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Teacher Liang's small class

2020-11-08 23:53:12 Official account Song Hua said

Among many design patterns , I think the template method pattern is easy to understand , It's also a skill worth studying in depth . The definition is as follows , In a multi-step business framework , Most of the steps are fixed , And it can be used in a variety of business scenarios , Variable steps are left to subclasses to implement independently , So we separate stability from change .


Using this model is a little bit careless , There will be some strange problems . The first is that multiple abstract methods modify the same variable , There is a strong correlation between methods , The second problem is that there are too many abstract methods defined . The latter is exactly what I want to talk to you about today , Follow my step and look down .


One 、 Why does this happen ?


I think so , The product manager told us from a business point of view , What steps does this function involve , What needs to be done at each step , Then we copy the concept and turn it into code , Considering that some steps involve multiple implementations , So there are a lot of extension methods left .


soon , New businesses need to reuse these processes , Steps required 1\2\3, But it doesn't take steps 4\5, I can't even understand the steps 4\5 What does it do , I don't know what conditions will be called , The original design is not simple enough .


How to understand the simple here ? Simplicity is from a human point of view . If , The definer and implementer come from two different teams , After the implementer has read the document, the definer needs to explain , To know how to implement abstract methods is right , So it's complex . This is my real feeling , Before that, I was doing capacity building for China Taiwan general purpose , When external empowerment is performed through extension points , The most important thing to think about is how to make the third party understand and start quickly .


To sum up , Copy requirements documents mechanically , Let the execution order be fully reflected in the code structure , It will lead to too many abstract methods in the template method pattern .


Two 、 What to do if something happens ?


What to do if there are too many abstract methods ? In fact, we can use the interface adapter mode to patch .


The actual practice is like this , Create an abstract class Wrapper, Implement all the methods , Method implementation does not require specific business implications . When we write a concrete implementation class , Inherit Wrapper class , Rewrite the methods it cares about , So you don't have to implement steps that you don't understand .


Using interface adapters for compatibility may not be enough , Consider the situation , Abstract methods in templates explicitly need to have return values , How to deal with it ?


It seems that we have to continue patching . such as , Define some default values , Discard these values in the business process . perhaps , Method default implementation runtime throw exception , Indicates that the operation is not supported , It has to be implemented by a third party . Neither of them is good . The first one has special logic , It's expected to be despised by many people . The second is a little more elegant , The disadvantage is that the debugging process can only find the exception .


To sum up , Using the interface adapter pattern , Let the subclass only care about the methods it needs , So the old code 、 Dirty code enlivens .


3、 ... and 、 How to avoid too many abstract methods ?


There are too many abstract methods in the template method pattern , The side shows that the process steps are complicated , Not simple enough . therefore , Another description of the problem is , How to avoid exposing too much detail when designing template method .


My advice is , Organize the business logic , Extract the same kind of behavior , Or merge methods that share a lot of information , Use a general term to summarize the link , Hide the details , Then load it in combination , Don't tile the code directly .


For example . In a request link , The intermediate method directly controls the cache clearing through the cache client 、 Cache value settings , Other links read cache values . This coding style , It will cache Key、 Cache components are exposed to third parties , This leads to difficulties in subsequent modifications . Normally, a new business class should be created , Specifically with the cache Key Dealing with .


To sum up , The key to avoiding too many abstract methods is how to avoid exposing too many details .

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