TakeUntil modifies the source sequence so that values are emitted only until the second sequence produces a value. At that time, TakeUntil will terminate immediately and ignore the remainder of the sequence. TakeUntil is often used to create a dynamic stop condition for an infinite sequence, e.g. grab frames from a video camera until a key is pressed.

If the source sequence terminates before the second sequence produces a value, TakeUntil will also terminate and cancel the subscription to the second sequence.