I need a second pair of eyes on why I'm getting an error when trying to assign the NSError to the one passed into the function:
// Response and Error Objs.
NSURLResponse *response = nil;
NSError *requestError = nil;
// Attempt authentication
[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&requestError];
// Error?
if (requestError != nil) {
*error = requestError; // Error happens here
return;
}
From Apple's Error Handling Programming Guide
Important: Success or failure is indicated by the return value of the
method. Although Cocoa methods that indirectly return error objects in
the Cocoa error domain are guaranteed to return such objects if the
method indicates failure by directly returning nil or NO, you should
always check that the return value is nil or NO before attempting to
do anything with the NSError object.
In this case, -sendSynchronousRequest:returnResponse:error: returns an NSData object. You should check to see if that is nil before proceeding with the error.
Did you forget the NSError:
if (requestError != nil) {
NSError *error = requestError; // Error happens here
return;
}
Or where/when is error declared/defined and whats the point of assigning one error objet to another? Why not just:
if (requestError != nil) {
NSLog(#"%#", [requestError localizedDescription]);
return;
}
Most likely *error doesn't point to a valid pointer; the sample isn't complete, so I can't say for certain. Perhaps *error is nil.
you can't use 'error' in the form you are trying to use. You can access error in NSURLConnection but not outside it.
Hence you need to define your error object before using it.
Lets say you have a method signature that accepts an NSError**.
This means that the method accepts a pointer to a pointer that is pointing to an NSError value.
So, I suspect because error is NULL, your basically asking that NULL object what the value is of the thing that it's pointing to. But of course, you cannot ask a NULL object for that information, that's why it crashes.
So, before you try to set the value of *error, you need to check that error is not NULL like so:
if (error == NULL) {
// the function caller has supplied us with a nil as an argument
// this indicates the caller does not care about the error
return;
}
The reason you have to check for NULL instead of nil is because you are checking a memory location not a cocoa object.
Related
What is the correct way to handle NSError** pointers?
- (BOOL)handleData:(NSDictionary *)data error:(NSError **)error {
// pass the error pointer to NSJSONSerialization
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:data options:options error:error];
// Check if NSJSONSerialization had errors
if (error) // <-- sometimes this works, sometimes it crashes...
return false;
...
return true;
}
- (void)someMethod {
NSError *error = nil;
BOOL result = [self handleData:dataDict error:&error];
if (error) {
// an error occurred
} else {
}
}
In this example someMethod passes a NSError reference to handleData:error. This is done by passing a pointer/address instead of the object (...error:&error)
The method handleData:error then passes this pointer to dataWithJSONObject:options:error (now without the &). Now I would like to check an error occurred, but what is the correct way to do this?
if (error)...
// This works if error == nil. However this is not always the case.
// Sometimes error is some address (e.g. 0x600001711f70) and *error == nil
// from the start of the method (passing error to NSJSONSerialization has no
// influence on this
if (*error)...
// This works in cases where error itself is not nil, but it crashes if
// error == nil
Why is error == nil in some cases and error != nil but *error == nil in others?
What is the correct way to pass error between the methods and to check if an error occurred?
The place to find the answers is Introduction to Error Handling Programming Guide For Cocoa. The conventions are:
A method may return an NSError object via an NSError ** parameter, such a method should also have a non-void return type and indicate success or failure via its return value. So, using your example, dataWithJSONObject:options:error: will return nil if it encounters an error and may return an error object via its third parameter.
Any method accepting an NSError ** parameter for error returns should accept either the address of an NSError * variable or NULL. The latter value means the user does not wish to have an error object returned. This means that the method accepting an NSError ** parameter must check the parameter value is not NULL before attempting to assign an error object via it.
So your method handleData:error: must be prepared to accept NULL and needs to test for it. Your code therefore must include something similar to:
// Check if NSJSONSerialization had errors
if (jsonData == nil)
{
// Error occurred, did it return an error object?
if (error != NULL && *error != nil)
{
// we have an error object
}
else
{
// we have an error but no error object describing it
}
}
else
{
// no JSON error
}
HTH
Sorry, if found a lot of threads like the but they were not about this Error**-thing.
I tried to 'design' my methods like the error-examples I found. But calling the second, the error is not pointing to nil, the debugger says error: summary string parsing error.
This is my controller-method:
-(void) refresh {
NSError *error;
ServerApi *serverApi = [mainModel newServerApi];
NSArray *newItems = [serverApi getNewItems: &error];
...
This is the called method:
- (NSArray *) getNewItems: (NSError **) error {
// Breakpoint here, error is: 'error: summary string parsing error'
...
NSURLResponse *response;
NSData *responseData = [NSURLConnection sendSynchronousRequest: request returningResponse: &response error: error];
I thought, I did the same as Apple with sendSynchronousRequest.... Their comment tells
error Out parameter (may be NULL) used if an error occurs
while processing the request. >>>>Will not be modified if the
load succeeds.<<<<
What did I do wrong and why does this work for Apples sendSynchronousRequest...?
The code is fine, as error does not need to be initialized "from outside".
Also, if you use ARC it will automatically initialize local object pointers to nil, so
NSError *error;
is no different than
NSError *error = nil;
under ARC.
While explicit initialization is still a good practice, that's not the source of any error here.
That being said,
summary string parsing error
is a lldb error. My hypothesis is that it gets confused by the double pointer, but I wouldn't worry too much.
By the way, you're doing a slight mistake in implementing this pattern.
Synchronous methods that may fail, should method to return a BOOL value indicating whether the computation was successful and then clients will check that value and subsequently inspect the error object in case it failed.
Checking the error object is in general a bad idea: even some Apple APIs can fail and yet return a nil error, so avoid doing that!
Remember to set your pointer to nil in refresh:
NSError *error = nil;
Also, remember, your checks should be:
Checking (*error) for nil (aka "there was no error passed") - operation may or may not have been successful;
Checking error for NULL (aka "there was no NSError* pointer passed, so don't assign it an object").
-(void) conn:(NSString *)method{
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurrentQueue, ^{
__block NSDictionary *resultBlock = nil;
dispatch_sync(concurrentQueue, ^{
/* Download the json here */
//Create webservice address
NSString *webService = [_baseURL stringByAppendingString:_webService];
//NSLog(#"%#", webService);
//Create error object
NSError *downloadError = nil;
//Create the request
NSMutableURLRequest *req = [self initRequest:webService method:method];
if(req != nil){
//Request the json data from the server
NSData *jsonData = [NSURLConnection
sendSynchronousRequest:req
returningResponse:nil
error:&downloadError];
if(downloadError!=nil){
NSLog(#"DOWNLOAD ERROR %#", downloadError);
}
NSError *error = nil;
id jsonObject = nil;
if(jsonData !=nil){
/* Now try to deserialize the JSON object into a dictionary */
jsonObject = [NSJSONSerialization
JSONObjectWithData:jsonData
options:kNilOptions
error: &error];
}
//Handel the deserialized object data
if (jsonObject != nil && error == nil){
NSLog(#"Successfully deserialized...");
if ([jsonObject isKindOfClass:[NSDictionary class]]){
resultBlock = (NSDictionary *)jsonObject;
//NSLog(#"Deserialized JSON Dictionary = %#", resultBlock);
}
else if ([jsonObject isKindOfClass:[NSArray class]]){
NSArray *deserializedArray = (NSArray *)jsonObject;
NSLog(#"Deserialized JSON Array = %#", deserializedArray);
} else {
/* Some other object was returned. We don't know how to deal
with this situation, as the deserializer returns only dictionaries
or arrays */
}
}
else if (error != nil){
NSLog(#"An error happened while deserializing the JSON data. %#", error);
}else{
NSLog(#"No data could get downloaded from the URL.");
//[self conn:method];
}
}
});
dispatch_sync(dispatch_get_main_queue(), ^{
/* Check if the resultBlock is not nil*/
if(resultBlock != nil){
/*Set the value of result. This will notify the observer*/
[self setResult:resultBlock];
}
});
});
}
Why do I get the following error?
An error happened while deserializing the JSON data. Error
Domain=NSCocoaErrorDomain Code=3840 "The operation couldn’t be
completed. (Cocoa error 3840.)" (JSON text did not start with array or
object and option to allow fragments not set.) UserInfo=0x20839f80
{NSDebugDescription=JSON text did not start with array or object and
option to allow fragments not set.}
When I change it to
/* Now try to deserialize the JSON object into a dictionary */
jsonObject = [NSJSONSerialization
JSONObjectWithData:jsonData
options:NSJSONReadingAllowFragments
error: &error];
}
I get the following error:
An error happened while deserializing the JSON data. Error
Domain=NSCocoaErrorDomain Code=3840 "The operation couldn’t be
completed. (Cocoa error 3840.)" (Invalid value around character 0.)
UserInfo=0x20888760 {NSDebugDescription=Invalid value around character
0.}
I changed my connection from LTE to wifi and now I get
504 error and NSLog(#"No data could get downloaded from the URL.");
You should fix these issues in your code first:
Properly check for errors in methods which provide a pointer to a reference to an NSError object as the last parameter, e.g.: - (BOOL) doSomething:(NSError**)error, or -(NSData*) doSomething:(NSError**)error
In order test for an error correctly, you have to check the return value of the method only. Those methods indicate an error condition with a "special return value". For example, they return NO or nil - as always specified in the documentation. Only after the method indicated an error, the provided error parameter contains a meaningful value - that is, it points to an NSError object created by the method. Note that this parameter may also become none NULL when the method succeeded, in which case that has no "meaning".
Web services usually can provide several formats of the requested resource. If you don't specify which format you want the server to encode the resource, you get a default format - which is not necessarily JSON.
In order to be explicit about the desired format of the resource, set a corresponding "Accept" header. For example, if you wish the format in JSON you would set a header: "Accept: application/json" in your request.
Web services may have reasons not to respond with the resource you requested. In order to be sure you got the response that you requested, you need to check the response for status code and MIME type in order to ensure you actually received a JSON response.
It seems, you are a bit uncertain about how to use dispatch functions to your advantage. If you use the synchronous convenient method sendSynchronousRequest:... You certainly need to wrap it in only one dispatch_async function. If you then want to set the result on the main thread, you certainly want to use dispatch_async, not dispatch_sync.
However, it would be an improvement if you would use sendAsynchronousRequest:... instead. And only if you would use NSURLConnection in asynchronous mode and implement the NSURLConnection delegate methods - which I strongly recommend - it would actually become great ;)
So, I think, once you fixed your code, you may be able to answer the original question yourself, or get better error responses from the server, or the error magically disappeared ;)
Xcode 4.3
I've read the SO questions on NSError**, so I wrote a simple test program that uses a slightly different syntax recommended by Xcode 4.3 (see __autoreleasing below), so I'm not 100% sure if this is correct, although the code does appear to function properly. Anyway, just a simple file reader, prints an error if the file can't be found.
Questions
Would like to know if the NSError initialization, argument passing using &, and error condition checking are correct.
Also, in the readFileAndSplit.. method, I noticed a big difference between if(!*error) and if(!error), in fact, if(!error) does not work when no error condition is raised.
File Reading Method w/Possible Error Condition
-(NSArray*) readFileAndSplitLinesIntoArray:(NSError *__autoreleasing *) error {
NSString* rawFileContents =
[NSString stringWithContentsOfFile:#"props.txt"
encoding:NSUTF8StringEncoding
error:error
NSArray* fileContentsAsArray = nil;
if(!*error)
fileContentsAsArray =
[rawFileContents componentsSeparatedByCharactersInSet:[NSCharacterSet newlineCharacterSet]];
return fileContentsAsArray;
Caller
SimpleFileReader* reader = ...
NSError* fileError = nil;
NSArray* array = [reader readFileAndSplitLinesIntoArray: &fileError];
if(fileError){
NSLog(#"Error was : %#, with code: %li",
[fileError localizedDescription],(long)[fileError code]);
}
There are a couple of issues.
First, As per Apple's Error Handling Programming Guide, you should be checking a method's return value to determine whether a method failed or not, and not NSError. You only use NSError to get additional error information in the event that the method failed.
E.g.:
NSArray* fileContentsAsArray = nil;
NSString* rawFileContents = [NSString stringWithContentsOfFile:#"props.txt"
encoding:NSUTF8StringEncoding
error:error];
if (rawFileContents)
{
// Method succeeded
fileContentsAsArray = [rawFileContents ...];
}
return fileContentsAsArray; // may be nil
Second, NSError out parameters are typically optional and may be NULL. But if you pass a NULL error variable into your method it will crash on this line:
if (!*error) {
because you're dereferencing a NULL pointer. Instead, you must always check for NULL before referencing a pointer, like so:
if (error && *error)
{
// Do something with the error info
}
However, if you rewrite the method as indicated above then you won't be accessing the error variable at all.
I got into the habit of coding my error handling this way:
NSError* error = nil;
NSDictionary *attribs = [[NSFileManager defaultManager] removeItemAtPath:fullPath error:&error];
if (error != nil) {
DLogErr(#"Unable to remove file: error %#, %#", error, [error userInfo]);
return;
}
But looking at the documentation It seems like I got this wrong.:
- (BOOL)removeItemAtPath:(NSString *)path error:(NSError **)error
If an error occurs, upon return contains an NSError object that describes the problem. Pass NULL if you do not want error information.
Technically there is no difference between nil and NULL so does this mean I'm actually turning this off and will never get a error message (even if the delete in the above example did fail) ?
Is there a better way to code this ?
Thanks.
First off, the following line doesn't really make sense:
NSDictionary *attribs = [[NSFileManager defaultManager]
removeItemAtPath:fullPath error:&error];
-removeItemAtPath:error: returns a BOOL value, not a dictionary.
I think I see what you’re wondering about with the NULL value. Notice carefully though, how there are 2 *'s in the error parameter in the method signature:
- (BOOL)removeItemAtPath:(NSString *)path error:(NSError **)error
That means a pointer to a pointer. When you pass in &error, you are passing in the address of the pointer to the NSError. (Ugh, someone else can probably help me out here, as my head still starts to swim when dealing with pointers to pointers). In other words, even though you have set error to nil, you aren't passing in error to the method, you're passing in &error.
So, here’s what the re-written method should look like:
// If you want error detection:
NSError *error = nil;
if (![[NSFileManager defaultManager] removeItemAtPath:fullPath
error:&error]) {
NSLog(#"failed to remove item at path; error == %#", error);
// no need to log userInfo separately
return;
}
// If you don't:
if (![[NSFileManager defaultManager] removeItemAtPath:fullPath
error:NULL]) {
NSLog(#"failed to remove item at path");
return;
}
Passing NULL means the following:
BOOL itemRemoved = [[NSFileManager defaultManager] removeItemAtPath:fullPath
error:NULL];
i.e., the error parameter is NULL. Internally, -removeItemAtPath:error: sees if a valid pointer was passed. If it’s NULL, it simply won’t report the error as an NSError instance — but the return value will indicate whether the method completed successfully.
Also, your test is wrong. You shouldn’t be using the error output parameter to detect if an error occurred because it might be set even if the method completes successfully. Instead, you should use the return value of the method to detect errors. If the return value (in this particular case) is NO, then use the error output parameter to get information about the error:
NSError *error = nil;
BOOL itemRemoved = [[NSFileManager defaultManager] removeItemAtPath:fullPath error:&error];
if (itemRemoved == NO) {
DLogErr(#"Unable to remove file: error %#, %#", error, [error userInfo]);
return;
}
Quoting the Error Handling Programming Guide,
Important: Success or failure is indicated by the return value of the method. Although Cocoa methods that indirectly return error objects in the Cocoa error domain are guaranteed to return such objects if the method indicates failure by directly returning nil or NO, you should always check that the return value is nil or NO before attempting to do anything with the NSError object.
Edit: As NSGod pointed out, -removeItemAtPath:error: returns BOOL, not NSDictionary *. I’ve edited my answer to reflect that as well.
No I do it the same way and it works just fine for detecting errors. You are not passing NULL to it you are passing a pointer to NULL to it which is a very different thing. Although another option you might want to add is.
if (error != nil){...
}else{
[NSApp presentError:error]
}