The Concat operator allows you to combine the output of multiple sequences of the same type into a single sequence. Concat subscribes to each sequence in turn, emits all the values from that sequence until successful termination, and then subscribes to the next sequence. Each sequence is guaranteed to only start after the previous one terminates.

The resulting sequence will terminate successfully when the last source sequence has terminated successfully, or exceptionally as soon as any sequence produces an error.

Higher-order operator

Concat also works as a higher-order operator, so it can take as input a sequence of observable sequences. In this case, it will subscribe to the first source sequence and start passing along all emitted values. As soon as that sequence terminates, it will subscribe to the next received sequence, either immediately if it arrived before termination of the first sequence, or as soon as a new observable sequence is emitted.

The higher-order variant is useful to queue execution of long-running operations, for example to sequence logic states in a task, or merging video files in a folder sequentially into a single frame sequence.