ReplaySubject

A variant of Subject that "replays" old values to new subscribers by emitting them when they first subscribe.

ReplaySubject has an internal buffer that will store a specified number of values that it has observed. Like Subject, ReplaySubject "observes" values by having them passed to its next method. When it observes a value, it will store that value for a time determined by the configuration of the ReplaySubject, as passed to its constructor.

When a new subscriber subscribes to the ReplaySubject instance, it will synchronously emit all values in its buffer in a First-In-First-Out (FIFO) manner. The ReplaySubject will also complete, if it has observed completion; and it will error if it has observed an error.

There are two main configuration items to be concerned with:

1. bufferSize - This will determine how many items are stored in the buffer, defaults to infinite.
2. windowTime - The amount of time to hold a value in the buffer before removing it from the buffer.

Both configurations may exist simultaneously. So if you would like to buffer a maximum of 3 values, as long as the values are less than 2 seconds old, you could do so with a new ReplaySubject(3, 2000).

Differences with BehaviorSubject

BehaviorSubject is similar to new ReplaySubject(1), with a couple of exceptions:

1. BehaviorSubject comes "primed" with a single value upon construction.
2. ReplaySubject will replay values, even after observing an error, where BehaviorSubject will not.