如何在对象内使用 objc_setAssociatedObject/objc_getAssociatedObject？

发布于 2024-09-01 23:03:47 字数 678 浏览 6 评论 0原文

如果我在类别实现中使用 objc_setAssociatedObject/objc_getAssociatedObject 在 setter 方法中存储模拟实例变量，那么我将如何访问 getter 方法中的键，因为 setter 方法中声明的任何变量都将超出 getter 方法的范围？

编辑：澄清一下，如果我要使用以下模式，我应该在哪里声明 STRING_KEY 以便我可以在 setter 和 getter 方法中使用它。

@interface NSView (simulateVar)
-(void)setSimualtedString:(NSString *)myString;
-(NSString *)simulatedString;
@end

@implementation NSView (simulateVar)

-(void)setSimualtedString: (NSString *)myString
{
    objc_setAssociatedObject(self, &STRING_KEY, myString, OBJC_ASSOCIATION_RETAIN);
}

-(NSString *)simulatedString
{
    return (NSString *)objc_getAssociatedObject(self, &STRING_KEY);
}

@end

原文

If I use objc_setAssociatedObject/objc_getAssociatedObject inside a category implementation to store a simulated instance variable in a setter method, how would I access the key in the getter method since any variables declared in the setter method would be outside the scope of the getter method?

Edit: To clarify, if I were to use the following pattern, where should I declare STRING_KEY so that I could use it in both the setter and the getter method.

@interface NSView (simulateVar)
-(void)setSimualtedString:(NSString *)myString;
-(NSString *)simulatedString;
@end

@implementation NSView (simulateVar)

-(void)setSimualtedString: (NSString *)myString
{
    objc_setAssociatedObject(self, &STRING_KEY, myString, OBJC_ASSOCIATION_RETAIN);
}

-(NSString *)simulatedString
{
    return (NSString *)objc_getAssociatedObject(self, &STRING_KEY);
}

@end

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热血少△年 2024-09-08 23:03:47

声明一个静态变量，以便您可以使用其地址作为键。
对 objc_setAssociatedObject 的调用采用 void* 并且实际上只使用静态变量的地址，而不是 NSString 的内容......这只是浪费内存。

您只需要添加：

static char STRING_KEY; // global 0 initialization is fine here, no 
                        // need to change it since the value of the
                        // variable is not used, just the address

Declare a static variable so that you can use its address as the key.
The call to objc_setAssociatedObject takes a void* and only the address of your static variable is actually used, not the contents of a NSString... that is only wasting memory.

You just need to add:

static char STRING_KEY; // global 0 initialization is fine here, no 
                        // need to change it since the value of the
                        // variable is not used, just the address

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尐籹人 2024-09-08 23:03:47

我知道这个问题已经过时了，但我认为为了完整性，还有另一种方法如何使用值得一提的关联对象。该解决方案利用@selector，因此不需要任何额外的变量或常量。

@interface NSObject (CategoryWithProperty)

@property (nonatomic, strong) NSObject *property;

@end

@implementation NSObject (CategoryWithProperty)

- (NSObject *)property {
    return objc_getAssociatedObject(self, @selector(property));
}

- (void)setProperty:(NSObject *)value {
    objc_setAssociatedObject(self, @selector(property), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

（灵感来自http://www.tuaw.com/2013/ 04/10/devjuice-better-objective-c-linked-objects/)

I know that this question is quit old but I think for completeness there is another way how to use associated objects worth mentioning. This solution utilizes @selector and therefore there is no need for any extra variable or constant.

@interface NSObject (CategoryWithProperty)

@property (nonatomic, strong) NSObject *property;

@end

@implementation NSObject (CategoryWithProperty)

- (NSObject *)property {
    return objc_getAssociatedObject(self, @selector(property));
}

- (void)setProperty:(NSObject *)value {
    objc_setAssociatedObject(self, @selector(property), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

(inspired by http://www.tuaw.com/2013/04/10/devjuice-better-objective-c-associated-objects/)

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凹づ凸ル 2024-09-08 23:03:47

对于关联的存储 void * 键，我喜欢这种方式：

static void * const kMyAssociatedStorageKey = (void*)&kMyAssociatedStorageKey;

这避免了可执行文件中存在另一个常量字符串，并且通过将其值设置为其自身的地址，您可以获得良好的唯一性，并且 < code>const 行为（因此，如果您执行会更改键值的操作，名义上您会收到编译器投诉），而没有任何额外的不必要的导出符号。

For associated storage void * keys, I like this way of doing it:

static void * const kMyAssociatedStorageKey = (void*)&kMyAssociatedStorageKey;

This avoids having another constant string in the executable, and by setting its value to the address of itself, you get good uniquing, and const behavior (so you'll nominally get a compiler complaint if you do something that would change the value of the key), without any extra unnecessary exported symbols.

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万劫不复 2024-09-08 23:03:47

在源文件的顶层声明一个静态（编译单元范围）变量。它可能有助于使其变得有意义，如下所示：

static NSString *MYSimulatedString = @"MYSimulatedString";

Declare a static (compilation unit-scope) variable at the top level of the source file. It may help to make it meaningful, something like this:

static NSString *MYSimulatedString = @"MYSimulatedString";

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娇纵 2024-09-08 23:03:47

相当接近。这是一个完整的例子。

.h-file

@interface NSObject (ExampleCategoryWithProperty)

@property (nonatomic, retain) NSArray *laserUnicorns;

@end

.m-file

#import <objc/runtime.h>

static void * LaserUnicornsPropertyKey = &LaserUnicornsPropertyKey;

@implementation NSObject (ExampleCategoryWithProperty)

- (NSArray *)laserUnicorns {
    return objc_getAssociatedObject(self, LaserUnicornsPropertyKey);
}

- (void)setLaserUnicorns:(NSArray *)unicorns {
    objc_setAssociatedObject(self, LaserUnicornsPropertyKey, unicorns, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

@end

就像普通属性 - 通过点符号访问

NSObject *myObject = [NSObject new];
myObject.laserUnicorns = @[@"dipwit", @"dipshit"];
NSLog(@"Laser unicorns: %@", myObject.laserUnicorns);

更简单的语法

或者您可以使用 @selector(nameOfGetter) 而不是创建静态指针。为什么？请参阅https://stackoverflow.com/a/16020927/202451。例子：

- (NSArray *)laserUnicorns {
    return objc_getAssociatedObject(self, @selector(laserUnicorns));
}

- (void)setLaserUnicorns:(NSArray *)unicorns {
    objc_setAssociatedObject(self, @selector(laserUnicorns), unicorns, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

Quite close. Here is a full example.

.h-file

@interface NSObject (ExampleCategoryWithProperty)

@property (nonatomic, retain) NSArray *laserUnicorns;

@end

.m-file

#import <objc/runtime.h>

static void * LaserUnicornsPropertyKey = &LaserUnicornsPropertyKey;

@implementation NSObject (ExampleCategoryWithProperty)

- (NSArray *)laserUnicorns {
    return objc_getAssociatedObject(self, LaserUnicornsPropertyKey);
}

- (void)setLaserUnicorns:(NSArray *)unicorns {
    objc_setAssociatedObject(self, LaserUnicornsPropertyKey, unicorns, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

@end

Just like a normal property - accessible with dot-notation

NSObject *myObject = [NSObject new];
myObject.laserUnicorns = @[@"dipwit", @"dipshit"];
NSLog(@"Laser unicorns: %@", myObject.laserUnicorns);

Easier syntax

Alternatively you could use @selector(nameOfGetter) instead of creating a static pointer. Why? See https://stackoverflow.com/a/16020927/202451. Example:

- (NSArray *)laserUnicorns {
    return objc_getAssociatedObject(self, @selector(laserUnicorns));
}

- (void)setLaserUnicorns:(NSArray *)unicorns {
    objc_setAssociatedObject(self, @selector(laserUnicorns), unicorns, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

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余罪 2024-09-08 23:03:47

接受的答案已经正确，但我想添加一个解释：“关键”的要点是获得一个唯一的（每个进程/程序）标识符，不会有任何意外冲突。

想象一下密钥被声明为 NSUInteger：很多人会使用 0、1 或 42 等标准值。特别是如果您正在使用某些库或框架，则很可能会发生冲突。

因此，一些聪明的苹果工程师想到了将键声明为指针，目的是声明一个变量并将指向该变量的指针作为键传递。链接器将为该变量分配一个在您的应用程序中唯一的地址，因此保证您获得唯一的、无冲突的值。

事实上，您可以传递任何您喜欢的值，指针不会取消引用（我已经测试过）。因此，传递 (void *)0 或 (void *)42 作为密钥确实有效，尽管这不是一个好主意（所以请不要这样做）。对于 objc_set/getAssociatedObject 来说，重要的是作为键传递的值在您的进程/程序中是唯一的。