of NSValue for the keys (NSNumber numberWithUnsignedLong: will do the trick on both 32 bit and 64 bit platforms).

回答4:

This is best achieved using the built-in ObjC feature Associated Objects (aka Associated References), in the example below just change to your category and replace associatedObject with your variable name.

NSObject+AssociatedObject.h

@interface NSObject (AssociatedObject)
@property (nonatomic, strong) id associatedObject;
@end

NSObject+AssociatedObject.m

#import <objc/runtime.h>

@implementation NSObject (AssociatedObject)
@dynamic associatedObject;

- (void)setAssociatedObject:(id)object {
     objc_setAssociatedObject(self, @selector(associatedObject), object, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (id)associatedObject {
    return objc_getAssociatedObject(self, @selector(associatedObject));
}

See here for the full tutorial:

http://nshipster.com/associated-objects/

回答5:

It mentioned in many document's online that you can't create create new variable in category but I found a very simple way to achieve that. Here is the way that let declare new variable in category.

In Your .h file

@interface UIButton (Default)

   @property(nonatomic) UIColor *borderColor;

@end  

In your .m file

#import <objc/runtime.h>
static char borderColorKey;

@implementation UIButton (Default)

- (UIColor *)borderColor
{
    return objc_getAssociatedObject(self, &borderColorKey);
}

- (void)setBorderColor:(UIColor *)borderColor
{
    objc_setAssociatedObject(self, &borderColorKey,
                             borderColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    self.layer.borderColor=borderColor.CGColor;
}

@end

That's it now you have the new variable.

回答6:

Why not simply create a subclass of UIViewController, add the functionality to that, then use that class (or a subclass thereof) instead?

回答7:

Depending on what you're doing, you may want to use Static Category Methods.

So, I assume you've got this kind of problem:

ScrollView has a couple of textedits in them. User types on text edit, you want to scroll the scroll view so the text edit is visible above the keyboard.

+ (void) staticScrollView: (ScrollView*)sv scrollsTo:(id)someView
{
  // scroll view to someviews's position or some such.
}

returning from this wouldn't necessarily require the view to move back, and so it doesn't need to store anything.

But that's all I can thinkof without code examples, sorry.

回答8:

I believe it is possible to add variables to a class using the Obj-C runtime.
I found this discussion also.