I am trying to write a simple multiplayers Swift program using Xcode 7 beta 5. I encountered the following error at runtime:
[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]'
*** First throw call stack
And then I tried and error and found that this piece of code somehow introduced this error:
func setupMatchHandler() {
/* This function handles invite as sent by other users */
GKMatchmaker.sharedMatchmaker().matchForInvite(GKInvite().self , completionHandler: { (invitedMatch , invitationError) -> Void in
if invitationError != nil {
// error out
print("Game Center error: \(invitationError)")
}
if invitedMatch != nil {
// success
print("invitation received!")
}
})
}
I wonder can any expert here shed light on what went wrong here? Thanks a million!
sam
Maybe a check for (GKInvite().self != nil) could help?
It seems that it's the only thing that you are inserting via matchForInvite is this one
Related
Say I have a func that returns an error:
func conditionalError() error {
if maybe {
return errors.New("here")
}
return nil
}
err := conditionalError()
but if we call it without handling the return value:
conditionalError()
is there a flag to get the compiler to complain, or maybe just an IDE setting in JetBrains or VSCode to warn about it?
There is an app for that! This type of functionality falls in the category of linters.
https://github.com/kisielk/errcheck
Update: I think now that there is no universal answer to this question. We can return both errors using the technique explained in the answer. I think that the most important thing here is not to forget the case when we have two errors and somehow handle it.
Notes: There are many questions on SO about how to return an error from deferred function. This is not a question here.
(In Go) What is the proper way to return an error from a deferred function when the function is already returning an error. For example
func errorMaker() (err error) {
defer func() {
err = errors.New("Deferred error")
}()
err = errors.New("Some error")
return
}
func main() {
err := errorMaker()
fmt.Printf("Error: %v\n", err)
}
In the code above the error returned by the deferred function overwrites the error returned by the function. What is the canonical way to return both errors? If another programmer uses my function what result might she expect from the function when the function returns 'two errors'?
Should I use Error wrapping for this?
Additional notes:
As #Volker says in his comment I write some application specific handling for this error. Because I know what should be done based on nature of the errors.
I think my question is - if I want to return all errors from the function what is the best way to combine them in my scenario?
Disclaimer: I don't know if the following advice can be seen as "standard" or "widely-accepted".
Should I use Error wrapping for this?
Short answer: yes (I would do so).
Go 1.12 and earlier
What I do when I need my errors to convey some specific meaning, without foregoing the error interface, I create a wrapper that implements the error interface - Error() string -. This wrapper contains all extra information I need.
If the caller is aware of the existence of those extra info, it can unwrap the error with a cast and find those info.
With the added benefit that unaware callers can just handle the error as a generic error.
type MyError struct {
DeferredError error
}
// Implements 'error' interface
func (e MyError) Error() string {
// format to string
}
func someFunc() error {
// might return an instance of MyError
}
...
// Caller code
err := someFunc()
if err != nil {
if myErr, ok := err.(*MyError); ok {
// here you can access the wrapped info
fmt.Println(myErr.DeferredError)
} else {
// otherwise handle the error generically
}
}
Go 1.13 onwards
With Go.13 you can use errors.As to unwrap an error. From the official docs:
[The method] As finds the first error in err's chain that matches target, and if so, sets target to that error value and returns true. The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.
var myErr *MyError
if errors.As(err, &myErr) {
// here you can access the wrapped info
fmt.Println(myErr.DeferredError)
} else {
// otherwise handle the error generically
}
As the docs say the myErr variable is populated as a side-effect of calling As.
I am using PFQueryTableViewController as part of ParseUI to load a table of objects based on the currentUser's geolocation. I have seen several other (older) forum posts (like here) detailing that the queryForTable function should return nil if a value like currentLocation:PFGeoPoint? is nil. Then wait on the background process in viewDidLoad to get the PFGeoPoint and loadObjects(), thus calling the queryForTable function again.
I am seeing others say that Swift's ParseUI libraries may not have a queryForTable function that allows nil returns.
var currentLocation:PFGeoPoint? = nil
override func viewDidLoad() {
if currentLocation == nil {
PFGeoPoint.geoPointForCurrentLocationInBackground {
(geoPoint: PFGeoPoint?, error: NSError?) -> Void in
if error == nil {
print("Got the user's current location!")
self.currentLocation = geoPoint
print("Reloading table using user's location")
self.loadObjects()
}
}
}
}
override func queryForTable() -> PFQuery {
if self.currentLocation != nil {
print("Generating query based on user's location!")
let query = PFQuery(className:self.parseClassName!)
query.whereKey("taskLocation", nearGeoPoint: self.currentLocation!, withinMiles: 50)
if(self.objects?.count == 0)
{
query.cachePolicy = PFCachePolicy.CacheThenNetwork
}
return query
}
print("User's current location is not known")
return nil
}
Obviously, this fails to build because the function is not (note the question mark):
override func queryForTable() -> PFQuery? {
...
}
My attempted workaround was to return PFQuery() instead of nil, but I believe it returns after the viewDidLoad self.loadObjects(). The behavior I see is a momentary flash of a table with cell results, and then an empty table.
Here is a link to the Google Group discussion about this very issue, with Hector Ramos saying that it works with Objective-C but not Swift (...yet?). I'm running the latest ParseUI as of this posting (1.1.6).
Is there an option yet to do this in Swift? If not, when?
If this isn't an option, what are some workarounds people have tried successfully?
I actually figured this issue out - nothing to do with the code itself. Apparently, when you set a fake location on the iOS Simulator, it also enforces that different location when you use a real iOS device plugged in. My results were not showing up because there legitimately wasn't an object with a PFGeoPoint near my location (because it thought I was in London!)
Anyway, moral of the story is to make sure you always know your preset location in both Simulator and physical iOS devices.
PS - The code above does work when my location is set correctly.
I got the follow error when attempting to build a parse.com project via Xcode 7.0/Swift 2.0:
Is there a work around for this?
What's the Swift 2.0 equiv.?
You can either:
user.signUpInBackgroundWithBlock { (succeeded: ObjCBool, error: NSError?) -> Void in
// do something
}
Or
user.signUpInBackgroundWithBlock { succeeded, error in
// do something
}
--
Note, Xcode can show you the appropriate types. For example, if I start to type and then let code completion show me the method, I see something like:
If I then hit enter and select the block: PFBooleanResultBlock? and hit enter again, I'll see:
That shows me precisely what types those two parameters are.
user.signUpInBackgroundWithBlock {success, error in
if error == nil {
//
}
else{
//
}
}
Above works fine with no errors.
Updating to 2.0 with Xcode 7 Beta 4
I have this code block
do
{
try AVAudioSession.sharedInstance().setActive(true)
} catch let err as NSError
{
println("Dim background error")
}
And its giving me the error (on the try line)
Errors thrown from here are not handled.
Is this a compiler error or is there something I am missing in my syntax?
I checked the docs and my code 'looks' correct.
What types of errors can AVAudioSession.sharedInstance().setActive(true) throw?
If it can only throw NSErrors, then there's no need of specifying this when catching the error. You could simply write:
do {
try AVAudioSession.sharedInstance().setActive(true)
} catch {
print("Dim background error")
}
If you want to use the error in your catch-scope, you can still access it. Swift automatically binds the thrown error to error, which can be used:
catch {
// do something with `error`
}
If the method throws multiple types of errors, and you only want to deal with the ones that are NSErrors, you can conditionally bind them:
catch let specialError as NSError {
// do something with `specialError`
}
You must ensure though, that every thrown error is handled. So assuming that the method can also throw a RandomError (which I just made up now), you would have to write:
catch let randomError as RandomError {
// do something with `randomError`
}
...in addition to the catch of the NSError.
Or you could of course use the general case:
catch {
// do something with `error`
}
So I assume your problem can be solved by removing let err as NSError, from your catch-statement.
May be a compiler bug. Anyway try removing let err as NSError ; catch alone is enough if you want to catch all errors.
Also, with Swift 2 you should use print, not println.
The following code compiles without errors with XCode 7 Beta 4:
import AVFoundation
class X {
func y() {
do {
try AVAudioSession.sharedInstance().setActive(true)
} catch {
print("Dim background error \(error)")
}
}
}
check this code that will you get idea on try/catch :
enum ErrorMessages :ErrorType {
case ErrorDescription
}
do
{
try AVAudioSession.sharedInstance().setActive(true)
} catch ErrorMessages.ErrorDescription
{
println("Present the error description here")
}