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Hello.

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In this video, we're going to talk about the topic 41.

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What is the decorator design pattern?

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The character is a design partner that dynamically adds extra functionality to an existing object without

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affecting the behavior of other objects from the same glass.

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Let's start with something simple here we have a class that reads information about people from some

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data source.

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This class is nice, simple and focused.

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At some point, we are asked to add an optional feature of logging.

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Human elements have been read that's added to this class.

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It was such a pretty class, and now it grew large and ugly.

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Well, never mind.

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At least it does what it's supposed to do.

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Soon after that change, we are asked to add one more optional feature to be able to limit data to some

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given count of people.

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Let's try to add this.

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This cult is terrible now.

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This class has a big chunks of logic which will or will not be executed depending on the flags.

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Its logic, so simple before, is no mercy and complex.

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It takes more parameters than it may need.

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For example, we want Mick Gallagher if logging is not enabled and we won't need the account to meet

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if the meeting is not enabled, it would be a nightmare to test this class as more and more extra features

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are required to be added to this class.

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It will keep growing, becoming an unmanageable mess that no one wants to work with.

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It's time to introduce the decorator design pattern.

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This pattern allows adding some behaviour to an object dynamically without touching its gold.

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If you know the open closed principle from solid, you know it's a good thing.

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It also allows us to keep the single responsibility principle happy first.

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That's reverted this class to how it was before changes.

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Beautiful in its simplicity.

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Now let's add a logging decorator across it will decorate the people, decorate their houses.

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The ability of logging, implementing the decorate or design pattern boils down to two steps making

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the decorator implement the same interface as the decorated object and making the decorator own an object

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implementing this interface.

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It will be the creative class itself or another decorator, which allows us to compose many decorators

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together.

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Let's see this in cold.

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As you can see, the decorator owns an object that it wants to decorate, it implements the same interface

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in the real method that comes from this interface.

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It goes well, whatever implementation is provided, but it adds only to something from itself.

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In this case, it writes to a dog.

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Remember that the decorated reader doesn't need to be the plain people.

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They carry their object.

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It can be anything implementing the AI people data reader interface, including unaware decorator.

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Of course, at some point, one of the decorators in the structure must own the basic decorated object

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of people data reader type.

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Now, let's add the decorator that will be limiting the count of returned person objects.

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Great.

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We can now compose those decorators to our liking.

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Let's create an object that reads people data, logs the original count and then limits it.

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Here, the real magic happens, each decorator takes an object implementing the AI people data reader

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interface as a parameter, but also implements this interface itself.

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It means we can pass a decorator as a parameter to other decorator stacking their functionalities.

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That's why this final object will be able to both log and limit the count of elements.

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Here you can see that the count has been limited and here that originally five elements has been right.

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Please be aware that the order of the decorators creation matters if we turn to this cold to this.

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The result will be different because the limiting the Reid method was executed before the logging decorators

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with method.

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From the point of view of the logging decorator, the amount of data will be free, not five as before,

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because the data was limited before it was given to the logging decorator.

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The features of logging and limiting are optional, but which the decorator pattern?

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It's easy to choose what we need.

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Let's create a people data reader that only logs some information, but it does not lead me to the count.

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Now, in the result, we see all five elements.

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In other words, the result was not limited because here we create only the logging the crater.

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And now let's create an object that does not log, but it does limited the data.

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As you can see, there is no log string in the result, which we have been seeing before.

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All right.

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As you can see, the decorator pattern allows us to easily art functionality to objects without touching

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the original glasses.

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So it's very much in line with the open closed principle.

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It also helps us to be in line with the single responsibility principle.

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As each class now has a very focused responsibility, the reader class only reads the data.

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The logging the great or simply adds the logging feature and the limiting the greater the data, those

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gases would be easy to test, maintain and generally pleasant to work with.

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During the interview, you can be asked What are the benefits of using the decorator or design pattern?

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The decorator pattern allows us to easily out functionality to objects without touching the original

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glasses.

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It allows us to keep glasses simple.

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It makes it easy to tuck functionalities together, building complex objects from simple glasses.

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It helps us to be in line with both open closed principle and single responsibility principle.

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The glasses extended with the decorating design pattern would be easy to test and maintain.

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Oh right.

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I hope you liked this pattern.

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Thank you for your attention, and I'll see you in the next video.