Introduction: Coroutines Under the Hood

A certain kind of person cannot accept that a car can just be driven. They need to open the car's hood to understand how it works. This chapter is purely explanatory. It tries to explain to a satisfactory level how coroutines work. The key lessons are:

• Suspending functions are like state machines, with a possible state at the beginning of the function and after each suspending function call.

• The number identifying the state and the local data is kept in the continuation object.

• Continuation of a function decorates a continuation of its caller function; as a result, all these continuations represent a call stack used when we resume or a resumed function completes.

If we want to learn some internals (simplified, of course), read on.

We could have implemented suspending functions in a few ways, but the Kotlin team decided on an option called the continuation-passing style. This means that continuations (explained in the previous lessons) are passed from function to function as arguments. By convention, a continuation takes the last parameter position. Let's have a look. ...