Introduction to Error Handling

Introduction to Error Handling

April 10, 2016

Project: Swift Concepts Explained

When programming, it is common to see functions that have the possibility of failing. For instance, consider the scenario where a user wants to send data over the internet, but doesn't have an internet connection. Should the application crash? Probably not. It makes more sense for the application to display a message to the user indicating that they aren't currently connected to the internet.

But how would an app developer know that a user isn't currently connected to the internet? What if the user really was connected to the internet, but just had airplane mode turned on?

Using the above example, we might handle errors like so:

if (errorOccurred){
  //handle error
  if (notConnectedError){
    //display message indicating that the user isn't currently connected to the internet
  } else if (airplaneModeError) {
    //prompt user to turn off airplane mode
  } else {
    //display default message that some unknown error occurred
} else {
  //continue as normal

Notice that we want to do something different for each specific error. Generally, error handling is characterized by detecting when an error occurred, determining the specific error, and taking the appropriate action depending on the specific error.

Error Handling in Swift

In Swift, when an error occurs in a function, the function is said to have thrown an error. Functions that can throw errors are called throwing function.

Defining Throwing Functions

We can indicate that a function has the possibility of throwing an error by using the throws keyword as follows:

func sendDataAcrossInternet() throws -> Int {
  //do something

We can define the types of errors that a function can throw by creating an enum that conforms to the ErrorType protocol. So let's say we wanted to define some errors that could be thrown by the sendDataAcrossInternet() function, we would do something similar to the following:

enum InternetError: ErrorType {
  case NotConnected
  case AirplaneModeEnabled

We can now define our sendDataAcrossInternet() function to throw specific errors like so:

func sendDataAcrossInternet() throws -> Int {
  if (userNotConnectedToInternet){
    throw InternetError.NotConnected
  } else if (userInAirplaneMode){
    throw InternetError.AirplaneModeEnabled

Calling Throwing Functions Using Do-Catch

Because throwing functions have the possibility of throwing an error, we have to call them in a special way.

Using what is called a do-catch block we can call a throwing function and handle all the possible errors that a specific throwing function can throw. For instance, to call the sendDataAcrossInternet() method, we would do the following:

do {
  try sendDataAcrossInternet()
  print("Data sent successfully.")
} catch InternetError.NotConnected {
  print("You are not currently connected to the internet.")
} catch InternetError.AirplaneModeEnabled {
  print("Disable airplane mode to proceed.")
  1. We are indicating the start of a do-catch block using the do keyboard.

  2. We are trying to call sendDataAcrossInternet() method using the try keyword. The try keyword indicates that the following function is a throwing function.

  3. This line will only be printed if the call to sendDataAcrossInternet() does not throw an error. If an error is thrown execution jumps to the appropriate catch block.

  4. This line will only be printed if the the call to sendDataAcrossInternet() throws the InternetError.NotConnected error.

  5. This line will only be printed if the the call to sendDataAcrossInternet() throws the InternetError.AirplaneModeEnabled error.

The do-catch block is similar to a switch statement in that only one of the above code blocks will be executed.

Calling Throwing Functions Using Try!

Alternatively, we can call a throwing function using the try! keyword. The try! keyword allows us to call a throwing function without having to consider the possible errors that could be thrown. The try! keyword should only be used when you know that a throwing function will not throw an error during runtime. (If an error is thrown during runtime, your program will crash.) For instance, if we had a throwing function called loadImage() and we are guaranteed that the image we want to load exists (maybe because it was included in the application), we could call the loadImage() function as follows:

let image = try! loadImage(pathToIncludedImage)

This covers the basics of error handling in Swift. For an in-depth look, take a look at Apple's great documentation.


If you have feedback on this tutorial or find any mistakes, please open issues on the GitHub Repository.

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