Observer mode vs publish subscribe mode

Preface

The name of observer mode , You must have heard about it . From our real life , Subscribe to wechat official account 、 Hospital registration and so on all belong to its practical application . In the world of programs , It is a design pattern for decoupling code , If you want to master and understand this design pattern , Today, come with me to explore its mysteries ！

Observer mode

When we talked about the event mechanism earlier , We've actually mentioned the observer model . The observer pattern actually implements a communication mechanism , That is, the mechanism by which the observation target notifies the observer . Under the premise of low coupling design , The observer does not need to directly call the internal methods or properties of the observation target to get the notification , But use “ subscribe ” To get notifications , The general flow chart is as follows ：

Combined with actual development examples , stay C# We only need to define the corresponding delegate and event , The use of observer mode , For detailed usage, please refer to my previous articles ： We can talk C# Delegate and event in , No more details here .

But in other languages , for example Java, It does not provide a mechanism like delegation or event for callback , Therefore, the implementation will be slightly cumbersome , The related class diagram is as follows ：

In this way of implementation , We need to look at the target , That is to say, the theme is abstracted , That is... In the picture Subject, Used to provide a collection of saved observers ( In the picture ArrayList), And the method of adding or deleting observers . But in the specific subject ( In the picture ConcreteSubject) in , Contains the state attribute , And the method of state change , When you change yourself , Would call notifyObserver Method to notify .

In the observer's implementation , There are also abstract observers (Observer) And specific observers (ConcreteObserver) Two characters . The former generally defines an interface , Used when a change notification is received , Update yourself . The latter will achieve concrete update Method , similar C# Callback method triggered after we subscribe to events in .

This is how we implement the observer pattern in both languages , As software engineering emphasizes “ There is no silver bullet ” That's what I'm saying , The observer model solves many problems , But there are also some disadvantages , The main contents are as follows ：

1、 When there are many observers , Notification by notification can be time consuming . As we implemented above , There is one. ArrayList To save all observers , If this ArrayList The elements in are large enough , Then the execution efficiency of the program at the time of notification is bound to be very low .

2、 If there is a circular dependency between the observation target and the observer , Observer mode triggers a loop call from both sides , Causing the system to crash .

3、 There is no mechanism to inform the observer of how the objective changes , The observer simply knows that the object of observation has changed . Of course, this is not absolute , When implementing a program, we can pass parameters or objects to “ inform ” The observer , Observe what changes have taken place in the target .

When we talk about observer mode , Repeatedly mentioned “ The theme ” and “ subscribe ” The concept of . Conceptually speaking , It seems that observer mode , It's about publishing - A subscription model , So is there any difference between the two ？ Let's move on to the next chapter to explore .

Publish subscribe mode

in fact , Publish subscribe mode , It is different from the observer mode . Back to our initial example , Subscription to WeChat official account , Seems to be paying direct attention to the official account , In essence , In the two behaviors of attention and push , It's all through “ WeChat public platform ” This is implemented by a third party . See here , Smart people should be able to understand the differences between publish and subscribe modes , you 're right , The publish subscribe mode is based on the observer mode , Joined the “ Event center ” The concept of ：

Through the event center , Publishers and subscribers are further decoupled , Publishers don't need to care about subscribers' subscriptions , Instead, you can directly publish the topic to the event center . And subscribers don't have to care about the publisher's own situation , I don't even know who the publisher is , Just receive the notification from the event center .

Publish subscribe mode is widely used in various message oriented middleware , As is common RabbitMQ,kafka etc. , The publisher publishes the topic , Consumers subscribe to the corresponding topics to consume the published messages , I believe you are not unfamiliar with these concepts .

Summary

thus , The topic we are going to explore today comes to an end for the time being . The observer pattern is so similar to the publish subscribe pattern , But it's different , After we have clarified our respective concepts and implementation methods , It will be found that these two modes are ubiquitous in our daily applications .

Design pattern itself is the precipitation and summary of some best practices and methodologies made by some excellent predecessors in the development process , So when you mention these contents , Don't assume that these things just stay in the conceptual stage , Unable to practice . But need to combine with their actual work , Think carefully about how design patterns should be applied , How to choose the right design pattern , Solve your own problems . Of course , There is never an optimal solution , Abusing design patterns will only greatly reduce the readability of the program , Make it difficult for subsequent maintenance . Active learning and flexible application , Here I wish you all can study with me , Common progress and growth , See you next time ！