A completable trait will either only emit a completed event, or it will emit an error event containing an error. It cannot emit next events. It also does not replay its completed or error events. We can convert a raw Observable sequence to Completable by calling asCompletable() on it. We will use a Completable when we only care if some operation has completed, such as a file read or some other complex operation that can fail.
The completable trait
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