Concept

In Combine, publishers can emit errors during execution.

When an error occurs, the publisher terminates the stream and sends a failure completion to the subscriber.

The Combine pipeline therefore has two possible completion states:

.finished
.failure(Error)

Error handling allows developers to intercept failures, recover from them, or replace them with fallback values.

Failure Types in Combine

Every publisher has two associated types:

Publisher<Output, Failure>

Where:

Output  → The type of values emitted
Failure → The type of error the publisher can produce

Example:

PassthroughSubject<String, Never>

This publisher:

emits String values
never produces errors

Throwing Errors with tryMap

Operators such as map cannot throw errors.

To allow throwing errors inside the pipeline, Combine provides: