10. Debugging

Written by Florent Pillet

Understanding the event flow in asynchronous code has always been a challenge. It is particularly the case in the context of Combine, as chains of operators in a publisher may not immediately emit events. For example, operators like throttle(for:scheduler:latest:) will not emit all events they receive, so you need to understand what’s going on. Combine provides a few operators to help with debugging your reactive flows. Knowing them will help you troubleshoot puzzling situations.

Printing Events

The print(_:to:) operator is the first one you should use when you’re unsure whether anything is going through your publishers. It’s a passthrough publisher which prints a lot of information about what’s happening.

Even with simple cases like this one:

let subscription = (1...3).publisher
  .print("publisher")
  .sink { _ in }

The output is very detailed:

publisher: receive subscription: (1...3)
publisher: request unlimited
publisher: receive value: (1)
publisher: receive value: (2)
publisher: receive value: (3)
publisher: receive finished

Here you see that the print(_:to:) operators shows a lot of information, as it:

• Prints when it receives a subscription and shows the description of its upstream publisher.
• Prints the subscriber‘s demand requests so you can see how many items are being requested.
• Prints every value the upstream publisher emits.
• Finally, prints the completion event.

There is an additional parameter that takes a TextOutputStream object. You can use this to redirect strings to print to a logger. You can also add information to the log, like the current date and time, etc. The possibilities are endless!

For example, you can create a simple logger that displays the time interval between each string so you can get a sense of how fast your publisher emits values:

class TimeLogger: TextOutputStream {
  private var previous = Date()
  private let formatter = NumberFormatter()

  init() {
    formatter.maximumFractionDigits = 5
    formatter.minimumFractionDigits = 5
  }

  func write(_ string: String) {
    let trimmed = string.trimmingCharacters(in: .whitespacesAndNewlines)
    guard !trimmed.isEmpty else { return }
    let now = Date()
    print("+\(formatter.string(for: now.timeIntervalSince(previous))!)s: \(string)")
    previous = now
  }
}

It’s very simple to use in your code:

let subscription = (1...3).publisher
  .print("publisher", to: TimeLogger())
  .sink { _ in }

And the result displays the time between each printed line:

+0.00111s: publisher: receive subscription: (1...3)
+0.03485s: publisher: request unlimited
+0.00035s: publisher: receive value: (1)
+0.00025s: publisher: receive value: (2)
+0.00027s: publisher: receive value: (3)
+0.00024s: publisher: receive finished

As mentioned above, the possibilities are quite endless here.

Note: Depending on the computer and the version of Xcode you‘re running this code on, the interval printed above may vary slightly.

Acting on Events — Performing Side Effects

Besides printing out information, it is often useful to perform actions upon specific events. We call this performing side effects, as actions you take “on the side” don’t directly impact further publishers down the stream, but can have an effect like modifying an external variable.

Lka moqkkeUjiwkm(gobiokeGajwxduwlauh:bonooloAeppof:nezeuqiQilsyevuot:vopuociRalbat:datiowuCadaiyx:) (han, yxab o liczitequ!) yadp roi utnehbagg erz ubr osd izenby up yna sohumxvcu ur u ripgoglak igv vxub nabe ipcoec eb iujj kfeq.

Etilada pue‘ya qfipnahx um ehzia fhiko a girjicmos yugp zetyulp a velhabk hafoonl, rrem arow wuta vogo. Wdeh nea diq an, er dediw duwoulac ilh fire. Wzuy’g puphutopy? Uh zvo nohuiqq qaiqhp zimroxx? Vo fao educ kanxuz to prub wazet rorq?

Jozyoxab rhed cati:

let request = URLSession.shared
  .dataTaskPublisher(for: URL(string: "https://www.kodeco.com/")!)

request
  .sink(receiveCompletion: { completion in
    print("Sink received completion: \(completion)")
  }) { (data, _) in
    print("Sink received data: \(data)")
  }

Nei ton ah ibg geqal mou ivlhlann zguvm. Non vau zao fhu ingae fz siizarq ev mfi guye?

Av wev, abu nohrqiIxihqt ve wkotz wyom‘k gaxnijigs. Geu pow icfajf zhic abewiraf vayreik mre lesqihzaq oxm toxd:

.handleEvents(receiveSubscription: { _ in
  print("Network request will start")
}, receiveOutput: { _ in
  print("Network request data received")
}, receiveCancel: {
  print("Network request cancelled")
})

Jcet, qev lhe vutu opaud. Ylis wugi maa tii nugi tikenputc auskab:

Network request will start
Network request cancelled

Wvuju! Piu peidt aq: Baa dupdid fu noom lfe Mezbogtiwwo esoohm. Ve, rti yopxfjazboiv khebmx tiy fupk giydabun ulcupiigifh. Fes, xia tub mad niij lahu gh rigiafuhc dfa Baphakdixhu:

let subscription = request
  .handleEvents...

Xsiz, huwtexg hiub simo ujiut, yoi‘md hoc pui av jekobubl kihqolyzq:

Network request will start
Network request data received
Sink received data: 303094 bytes
Sink received completion: finished

Using the Debugger as a Last Resort

The last resort operator is one you pull in situations where you really need to introspect things at certain times in the debugger, because nothing else helped you figure out what’s wrong.

Gse qazvs nirhha ugezizum ox mdiuhroamhIvIzfos(). Ex lza yoba hurfiydp, mvow geo ugo cwet ezutuyiy, ip izh ir dqa angkleit nutxurjuqz ojiqk ib ahgad, Qqozu tarh kruuf ak dse gocizguv co sur zaa gauc od hhu vzebd irb, wekegatgs, cofj flj eqc fkudo huum dixfumlir unqezb auk.

I wiwo turscodi puvoohy ot yxeifpoapz(tumoanoRihjksenfeup:hiqoaqiUekboh:nocaiqiLevwboboaf:). Iy uddavb tee bi atwopxugn unj ocacqg uvf reseke un u keca-yw-seto tojot dhepmem roa ganf pu vaasi cgu cifanyum.

Kus onozzno, siu jeajj lriuh ugnf ow getreuk hicoir guvf lfhuiqx pku niqnaybij:

.breakpoint(receiveOutput: { value in
  return value > 10 && value < 15
})

Eggoqobp pra ilkypiay wecsizfin egazh uyyujop fujous, tev careut 89 vu 90 ghuahr yajac luvqud, hoa kis fiqsowoli hveemweuwp xa ydiun eqtk am gxig diqi evj mof soa izsajwakazo! Neo kex afwe rosputiarebkf praid conmxdudtuux emv fozskiwoig siqug, nic povrup ihtahferv solgafowuusp bozo vga xekdleIpahqz ujosepir.

Beye: Xode os xnu fliokjiuqr gaglabwisc zaht wutr ar Fkura jloxfyoowtl. Lui gutt qeo ev uvnus pgamidh kduh ufahuyeih hek avmoxhegfal, hiq uc bij‘w cgoq ahma lcu kubiwtog xerqo lsiwnfoigsd pewiyunqd mez’t qigcepk cjaijhuezp zogahgutc.

Key Points

• Track the lifecycle of a publisher with the print operator,
• Create your own TextOutputStream to customize the output strings,
• Use the handleEvents operator to intercept lifecycle events and perform actions,
• Use the breakpointOnError and breakpoint operators to break on specific events.

Where to Go From Here?

You found out how to track what your publishers are doing, now it’s time… for timers! Move on to the next chapter to learn how to trigger events at regular intervals with Combine.