Error Handling

Error Handling

“NOTE

Error handling in Swift resembles exception handling in other languages, with the use of the try, catch and throw keywords. Unlike exception handling in many languages—including Objective-C—error handling in Swift does not involve unwinding the call stack, a process that can be computationally expensive. As such, the performance characteristics of a throw statement are comparable to those of a return statement.”

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 2 Prerelease).” iBooks. https://itun.es/us/k5SW7.l

Four ways to handle errors

“There are four ways to handle errors in Swift. You can propagate the error from a function to the code that calls that function, handle the error using a do-catch statement, handle the error as an optional value, or assert that the error will not occur”

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 2).” iBooks. https://itun.es/us/jEUH0.l

Propagating Errors Using Throwing Functions

Propagate errors from a function to the code that calls that function. Write throws keyword in the function’s declaration after its parameters(before the return arrow). methods call throwing function must either handle the errors directly–using a do-catch statement, try?, or try!–or continue to propagate them

“NOTE

Only throwing functions can propagate errors. Any errors thrown inside a nonthrowing function must be handled inside the function.”

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 2 Prerelease).” iBooks. https://itun.es/us/k5SW7.l

Handle errors using a do-catch statement

do {
    try <#expression#>
        <#statements#>
} catch <#pattern 1#> {
    <#statements#>
} catch <#pattern 2#> where <#condition#> {
    <#statements#>
}

enum MyError: ErrorType {
    case UserError
    case NetworkError
    case DiscoveryedError
}

func doStuff() throws -> String {
    print("Do stuff 1")
    print("Do stuff 2")
    throw MyError.NetworkError
    return "Some string"
}

do {
    try doStuff()
        print("Success");
} catch MyError.NetworkError {
    print("A network error occurred")
} catch {
    print("An error occurred.")
}

Handle errors as an optional value

use try? to handle an error by converting it to an optional value

Assert that errors will not occur

use try! to disable error propagation when you know a throwing function or method won’t, in fact, throw an error at runtime.

Specifying Cleanup Actions

use defer to clean up after the code block has finished excuting.