Error Handling

We can define custom errors that will occur with an enum ourselves. In the example below, the error types that may occur when entering our credit card information are added with the name CreditCardError.

enum CreditCardError: Error {
    case insufficientFunds
    case issuerDeclined
    case invalidCVC
}

If a method we wrote can generate an error, a throws keyword is added to the method's definition.

Below is an example of the syntax.

func processPayment(creditcard: String) throws {
}

Sometimes, while writing our code, the software developer may need to throw the error.

Let's imagine. We write a payment system in a bank. 

And the customer's balance is not enough when paying by credit card. In this case, we can throw an error that the fund is not sufficient.

throw CreditCardError.insufficientFunds

When we want to use methods that may cause errors in our application, we can capture possible errors by taking these codes into do-catch blocks.

While the codes that may cause errors are located in the do block, possible errors will be captured in the error catch block.

Let's look at the example below

do {
   //Code block that may throw an Error
}
catch let error {
    print(error)
}

Except for the try keyword. We have try! or try? usages. 

Let's look at our example.

// if an error was thrown, app will crash
let result = try! doSomething() 


if let result = try? make() {
    //If the make method is successful, the result is unwrap. It is a very elegant use.
} else {
    //make() threw an error.
}

If the method we call can throw multiple errors, we can filter and direct our application flow according to the type of error.

In the example below, each catch block handles different errors that may occur.

do {
    try doSomething()
} catch CustomError.errorOne {
  //handle errorOne Error
} catch CustomError.errorTwo(let customErr) {
   //handle errorTwo Error with customErr
}

Congratulations 🎉

In this lesson, we learned how to handle application errors with Swift.

See you in the next lesson.