#warning Do not override methods in categories

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In iOS development, category is a very useful thing. (Pure text, short and concise)

We can add methods and properties to a class. For example, NSDateFormatter has a very large overhead of creating a new one every time, which is particularly evident in tableview. With category, we can write a singleton for it to avoid opening up new memory for it every time.

So, we thought, what would happen if we used category to override the method of this class? It was actually feasible and it was indeed overwritten... However, as the future flowers of our motherland, we cannot do this!

Here are the reasons:

1. Category cannot replace subclasses. It cannot call the parent class's method implementation through super like a subclass. If a category overrides a method in the current class, the original method implementation in the current class will never be executed, which is fatal in some methods. (ps: Here, the +(void)load method is a special case. It will be executed in the category after the current class is executed.)

2. At the same time, a category cannot reliably override the same method of the same class in another category. For example, if UIViewController+A and UIViewController+B both override viewDidLoad, we cannot control who overrides whom.

3. By observing the header files, we can find that many classes in the Cocoa framework implement functions through categories. You may inadvertently overwrite one of these methods, which may sometimes cause some unresolved exceptions.

4. The creation of category is just to make it easier for developers to expand a class. Its original intention is not to let you change a class.

in conclusion:

To override methods, of course we first recommend overriding the parent class method through the subclass. In some cases where it is inconvenient to override, we can also use runtime to perform method swizzling in the category to achieve it.