This is Understanding Combine, written by Matt Neuburg. It is a work in progress. Corrections and suggestions are greatly appreciated (you can comment here). So are donations; please consider keeping me going by funding this work at Or buy my books: the current editions are iOS 13 Programming Fundamentals with Swift and Programming iOS 13. Thank you!


.breakpoint (Publishers.Breakpoint) helps to solve a curious problem that can arise when you’re trying to debug a pipeline: pausing at a breakpoint. The difficulty is that your pipeline is usually just one statement; there’s nowhere to pause within it by normal means. If an operator takes a function, you can pause in that function; but in an asynchronous world, that might not be much help. .breakpoint lets you pause at the operator itself, within the pipeline chain.

It takes three parameters, all of them functions, all of them optional, similar to the first three parameters of .handleEvents:

  • receiveSubscription: — takes a Subscription

  • receiveOutput: — takes a value from upstream

  • receiveCompletion: — takes a completion (.failure or .finished)

These functions return a Bool. If you return true from one of them, Xcode will pause in the debugger when we reach this operator. The function is a function, so you get to decide in real time whether or not to pause. You can also print to the console before returning.

However, you’re not actually pausing at a breakpoint per se. You’re pausing in LLDB, but the context is not useful; it will appear as assembler. There are no frame variables. There is no execution pointer in your source code. The only thing you can do after pausing is resume (by clicking the Continue button, or saying continue at the debugger console). Nevertheless, by a judicious use of printing and pausing, you can use .breakpoint calls to step through your pipeline and trace what it does.

.breakpointOnError is a simpler form of .breakpoint; it doesn’t take a function, and it pauses only when a failure comes down from upstream.

WARNING: Do not ship code containing .breakpoint calls! Your app will crash if a .breakpoint operator is encountered.

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