Groups the elements from the source stream by using a key selector, and maps each of these groups by using a map function.
stream: Input stream
keySelector: Function that specifies the key for each element in
streamSelector: Function to apply to each resulting group
A function that accepts a key and returns a stream for the group of that key.
A stream with the list of active keys
A more typical list example. The list component can bind the list of keys while the item component binds the stream for each item, eliminating unnecessary renders: