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Uncaught Error/Exception Handling in Swift
(2 answers)
Closed 5 years ago.
Is it somehow possible to handle a fatal error (of any kind) in iOS? I am not looking for some magical way to make the app stay alive, just some way to inform the user that something went wrong before the app disappears (like an alert for example).
A fatal error is a runtime error. You do not catch fatal errors, because they indicate a programming error, you should fix your code instead. Crashes are built in such a way so that you cannot stop them unless you fix the error in your code. Letting the user know that something went wrong cannot be done and wouldn't help anyway, unless you make it work properly.
To prove that they cannot be stopped, unless you change your code, we can put something that would cause a fatal error in a do-try-catch structure:
do{
var car: String? = nil
try print(car!)
}
catch{
print("Something went worng")
}
Obviously, this still crashes with:
fatal error: unexpectedly found nil while unwrapping an Optional value.
So you must repair your program instead.
While there is no way to inform the user directly of the error, since you would be printing in the console, using a guard statement will help you to avoid fatal errors by checking if the optional value exists in the first place, and returning early if the optional value does not exist. This will save your app from crashing at times.
When something goes wrong like unwrapping nil, two situation may happen. If there are correct try catch around and the error is caught, then your app will continue working. Otherwise, you will end up having a fatal error that crash your app. So a fatal error is a consequence of lacking exception handling and is not something you can stop and show user something before the crash happens.
Just like others mentioned, fatal errors should NEVER occur. Do everything you can to prevent that error from happening.
You can prevent them using do catch blocks and by handling optionals correctly. I would recommend you to read Apple's documentation on that! Good luck!
Related
I'm attempting to mod a game I've been playing recently, and I've encountered an error when writing some code that adds a new mechanic. The problem itself isn't very complicated, but I can't seem to figure out what the solution is in this context. I'm pretty new to coding, especially with lua, so I apologize if the solution is really obvious. I've looked around for some answers, but again, nothing I can find seems to help me in this specific context. The game I'm trying to mod is called Project Zomboid, if that helps.
When attempting to start the game with the mod enabled, the error "Object tried to call nil in isBloodthirsty" pops up. Here's the snippet of code that's causing the error:
local function isBloodthirsty(player)
if player:getDescriptor():getTrait() ~= "bloodthirsty" then
return false else
return true
end
end
Ignoring how poorly written it is, the code was supposed to check if the player had a certain trait, and if so, then set the value isBloodthirsty to true. I think the error is caused by lua not recognizing the value "bloodthirsty", but I'm not sure what I should put instead. If anybody has an idea of what I'm doing wrong, I'd greatly appreciate some help. If it's helpful, I can post the rest of the code.
Thanks to all the great help from the stack overflow community, I managed to figure out what my problem was. The code I had written wasn't working because lua didn't recognize "bloodthirsty" as a valid trait string. My solution was to mix up the code a bit and frame the trait as a profession instead (a profession is kind of like a collection of traits within the game). The following code worked:
local function bloodthirstyStart(player)
if player:getDescriptor():getProfession() ~= "bloodthirsty" then
return
end
My application is throwing an exception while loading the data and it's showing a message that "requested operation is cancelled due to object is garbage collected".
sometimes My application is working fine but unexpectedly in the middle it,s throwing this exception.
Can any one guide me to solve this issue.
Thanks
Please check this once https://forums.xamarin.com/discussion/18910/navigation-popasync-not-freeing-page
I don't think it will work or not but please check your xaml file once the issue in the design.If you are manually adding any controls like buttons,labels with bindings (any controls) from your ViewModel or any file it should throw this type exception I also faced this.In my case below line of code worked for me
await Task.Delay(your seconds);
before or after adding controls
There is a chance that you create some background task in your app and run and in this task, you get the null object.
In this case, you will need to:
put a debugger on System.NullReferenceException and find the exact issue location. please look below image.
Hi I'm trying to verify the user email with parse, I been searching the entire web for hours and have found nothing. Everything else with parse is working great! but when I trying to check if the email verification is true, I get this error:
fatal error: unexpectedly found nil while unwrapping an Optional value
basically I'm trying to check that first so I can change to a different viewController, the code that I'm using right now is this...
if user.objectForKey("emailVerified")!.boolValue == true
{
performSegueWithIdentifier("tarjetaCredito",sender: self)
}
I been trying all type of code and I always get the same error, any help? I'm using Xcode 7.0 beta 6
When you see “unexpectedly found nil” in Swift, you can trace it to an exclamation point. (Whether it’s a bare ! or as! or try!, the exclamation point essentially says, “Hey compiler, I know you think an error could happen here, but I know it will never actually happen, so don’t force me to deal with the error case.”)
In your code, the ! is right here:
if user.objectForKey("emailVerified")!.boolValue == true
⬆︎
…which means that the result of the expression user.objectForKey("emailVerified") must be nil. Figure out why that expression is nil, and you’ll have your answer.
If user is a Dictionary, then perhaps there is no value for the key "emailVerified", and perhaps you are using the wrong key string. (Aside: you may be able to say user["emailVerified"] instead, depending on what the type of user is.)
Whatever the cause, printing out the value of user on the line before the crash is likely to help you diagnose the problem:
print("----------> user = ", user)
if user.objectForKey("emailVerified")!.boolValue == true
...
In general, it is often good practice in Swift to avoid using ! — or at least carefully consider why you are using it. If you determine that it is sometimes valid for that expression to evaluate to nil — for example, it might be nil if the email is not validated yet — then you might use ? instead; for example:
if user.objectForKey("emailVerified")?.boolValue == true
I'm updating a node and pushing it:
remote_graph.push(node)
push() seems to return nothing. How can I tell if the push works? In my test code, I ought to be violating a unique constraint in Neo4J.
How can I tell using py2neo? I expected an exception.
When I enter the equivalent cypher into the Neo4J web tool, I get the following exception:
Node 322184 already exists with label VERSION and property "version"=[1.436818928448956E9]
which is what I expected.
Edit -
What I expected to get back was an indicator of whether the operation worked or not. I think push() accepts an array of nodes, so an array of results would be sensible. I don't know what the indicator would have within it since I don't know what is available. An array of strings would be fine, with each string being a failure reason, or "OK".
Generally speaking, the design of this API is: if it returns OK, you can assume everything has worked as expected, if an error is raised, that error will contain details of what went wrong. Therefore, absence of error should usually be interpreted as a signal of success.
That said, if you believe that your push has failed and no error has been raised, there is a bug in py2neo. For debugging, you can check the state of the database after your push by using the browser and then if you're able to recreate this scenario in a standalone piece of code, please raise an issue on GitHub and I will be happy to fix it.
Summary:
I want to see more detailed XML/SVG parsing error messages. I want to know where the errors are happening. How can I do this?
Background:
I'm working with some complicated javascript-generated SVG in Firefox. As I'm developing, sometimes while hunting down a big I'll see errors in the Firefox error console (or firebug) "Unexpected value NaN parsing y attribute". This is pretty clear. However, there's no line number, no code shown in Firebug - basically no way to track down where this error occurs.
With simple JS, it's a matter of tracking down the bad code. However, as my JS gets more complicated, I really need to be able to see which of hundreds of potential lines is causing this.
Ideally, I'd like to see this parsing error the same way I see JS errors or HTML errors:
Unexpected value NaN parsing y attribute.
Line 103: svgElement.setAttribute('x', some_bad_js_variable);
Is there any way to do this? Even knowing which SVG element is being affected would help, anything besides "There was an error somewhere". Thanks!
Nearly three years later and using Firefox 29.0.1, I have the same difficulty. I ended up commenting out successive blocks of code until I found the offending line.
FWIW, in my case Firefox didn't like the fact that I had created a node with blank attributes:
<clipPath id="chart_area">
<rect x="" y="" width="" height=""/>
</clipPath>
Once I removed the attributes or set them to any value, the problem went away. I was surprised because I'd expected the error to be in the Javascript instead. I hope this helps someone else.
Raise a bug in bugzilla and ask for the element tag name to be added to the error message: https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&component=SVG
Adding a line number would be more difficult. If want that too then create another bug specifically for it as you're less likely to get it.