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I have a situation where my code needs to make one network call to fetch a bunch of items, but while waiting for those to come down, another network call might fetch an update to those items. I'd love to be able to enqueue those secondary results until the first one has finished. Is there a way to accomplish that with Combine?

Importantly, I am not able to wait before making the second request. It’s actually a connection to a websocket that gets made at the same time as the first request, and the updates come over the websocket outside of my control.

After examining Matt’s thorough book on Combine, I settled on .prepend(). But as Matt warned me in the comments, .prepend() doesn’t even subscribe to the other publisher until after the first one completes. This means I miss any signals sent prior to that. What I need is a Subject that enqueues values, but perhaps that’s not so hard to make. Anyway, this is where I got:

Initially I was going to use .append(), but I realized with .prepend() I could avoid keeping a reference to one of the publishers. So here’s a simplified version of what I’ve got. There might be syntax errors in this, as I’ve whittled it down from my (employer’s) code.

There’s the ItemFeed, which handles fetching a list of items and simultaneously handling item update events. The latter can arrive before the initial list of items, and thus must be sequenced via Combine to arrive after it. I attempt to do this by prepending the initial items source to the update PassthroughSubject.

Below that is an XCTestCase that simulates a lengthy initial item load, and adds an update before that load can complete. It attempts to subscribe to changes to the list of items, and tries to test that the first update is the initial 63 items, and the subsequent update is for 64 items (in this case, “update” results in adding an item).

Unfortunately, while the initial list is published, the update never arrives. I also tried removing the .output(at:) operators, but the two sinks are only called once.

After the test case sets up the delayed “fetch,” and subscribes to changes in feed.items, it calls feed.handleItemUpatedEvent. This calls ItemFeed.updateItems.send(_:), but unfortunately that is lost to oblivion.