How to implement observer mode

Observer mode ： When an object state sends a change , It will automatically notify the objects that depend on it . Observer model belongs to behavioral model .

Realization ： Instantiate the topic object , The observer then registers with the subject object , Traverse the registration , Let the subject object implement the operations provided by its interface , Tell the observer to do something .

Subject interface , The theme , Observer interface , The observer

An observer can have multiple observers , The subject is the observed

Topics provide interfaces for registration and notification , The observer provides an interface for his own operation .( These observers have the same interface .) The observer uses the topic interface to register... With the topic , The theme uses the observer interface to inform the observer . The coupling is pretty low .

The advantages of the observer model ：

Subject and observer are loosely coupled . A topic only knows that its observer is an instance of a class that implements the observer , But I don't know which class it is , Among observers , Only know that the subject it depends on is an instance of a class that implements the subject , But you don't need to know which class it is .

The observer mode satisfies the open close principle , For extension development , Turn off for changes . When the requirements of the application change , Without modifying the source code or binary code of the software entity , It can extend the function of the module , Make it meet new needs .

The disadvantages of the observer model ：

If an observed object has many direct and indirect observers , It will take a lot of time to inform all the observers

If there is circular dependence between the observed , The observed will trigger a circular call between them , Causing the system to crash

Although the observer pattern can let the observer know that the observed object has changed at any time , But the observer mode has no corresponding mechanism to make the observer know how the observed object changes ：

Observer mode uses scenarios ：

When the data of an object is updated, other objects need to be notified , But this object doesn't want to be tightly coupled to other objects .

When an object's data is updated , This object needs to let other objects update their own data , But this object doesn't know how many objects update data .

For example, microblog subscription , When we subscribe to someone's microblog account , This man has released new information , Will inform us , But he doesn't know or need to know how many people to notify .