When the function that returns a Future value completes successfully (calls back) signaled in the code by then, a callback function handleValue is executed that receives the value returned. If an error condition took place, the callback handleError handles it. Let's say this function is getFuture(), with Future as the result and a return value of type T, then this becomes equivalent to the following code:

Future<T> future = getFuture();
future.then(handleValue)
.catchError(handleError);

handleValue(val) {
  // processing the value
}

handleError(err) {
  // handling the error
}

The highlighted code is sometimes also written as follows, only to make the return...