In simple words, you can understand by keyword as provided by.
From the perspective of property consumer, val is something that has getter (get) and var is something that has getter and setter (get, set). For each var property there is a default provider of get and set methods that we don’t need to specify explicitly.
But, when using by keyword, you are stating that this getter/getter&setter is provided elsewhere (i.e. it’s been delegated). It’s provided by the function that comes after by.
So, instead of using this built-in get and set methods, you are delegating that job to some explicit function.
One very common example is the by lazy for lazy loading properties.
Also, 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
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.
class MyClass: SomeInterface by SomeImplementation, SomeOtherInterface
This code is saying:
‘I am class MyClass and I offer functions of interface SomeInterface which are provided by SomeImplementation.
I’ll implement SomeOtherInterface by myself (that’s implicit, so no by there).’