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What is dependency injection (DI)

Dependency injection (DI) is essentially a dependency on a class that is automatically injected through a constructor.

Generally speaking, you are currently operating a class, but some of the methods or functions of this class can be accomplished not only by this class, but by another class.

The most direct sign is when the parameter data is passed as the object. Strictly speaking, you want to operate on another class in another class, and the two classes form an interdependent relationship, and the way to pass parameters is called injection.

Second, the reasons for the emergence of dependency injection

In the beginning, when php needs to use another class in one class, it will do the following

For example, if I need to use the adapter class in the container class, I need to instantiate it before using it.

If you need to use a large number of external classes, this will result in a high degree of coupling, which can easily lead to later maintenance difficulties.

Popularly speaking, container can not work without external classes, which is called too high coupling.

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