How do you see what a Combine pipeline is doing: handleEvents, print, breakpoint and the order of subscription, request, output, completion and cancel?

Combine pipelines are hard to step through, so you instrument them. handleEvents lets you observe each lifecycle event without changing the stream: receiveSubscription, receiveRequest (the demand), receiveOutput, receiveCompletion and receiveCancel. In the check the order is “subscription”, then “request unlimited” (a sink asks for unlimited demand), then each “output”, and when the stored cancellable is removed the pipeline prints “cancel”; the later send(2) prints nothing, because nobody is subscribed. That tells you, for any bug, whether the pipeline was subscribed at all, whether it was cancelled early (often because the AnyCancellable was released, Q310), and what demand it asked for.

Other tools: .print("tag") logs every event to the console with a prefix (quick but noisy); .breakpointOnError() and .breakpoint(receiveOutput:) trap in the debugger on a condition; and .share() or a subject placed deliberately can make a stage inspectable. Put handleEvents between stages to bisect where a value stops, and strip them before shipping (or compile them out with #if DEBUG), because they add overhead and log noise. When a sink never fires, the usual causes are: the cancellable was not stored, the upstream never emits (a subject you forgot to send to), the stream already completed or failed, or receive(on:) is delivering later than you expect.