setting new properties in category interface/implementation

Question

Ok, so I have this, but it wont work:

@interface UILabel (touches)

@property (nonatomic) BOOL isMethodStep;

@end


@implementation UILabel (touches)

-(BOOL)isMethodStep {
    return self.isMethodStep;
}

-(void)setIsMethodStep:(BOOL)boolean {
    self.isMethodStep = boolean;
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    if(self.isMethodStep){
        // set all labels to normal font:
        UIFont *toSet = (self.font == [UIFont fontWithName:@"Helvetica" size:16]) ? [UIFont fontWithName:@"Helvetica-Bold" size:16] : [UIFont fontWithName:@"Helvetica" size:16];

        id superView = self.superview;
        for(id theView in [(UIView *)superView subviews])
            if([theView isKindOfClass:[UILabel class]])
                [(UILabel *)theView setFont:[UIFont fontWithName:@"Helvetica" size:16]];

        self.font = toSet;
    }
}

@end

If I take out the getter and setter methods then it doesn't work it tells me I need to create some getter and setter methods (or use @synthesize - but putting @synthesize in the @implementation throws an error too). But with the getter and setter methods I get an EXC_BAD_ACCESS and a crash. Any ideas? Thanks

Tom

This guy 'Dave DeLong' posted an answer that you should check out... it's probably the "correct" way to do what you want... :) [here's the link...](http://stackoverflow.com/questions/4146183/instance-variables-for-objective-c-categories) — Alex Zak, Jul 04 '11 at 15:54
Associative reference is solution... find nice tutorial here http://www.techpaa.com/2012/04/adding-properties-to-categories-and.html — ShivaPrasad, Apr 17 '12 at 11:47
Also, your `isMethodStep` getter is an infinitely recursive function. That's why you're getting an EXC_BAD_ACCESS crash. `self.isMethodStep` is equivalent to `[self isMethodStep]`. — Drew H, Oct 06 '14 at 18:43

score 40 · Accepted Answer · edited Jul 02 '17 at 09:32

40

It is not possible to add members and properties to an existing class via a category — only methods.

https://developer.apple.com/library/content/documentation/General/Conceptual/DevPedia-CocoaCore/Category.html

One possible workaround is to write "setter/getter-like" methods, that uses a singleton to save the variables, that would had been the member.

-(void)setMember:(MyObject *)someObject
{
    NSMutableDictionary *dict = [MySingleton sharedRegistry];
    [dict setObject:someObject forKey:self];
}

-(MyObject *)member
{
    NSMutableDictionary *dict = [MySingleton sharedRegistry];
    return [dict objectforKey:self];
}

or — of course — write a custom class, that inherits from UILabel

Note that nowadays an associated object can be injected during runtime. The Objective C Programming Language: Associative References

edited Jul 02 '17 at 09:32

Cœur

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answered Nov 04 '10 at 18:46

vikingosegundo

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"or — of course — write a custom class, that inherits from UILabel" would be much less faffy. :) bad news eh? ah... well, thank you. :) – Thomas Clayson Nov 04 '10 at 19:10
You should use [NSValue valueWithPointer:self] as the key, not self. – Dustin Jan 28 '14 at 19:00
@Dustin: as I pointed out in my edit, there is no need to use this ancient code nowadays anymore — if it ever was wise to fake properties at all. Thanks for the downvote. – vikingosegundo Jan 28 '14 at 19:57
Avoid associated objects. Bad form. – ZaBlanc Feb 10 '14 at 18:07
@ZaBlanc, I agree. actually I never used them. But the question is about properties in categories. than they should be mentioned. – vikingosegundo Feb 10 '14 at 18:38
what do `MySingleton` and `MyObject` represent, can you give an example? – gabbler Jan 13 '17 at 09:14
any singleton, any object. – vikingosegundo Jan 13 '17 at 09:39

Kirow · Answer 2 · 2019-06-18T21:10:26.707

Checked all answers and did not find the most common solution:

#import <objc/runtime.h>

static void const *key;

@interface ClassName (CategoryName)
@property (nonatomic) BOOL myProperty;
@end

@implementation ClassName (CategoryName)
- (BOOL)myProperty {
    return [objc_getAssociatedObject(self, key) boolValue];
}

- (void)setMyProperty:(BOOL)value {
    objc_setAssociatedObject(self, key, @(value), OBJC_ASSOCIATION_RETAIN);
}
@end

swift:

private struct AssociatedKeys {
    static var keyName = "keyName"
}

extension Foo {
    var bar: Any! {
        get {
            return objc_getAssociatedObject(self, &AssociatedKeys.keyName)
        }
        set {
            objc_setAssociatedObject(self, &AssociatedKeys.keyName , newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}

Perfect solution! Thank you! – Paula Vasconcelos Gueiros Nov 30 '16 at 19:54 — Paula Vasconcelos Gueiros, Nov 30 '16 at 19:54

score 1 · Answer 3 · answered Feb 20 '15 at 11:54

There is actually a way, which may not be ideal, but does work.
For it to work, you will need to create a category for a class X and can only be used on subclasses of the same X (e.g. category UIView (Background) can be used with class MyView : UIView, but not directly with UIView)

// UIView+Background.h

@interface UIView (Background)

@property (strong, nonatomic) NSString *hexColor;

- (void)someMethodThatUsesHexColor;

@end

// UIView+Background.h

@implementation UIView (Background)

@dynamic hexColor; // Must be declared as dynamic

- (void)someMethodThatUsesHexColor {
    NSLog(@"Color %@", self.hexColor);
}

@end

Then

// MyView.m

#import "UIView+Background.h"

@interface MyView : UIView

@property (strong, nonatomic) NSString *hexColor;

@end

@implementation MyView ()

- (void)viewDidLoad {
    [super viewDidLoad];
    [self setHexColor:@"#BABACA"];
    [self someMethodThatUsesHexColor];
}

@end

Using this method, you will need to "redeclare" your properties, but after that, you can do all of its manipulation inside your category.

score 1 · Answer 4 · answered Feb 20 '18 at 10:01

You could inject an associated object during runtime.

#import <objc/runtime.h>

@interface UIView (Private)

@property (nonatomic, assign) CGPoint initialTouchPoint;
@property (nonatomic, strong) UIWindow *alertWindow;

@end

@implementation UIView (Private)

@dynamic initialTouchPoint, alertWindow;

- (CGPoint)initialTouchPoint {
    return CGPointFromString(objc_getAssociatedObject(self, @selector(initialTouchPoint)));
}

- (void)setInitialTouchPoint:(CGPoint)initialTouchPoint {
    objc_setAssociatedObject(self, @selector(initialTouchPoint), NSStringFromCGPoint(initialTouchPoint), OBJC_ASSOCIATION_RETAIN);
}

- (void)setAlertWindow:(UIWindow *)alertWindow {
    objc_setAssociatedObject(self, @selector(alertWindow), alertWindow, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (UIWindow *)alertWindow {
    return objc_getAssociatedObject(self, @selector(alertWindow));
}

@end

Maxim Chetrusca · Answer 5 · 2013-06-28T08:43:01.777

0

EDIT: Warning: This property would have a unique value for all the instances of the class.

This worked for me, but only because I had only one instance of this class in my app.

#import <AVFoundation/AVFoundation.h>

@interface AVAudioPlayer (AstroAVAudioPlayer)

@property (nonatomic) BOOL redPilot;

@end


#import "AVAudioPlayer+AstroAVAudioPlayer.h"

@implementation AVAudioPlayer (AstroAVAudioPlayer)

BOOL _redPilot;

-(void) setRedPilot:(BOOL)redPilot
{
    _redPilot = redPilot;
}

-(BOOL) redPilot
{
    return _redPilot;
}

@end

edited Jun 28 '13 at 08:43

answered Jun 01 '13 at 14:48

Maxim Chetrusca

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This answer should be flagged as incorrect. The implementation above only allocates one property called _redPilot for all instances of AVAudioPlayer. In other words, if you have three instances of AVAudioPlayer, then avAudioPlayer1.redPilot == avAudioPlayer2.redPilot == avAudioPlayer3.redPilot always. – noobular Jun 27 '13 at 21:26
1

@nobular, You're right, Sir. Indeed in my case I had only one instance of AVAudioPlayer, so I wasn't caring much about that. I answered this here anyway, because I thought this might be helpful for someone, for example someone like me. I would edit the answer so it warns about the above-mentioned issue. – Maxim Chetrusca Jun 28 '13 at 08:38
Here's a technique that I've used in the past for solving the lack of properties on categories: http://www.techpaa.com/2012/04/adding-properties-to-categories-and.html – noobular Aug 04 '13 at 07:00
1

In my case, this is a perfect solution. I'm trying to add a NSDateFormatter to the category of NSDate (maybe I'm crazy) to have only one instance of the formatter in all of my NSDate objects. What do you think about it? – Floydian Sep 04 '13 at 15:01
@Floydian, you know, there is a saying: "It's not a bug. It's a feature." :) So if you want such a feature, go for it, if it does the right job for you. – Maxim Chetrusca Sep 05 '13 at 16:27

score 0 · Answer 6 · answered Mar 25 '14 at 16:23

A solution that I found to this was to just give each object that you want flagged a unique tag.

I made a UILabel category to add custom fonts to all my labels but on some i wanted them to be bold so i did this ->

- (void) layoutSubviews {
    [super layoutSubviews];
    [self addCustomFont];   
}

- (void) addCustomFont {
    if (self.tag == 22) {
        [self setFont:[UIFont fontWithName:SEGOE_BOLD size:self.font.pointSize]];
    }else{
        [self setFont:[UIFont fontWithName:SEGOE_LIGHT size:self.font.pointSize]];
    }
}

mikeho · Answer 7 · 2015-10-19T21:53:35.937

It seems as if since Xcode 7 (7.0.1, 7A1001), properties are supported in categories. I noticed that Xcode generates categories now for Core Data subclasses.

For example, I got the files:

Location+CoreDataProperties.h

#import "Location.h"

NS_ASSUME_NONNULL_BEGIN

@interface Location (CoreDataProperties)

@property (nullable, nonatomic, retain) NSNumber *altitude;
@property (nullable, nonatomic, retain) NSNumber *latitude;
@property (nullable, nonatomic, retain) NSNumber *longitude;

@end

NS_ASSUME_NONNULL_END

Location+CoreDataProperties.m

#import "Location+CoreDataProperties.h"

@implementation Location (CoreDataProperties)

@dynamic altitude;
@dynamic latitude;
@dynamic longitude;

@end

So looks like properties in categories might work now. I haven't tested on non-Core Data classes.

What I've noticed is that they do include the category file back into the original class:

Location.h

@interface Location : NSManagedObject

@end

#import "Location+CoreDataProperties.h"

This allows the original class to edit the properties specified by the category.

setting new properties in category interface/implementation

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