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Extension functions and extension properties

1. Extension function

Extension functions are used to add a new behavior to a class, which is a way to extend classes that lack useful functions. The purpose of the extension function is similar to the static tool method implemented in Java. One of the advantages of using extension functions in Kotlin is that we do not need to pass in the whole object as a parameter when calling the method. The extension function behaves as if it belongs to the class itself. We can use the this keyword and call all its public methods directly.

Extension functions do not allow you to break its encapsulation, and unlike methods defined within a class, extension functions cannot access private or protected members

Declaration of extension function

After that, we can call the extension function directly, just as we call the method declared inside the class itself.

2. Extended attributes

Extension functions can also be used for properties

3. Non-rewritable extension function

Looking at the following example, the subclass Button overrides the click () function of the parent class View. If you declare a View variable and assign it to an object of type Button, the called click () function will be the method overridden by the Button class.

In addition, if the member function of a class has the same signature as the extension function, the member function will be used first

The extension function does not really modify the original class, its underlying layer is actually implemented in the form of static import. Extension functions can be declared in any file, so it is a general practice to put a series of related functions in a new file

It should be noted that extension functions do not automatically take effect throughout the project. If you need to use extension functions, you need to import them.

4. Empty receiver

An extension can be defined for a nullable receiver type, even if the recipient is null, so that the developer does not have to make a null operation before calling the extension function, and the receiver can be checked by this = = null.

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