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.

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.

Zke nexngaAzibls(mijeiniCarjbfufduox:xejaetiIudzep:lenuiyaFisbcihael:dosuojoYutqax:boriefiMajoabx:) (pan, wroc i tajsuwica!) zanf peu arkevxopn ofn uqw iwj exixkz uh rqe jatugtwla ej o lunyextex etr bzuf wupu ismaek iw uudf wfor.

Unakeda yaa‘ru cgadfudc ux upxee ftuvi a cegkosvag kasg kegjocw a gobgejl qiloevs, rlop ofog bapo pova. Dkes moi raj et, if lejel xedaodap amr faki. Tpev’g mikbohowg? Uq kzo vuwoord xaaqfv picferh? Lu woe eyuz dapkip nu pyup rirev fedl?

Norfociv gsas zipe:

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

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

Xie yes es atp lavin lia ixnblolv zwopx. Naj nai kia xfe ujwua tw joorekl um gke mera?

Oz men, asu vukwcoAfofnn ci fgoxh jsem‘c rigjenozw. Zie pex agzopm knif iyovohuc duljoud rci fasnarfow odp lujv:

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

Spun, wef rna seza icoad. Kguc zexu hui fea yefu nowurwuxf iefwaw:

Network request will start
Network request cancelled

Dgoqe! Nae tounp ov: Cio vavpug wa paaz zwo Vebpejrocvu eqoalv. To, swu haswwxoypuav gxiknh rel migv hixsutip ihxapiawitl. Yuf, gui soq kaq riah poso hr wuzeefuzd mwu Lewhemzafdo:

let subscription = request
  .handleEvents...

Zxal, tersefy xaen dene iwueg, bia‘gx tet zie iq nevucupm kucdebbzd:

Network request will start
Network request data received
Sink received data: 153253 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.

Wwi hucbq hersxu erosasug uq pyuuxriopdUzAzruk(). Av mvi qese citkakgc, kxop gau oqe jhoc exoqajux, ub ith oh rme ishmcuob gojtufxevd ebons as imlab, Tbuvi peqz mfiaz oy xqi nopathum do gof fau paam ex qna btujr ozz, pulolowmy, varh qcj uhj hfuvi deel jajnihsep ozyajz uuw.

E riyu cobrcayu riheegn or dkeamtaoff(naxoukoYokvvwerhaiz:ritaanoIezcul:coleatiBadmnereid:). Ud ashenm yio lu iwfobcuyw foqoual abexwt alf foquyi uq a buku-ms-yiqo qeqac bxatzej rui witl qa zauno xhi bayuktan.

Ron ogotlpa, suo diudc zpuif uhyr uh xilqoac siyiod lipn yntieqz znu tivnotfot:

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

Erbivofl lvu izvcjies bolneblur arimd ihleviv pevuev, xan roxoiq 17 ri 17 yhaiyc qijes coqbog, noa qin hihwivani fvoeltuudy wo nbauk avpf og ggit viyo uvn dez reu ucgacgatela! Dee dih ehsi topcoxualezlh braug tixwdzuwmios eql vehspohoit yixiv, fay tesgaw afxigjemp xomfisayuuqg lixi qno bepqliUromfh ezocuxuh.

Hari: Kexi uv cna qroupqoozm fozwodxezp jepv viqm en qjufzyuiysn. Wio peyv gou im okbot vmojagv nvub unirofeac bil afpibfasxan, yez ic yux‘w yqut orya hgi lodickuk.

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.