I've been looking up how to use the guard keyword in Swift. Recently a developer told me that the code below will print "success" if there's no error in the closure.
for attachment in attachments! {
attachment.fetchData { (data, error) in
guard let error = error else {
print(“success”)
return
}
print(error.localizedDescription)
}
I'm a bit confused by his statement. After reading the closure and guard keyword documentation from Apple, it looks to me like his code will print out "success" only when there is an error.
I feel like he's using it in reverse, but I may be wrong. Can someone break it down for me and explain if success is printed when there is or is not an error?
Thank you.
The use of guard to unwrap the error is very misleading. You should use it to unwrap your data and make sure there is no error and provide an early exit to your method in case of error.
Just change your guard statement to:
guard let data = data, error == nil else {
print(error ?? "")
return
}
Related
I'm following a tutorial on youtube on how to create a Pokedex in Swift. I'm following all the same steps and my file arrangement is the same but I can't get rid of this stupid error.
URLSession.shared.dataTask(with: url) { (data, response, error) in
guard let data = data else { return }
guard let pokemon = try? JSONDecoder().decode([Pokemon].self, from: data)
I'm getting an error on the third line
You aren't showing the rest of your statement, but here's what I can infer is happening:
Every guard statement has to be in this format:
guard *CONDITION* else {
//some code
return //or otherwise leave the current scope
}
You have the guard and the condition, but you're probably not following it with else. Like the statement above, it should look something like (note the else at the end of the line):
guard let pokemon = try? JSONDecoder().decode([Pokemon].self, from: data) else {
//do something to handle the error
return
}
The documentation for guard can be found on this page: https://docs.swift.org/swift-book/ReferenceManual/Statements.html
Leo Dabus in the comments points out that it would be good to have a more robust solution to the decoding. You may want to replace your guard statement with something along these lines:
do {
let pokemon = try JSONDecoder().decode([Pokemon].self, from: data)
} catch {
//handle the error
print(error)
}
In this scenario, you have access to error, which would give you some information as to why the decoding failed, whereas using the try? and guard statement, it could fail mysteriously.
Im tring to make a network call and instead of using callback I try to use delegate instead.using Result type where .Sucsess is T: Decodable and .failure is Error. passing my model in the .Sucsess is working but when trying to pass an error I get a compile error "Generic parameter 'T' could not be inferred" what am I missing ?
protocol NetworkServiceDelegate: class {
func decodableResponce<T: Decodable>(_ result: Result<T, NetworkError>)
}
let dataTask:URLSessionTask = session.dataTask(with: url) { (dataOrNil, responceOrNil, errOrNil) in
if let error = errOrNil {
switch error {
case URLError.networkConnectionLost,URLError.notConnectedToInternet:
print("no network connection")
self.delegate?.decodableResponce(Result.failure(.networkConnectionLost))
case URLError.cannotFindHost, URLError.notConnectedToInternet:
print("cant find the host, could be to busy, try again in a little while")
case URLError.cancelled:
// if cancelled with the cancelled method the complition is still called
print("dont bother the user, we're doing what they want")
default:
print("error = \(error.localizedDescription)")
}
return
}
guard let httpResponce:HTTPURLResponse = responceOrNil as? HTTPURLResponse
else{
print("not an http responce")
return
}
guard let dataResponse = dataOrNil,
errOrNil == nil else {
print(errOrNil?.localizedDescription ?? "Response Error")
return }
do{
//here dataResponse received from a network request
let decoder = JSONDecoder()
let modelArray = try decoder.decode([Movie].self, from:
dataResponse) //Decode JSON Response Data
DispatchQueue.main.async {
self.delegate?.decodableResponce(Result.success(modelArray))
}
} catch let parsingError {
print("Error", parsingError)
}
print("http status = \(httpResponce.statusCode)")
print("completed")
}
this line generates the error, it dosnt metter if I pass my enum that cumfirms to Error or trying to pass the error from the dataTask
self.delegate?.decodableResponce(Result.failure(.networkConnectionLost))
Well, you have two problems, having to do with the question "what type is this?" Swift is very strict about types, so you need to get clear about that.
.networkConnectionLost is not an Error. It is an error code. You need to pass an Error object to a Result when you want to package up the error. For example, URLError(URLError.networkConnectionLost) is an Error.
The phrase Result<T, NetworkError> makes no sense. Result is already a generic. Your job is to resolve the generic that it already is. You do that by specifying the type.
So for example, you might declare:
func decodableResponce(_ result: Result<Decodable, Error>)
It is then possible to say (as tests):
decodableResponce(.failure(URLError(URLError.networkConnectionLost)))
or (assuming Movie is Decodable):
decodableResponce(.success([Movie()]))
That proves we have our types right, and you can proceed to build up your actual code around that example code.
I am attempting to use SwiftSoup to scrape some HTML. This example, based on the SwiftSoup github documentation, works fine…
func scrape() throws {
do {
let htmlFromSomeSource = "<html><body><p class="nerp">HerpDerp</p><p class="narf">HoopDoop</p>"
let doc = try! SwiftSoup.parse(htmlFromSomeSource)
let tag = try! doc.select("p").first()!
let tagClass = try! tag.attr("class")
} catch {
print("oh dang")
throw Abort(.notFound)
}
print(tagClass)
}
… Up until I mess with the selector or attribute targets, at which point everything crashes thanks to the implicitly unwrapped optionals (which I assume was just quick-and-dirty code to get smarter people started). That do/catch doesn't seem to help at all.
So what's the Right way? This compiles...
print("is there a doc?")
guard let doc = try? SwiftSoup.parse(response.body.description) else {
print("no doc")
throw Abort(.notFound)
}
print("should halt because there's no img")
guard let tag = try? doc.select("img").first()! else {
print("no paragraph tag")
throw Abort(.notFound)
}
print("should halt because there's no src")
guard let tagClass = try? tag.attr("src") else {
print("no src")
throw Abort(.notFound)
}
... but again if I mess with the selector or attribute it crashes out, "Unexpectedly found nil while unwrapping an Optional value" (after "is there a doc?"). I thought guard would halt the process when it encountered a nil? (If I convert "try?" to "try" the compiler complains that "initializer for conditional binding must have Optional type"…)
If you declare the function as throws you don't need a do - catch block inside the function. Just remove the block and the exclamation marks after try to pass through the errors to the caller function.
func scrape() throws { // add a return type
let htmlFromSomeSource = "<html><body><p class="nerp">HerpDerp</p><p class="narf">HoopDoop</p>"
let doc = try SwiftSoup.parse(htmlFromSomeSource)
guard let tag = try doc.select("p").first() else { throw Abort(.notFound) }
let tagClass = try tag.attr("class")
// return something
}
I'm not very familiar with error handling and so any advice would be really appreciated.
My code makes multiple calls recursively to the Apple API for calculating a route as it needs to calculate the distance for multiple options.
do {
directions.calculate(completionHandler: {(response, error) in
let response = (response?.routes)! //This line bugs out
for route in response {
//code
completion(result, error)
}
})
}
catch {
print(error.localizedDescription)
}
It tends to crash on the 5th or 6th time, and I wondered if there was a way to stop the application crashing and notify instead.
Thanks
There's no point in using a do-catch block, since there's no throwable function in your code, so you won't be able to catch any errors. In Swift you can only catch errors thrown by a function marked throws, all other errors are unrecoverable.
You should safely unwrap the optional response, since it might be nil, in which case the force unwrapping would cause an unrecoverable runtime error that you have already been experiencing.
You can use a guard statement and optional binding to safely unwrap the optional response and exit early in case there's no response.
directions.calculate(completionHandler: {(response, error) in
guard let response = response, error == nil else {
completion(nil,error)
return
}
for route in response.routes {
....
completion(result, nil)
}
})
I got confused error handling in swift3. I try to do like "if XX function got error then try YY function"
Let me show you what I try:
class MyClass {
enum error: Error
{
case nilString
}
func findURL() {
do {
let opt = try HTTP.GET(url_adr!)
opt.start { response in
if let err = response.error {
print("error: \(err.localizedDescription)")
return //also notify app of failure as needed
}
do
{
/* This is func1. and got error. I want to if this function has error then go next function. */
try self.stringOperation(data: response.description)
}
catch{
print("doesn't work on func1. trying 2nd func")
self.stringOperation2(data:response.descritption)
}
}
} catch let error {
print("got an error creating the request: \(error)")
}
}
func stringOperation(data:String)throws -> Bool{
do{
/** 1 **/
if let _:String = try! data.substring(from: data.index(of: "var sources2")!){
print("its done")
}else{
throw error.nilString
}
IN 1: I got this fatal error on this line:
"fatal error: unexpectedly found nil while unwrapping an Optional value" and program crashed.
I googled error handling try to understand and apply to in my code. However not succeed yet. Can someone explain where did I wrong?
Additional info: I got String extension for .substring(from:...) , and .index(of:"str"). So these lines doesn't got you confused.
As a general rule, try avoiding using force unwrapping (!), where you have
if let _: String= try! data.substring...
Instead use
if let index = data.index(of: "var sources2"),
let _: String = try? data.substring(from: index) { ... } else { ... }
That way you remove the two force unwraps that may be causing your crash. You already have the if let protection for catching the nil value, so you can make the most of it by using the conditional unwrapping.