objcsetAssociatedObject

objc_setAssociatedObject is a function in the Objective-C runtime that allows developers to associate arbitrary data with an Objective-C object. This is useful for adding custom properties to existing classes without subclassing or modifying the original class definition. The function is part of the Objective-C runtime library and is declared in the objc/runtime.h header file.

The function signature for objc_setAssociatedObject is as follows:

void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy);

Here's a breakdown of the parameters:

- object: The Objective-C object to which the data will be associated.

- key: A unique key used to identify the associated data. This is typically a static variable or a unique pointer.

- value: The data to be associated with the object. This can be any Objective-C object.

- policy: The association policy, which determines the lifetime of the associated object. The available policies are OBJC_ASSOCIATION_ASSIGN, OBJC_ASSOCIATION_RETAIN_NONATOMIC, OBJC_ASSOCIATION_COPY_NONATOMIC, OBJC_ASSOCIATION_RETAIN, and OBJC_ASSOCIATION_COPY.

To retrieve the associated data, the objc_getAssociatedObject function is used. This function takes the same object and key parameters and returns the associated data.

objc_setAssociatedObject is commonly used in categories to add properties to existing classes. For example, a category on UIView might use objc_setAssociatedObject to add a custom property for storing a background color. This approach allows developers to extend the functionality of existing classes without modifying their original implementation.