A class Derived can inherit from an interface Base and delegate all of its public methods to a specified object The Delegation pattern has proven to be a good alternative to implementation inheritance, and Kotlin supports it natively requiring zero boilerplate code. As another example, some of the dependency injectors for Kotlin support this model by delegating the getter to receiving a value from a registry of instances managed by the dependency injection engine.Īnd Interface/Class delegation is the other use: Here you delegate the getter/setter to another class that does the work and can contain common code. Storing properties in a map, not in separate field each.
Observable properties: listeners get notified about changes to this property, Examples include lazy properties: the value gets computed only upon first access, There are certain common kinds of properties, that, though we can implement them manually every time we need them, would be very nice to implement once and for all, and put into a library.
In the Kotlin reference you will find two uses for by, the first being Delegated Properties which is the use you have above: I'll implement SomeOtherInterface by myself (that's implicit, so no by there).' 'I am class M圜lass and I offer functions of interface SomeInterface which are provided by SomeImplementation. class M圜lass: SomeInterface by SomeImplementation, SomeOtherInterface In the class definition, it follows the same principle, it defines where some function is provided, but it can refer to any set of methods/properties, not just get and set. One very common example is the by lazy for lazy loading properties.Īlso, if you are using dependency injection library like Koin, you'll see many properties defined like this: var myRepository: MyRepository by inject() //inject is a function from Koin So, instead of using this built-in get and set methods, you are delegating that job to some explicit function. It's provided by the function that comes after by. For each var property there is a default provider of get and set methods that we don't need to specify explicitly.īut, when using by keyword, you are stating that this getter/getter&setter is provided elsewhere (i.e. In simple words, you can understand by keyword as provided by.įrom the perspective of property consumer, val is something that has getter (get) and var is something that has getter and setter (get, set).
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