Swift Try Catch and Error Handling – Code Examples

Learn how to handle your errors and crashes gracefully in Swift with try catch blocks and more! You’ll see the most common error handling code samples below. If you’d like a longer read, check out the Swift error handling documentation.

Swift Try Catch

When you’re coding, you’ll see methods that have the throws keyword. This indicates that the method may raise an error and as a best practice, you should gracefully handle the error and show an appropriate error message to the user. If you don’t catch the error, then your app will crash!

Here’s an example of the Swift do-try-catch syntax:

do {
    // Create audio player object
    audioPlayer = try AVAudioPlayer(contentsOf: soundURL)
            
    // Play the sound
    audioPlayer?.play()
}
catch {
    // Couldn't create audio player object, log the error
    print("Couldn't create the audio player for file \(soundFilename)")
}

Let’s walk through the code:

do – This keyword starts the block of code that contains the method that can potentially throw an error.

try – You must use this keyword in front of the method that throws. Think of it like this: “You’re trying to execute the method.

catch – If the throwing method fails and raises an error, the execution will fall into this catch block. This is where you’ll write code display a graceful error message to the user.

Use this pattern to handle any potential errors caused by a method that throws.

Using Try? and Try!

You can still call a method that throws without using the do-try-catch syntax.

// Create audio player object
audioPlayer = try? AVAudioPlayer(contentsOf: soundURL)
            
// Play the sound
audioPlayer?.play()

If you use the try? keyword instead of the basic try, you’ll get nil instead of the thrown error. In the code above, if the AVAudioPlayer object can’t be created, then nil will be assigned to the audioPlayer variable.

Finally, if you’re 100% sure that there will be no error thrown, even though it’s a method that is marked as throws, then you can use the try! keyword instead of the basic try.

// Create audio player object
audioPlayer = try! AVAudioPlayer(contentsOf: soundURL)
            
// Play the sound
audioPlayer?.play()

Doing this will completely disregard the fact that an error might be thrown by the method and if an error actually occurs and you don’t have the do catch blocks, then your app will probably crash.

Creating your own Swift Error type

If you’d like to define your own error type, you can use an enum and the Error protocol like this:

enum BankAccount: Error {
    case insufficientFunds
    case invalidTransaction
    case duplicateTransaction
    case unknown
}

Throwing Errors

After you define your own error type, you can start using it in your methods like this:

func withdrawCash (amount: Int) throws {
    if funds - amount < 0 {
        throw BankAccount.insufficientFunds
    }
    else {
        funds -= amount
    }
}

Using the throw keyword, your method will return and raise the error. Notice that in your function declaration, you also need to indicate that the function can raise errors by specifying the throws keyword.

When you throw the error, it’s like the return keyword. Execution stops at the throw keyword and returns to the caller.