OC-Category

The main purpose of a category is to prevent a class with many methods from being too long
It is possible to spread the methods in a class across multiple categories
A category belonging to a class can be seen as grouping the methods of a class

Usage

@interface class name (category name)
method declaration
@end
@implementation class name (category name)
method definition
@end
The main purpose is toDisperse many class methods in multiple categories Declare all categories in .h file but define different categories in different .m files

About adding a property declaration @property to a category

Attention!Variables cannot be directly declared in the declaration of the category, only by writing @property int a; then in the normal state @preperty will automatically define the set and get methods, but if the @property is declared in the category, it will not be automatically defined in the definitionCreate and cannot write @synthesize in the category definition, you need to manually implement it yourself because it prevents random access to instance variables defined in different files of the same class

About overriding the same method in multiple categories

The last edit (for example, adding a space to the method of this category) is the method in which category is to call the method in which category. The overridden method must be able to overwrite the original method (that is, in thethe one defined in the class)

A class extension is a category without a name

Class extension can declare instance variables but class extension can only declare
The implementation must only be in the main implementation
#interface A()//There is nothing to write in parentheses
{
int a;
}
-()say;

Linked references

Multiple references can be added to an object
Must use certain non-changing addresses as keys such as static local variables

import<objc/runtime>

void objc-setAssociatedObject(id object,void *key,id value,objc_AssociationPolicy policy)

Add the address specified by key as the key value for the object object, associative reference with value as the value, plicy specifies the storage strategy of the associated reference
By specifying the value as nil, the association of the key can be deleted

id objc_getAssociatedObject(id object,void *key)

Returns the object associated with object with key as key If it is not associated with any object, it will return nil

Instance