Objective-c adding "attributes" to a category--associative objects

Add a "property" to a category

We know that you can add methods to a category, but you can't add properties . Do we have another way to do that?
Let's take a look at the following code:

@interface UIView (nl_Frame)@property (nonatomicassignCGFloat nl_width;@end

  @implementation  uiview  (nl_ Frame ) -(void ) Setnl_width: (cgfloat ) nl_width {cgrect  frame = self  .frame ;  Frame.size  .width  = nl_width; self  .frame  = frame;} -(cgfloat ) nl_width {return  cgrectgetwidth ( Span class= "Hljs-keyword" >self  .frame );}  @end   

This adds a width "attribute" to UIView: Nl_width, and implements the appropriate getter and setter methods ( nl_width ,) for it setNl_width: . These two methods actually access the frame "Properties", why give a double quote here also? Yes, it frame is also defined in the category:

@interface UIView(UIViewGeometry)@property(nonatomic) CGRect            frame;//...@end

As you can see, the "attribute" of this definition in the classification actually implements the corresponding method and achieves the purpose by accessing other properties in the method. This is often used to simplify certain operations, such as defining our classification, to get the width of the view as long as view.nl_width it is available, not CGRectGetWidth(view.frame) to get the width, and to improve readability.

Let's take a look at this requirement: in SQLite, the first table will define a primary key by default if it is not given a primary key rowid . Let's put this rowid directly NSObject inside, as a property, then any object will have this primary key rowid . But this rowid is not the same as the above nl_width . By accessing other properties to achieve the goal. What should I do then?

Associating objects

The protagonist of this section has appeared: related objects
Before you can use the associated object, you have to introduce the header file:

#import <objc/runtime.h>

You can find three functions in this header file that allow you to associate any key value to an object at run time:

objc_setAssociatedObject       // 设置关联对象objc_getAssociatedObject       // 获取关联对象objc_removeAssociatedObjects   // 移除关联对象

Now that we have this tool, let's look at it again:

 @interface  nsobject  (nl_ SQLite )   @property  (nonatomic , Span class= "Hljs-keyword" >assign ) Nsuinteger rowid;  @end   

 @implementation  nsobject  ( Nl_sqlite )  static  void  *nl_  Sqlite_rowid_key = &nl_sqlite_rowid_key;-(void ) Setrowid: (NSUInteger) rowID { Objc_setassociatedobject (self , Nl_sqlite_rowid_key, @ (ROWID), Objc_association_ retain_nonatomic);} -(Nsuinteger) rowid {return  [Objc_getassociatedobject ( self , Nl_sqlite_rowid_key) unsignedlongvalue];}  @end   

The above code NSObject adds an "attribute" to the associated object rowid . When the associated object is in use, we need to provide a pointer, that is key , to identify the object being associated. What we have here key is a null pointer: nl_sqlite_rowid_key . Of course, you can also @selector(rowid) come as key (commonly used).
So, you can use it this way:

id person = [NSObject new];person.rowid1;

It's cool! You can add "properties" to an existing class in the future. This is a very powerful feature yo, if you look at some of the powerful third party libraries, you will find that this is a common technique.

Why classification Cannot add attributes

Pending write

Fundamentals of Associated objects

Pending write

Objective-c adding "attributes" to a category--associative objects