Groups the elements of an observable sequence according to a specified key selector function and comparer and selects the resulting elements by using a specified function.
Function): A function to extract the key for each element.
Function): A function to map each source element to an element in an observable group.
Function): A function to signal the expiration of a group.
Any): Used to compare objects. If not specified, the default comparer is used.
Observable): A sequence of observable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.
If a group's lifetime expires, a new group with the same key value can be created once an element with such a key value is encoutered.