I have 3 entities that I generated with MOGenerator, I'd like to be able to get one of them back from their objectID
I tried this :
- (void)aMethod: (SpecialEntity1ID *)entityID
{
//This is a method from MagicalRecord but it doesn't matter(I think...).
NSManagedObjectContext *context = [NSManagedObjectContext MR_contextWithParent:[NSManagedObjectContext MR_defaultContext]];
SpecialEntity1 *entity1 = [context objectRegisteredForID:entityID]
//But this returns an NSManagedObject so it doesn't work...
}
Could someone help me get this object back with its ID ?
Since I don't know how to do it with the ID I'm currently working around it by making a method with an NSStringas a paramater instead of SecialEntity1ID that defines one of the attribute of this object (and is unique) and fetching the object.
I think getting back with his ID is better so any idea ?
You want to use existingObjectWithID:error: method of your NSManagedObjectContext and typecast the return type if you are 100% sure what it will be. I'd keep it generic i.e. let it return an NSManagedObject and then test its class elsewhere if you want to determine whether it belongs to a particular class.
- (Object*)retrieveObjectWithID:(ObjectID*)theID
{
NSError *error = nil;
Object *theObject = (Object*)[[NSManagedObjectContext contextForCurrentThread] existingObjectWithID:theID error:&error];
if (error)
NSLog (#"Error retrieving object with ID %#: %#", theID, error);
return theObject;
}
is there a shorter way to do this?
iam fetching all core data objects with some predicate and then parsing them to JSON, using RestKit and defined mapping... i still want to use RestKit, but cant it be done more simple?
- (NSArray *)serializedManagedObjectClass:(Class)managedObjectClass updatedSinceRevision:(long long)revision {
NSArray * managedObjects = [managedObjectClass MR_findAllSortedBy:#"createdDate" ascending:YES withPredicate:[NSPredicate predicateWithFormat:#"revision > %lld", revision]];
NSError * error = nil;
NSMutableArray * serializedObjects = [[NSMutableArray alloc] initWithCapacity:[managedObjects count]];
for (NSManagedObject * managedObject in managedObjects) {
id object = [NSMutableDictionary new];
RKObjectMappingOperation * op = [RKObjectMappingOperation mappingOperationFromObject:managedObject toObject:object withMapping:[self.objectManager.mappingProvider serializationMappingForClass:managedObjectClass]];
BOOL success = [op performMapping:&error];
if ((success) && (error == nil) && (object)) {
[serializedObjects addObject:object];
} else {
ALog(#"Serialization problem with %#, possible error %#", managedObject, error);
}
}
return serializedObjects;
}
You don't need to map each object individually. As long as you have set the serialization mapping for that class, RestKit will handle the serialization when you try to send instances of that object. It looks like you have already created and assigned the mapping for the class.
Now, just add your array of managed objects to your dictionary of parameters that are being sent to the server. RestKit should then map each object according to the mapping provided.
why is "error:&error" used here (objective-c)
NSError *error = nil;
NSArray *array = [moc executeFetchRequest:request error:&error];
wouldn't an object in objective-c be effectively pass-by-reference anyway?
The argument type for error: is NSError** (i.e. a pointer to a pointer to an object). This permits the moc object to allocate and initialize a new NSError object as required. It is a common pattern, especially in Cocoa.
The NSError documentation gives some indication of the motivation for this approach:
Applications may choose to create subclasses of NSError to provide better localized error strings by overriding localizedDescription.
Passing in an NSError** argument allows that method to return any subclass of NSError that makes sense. If you passed in NSError*, you would have to supply an existing NSError object, and there would be no way for the method to return a different object from the one you passed in.
To be clear, the method could look something like this:
- (NSArray*)executeFetchRequest:(Request *)request error:(NSError**)error {
...
if ((error != NULL) && (some_error_condition)) {
*error = [[[SomeNSErrorSubclass alloc] init...] autorelease];
return nil;
}
}
Note that this also allows the calling code to ignore errors by simply passing in NULL for the error: parameter, as follows:
NSArray *array = [moc executeFetchRequest:request error:NULL];
Update: (in response to questions):
There are two reasons why the argument type has to be NSError** instead of NSError*: 1. variable scoping rules, and 2. NSError instances are imutable.
Reason #1: variable scoping rules
Let's assume that the function declaration were to look like this:
- (NSArray*)executeFetchRequest:(Request *)request error:(NSError*)error;
And we were to call the function like this:
NSError * error = nil;
[someArray executeFetchRequest:someRequest error:error];
if (error != nil) { /* handle error */ }
When you pass in a variable this way, the function body will not be able to modify the value of that variable (i.e. the function body will not be able to create a new variable to replace the existing one). For example, the following variable assignments will exist only in the local scope of the function. The calling code will still see error == nil.
- (NSArray*)executeFetchRequest:(Request *)request error:(NSError*)error {
...
error = [[[NSError alloc] init...] autorelease]; // local only
error = [[[SomeNSErrorSubclass alloc] init...] autorelease]; // local only
}
Reason #2: instances of NSError are immutable
Let's keep the same function declaration, but call the function like this:
NSError * error = [[[NSError alloc] init...] autorelease];
[someArray executeFetchRequest:someRequest error:error];
if (error != nil) { /* handle error */ }
First of all, the variable scoping rules guarantee that error can not be nil, so the if (error != nil) { ... condition will always be true, but even if you wanted to check for specific error information inside the if block, you would be out of luck because instances of NSError are immutable. This means that once they are created, you cannot modify their properties, so the function would not be able to change the domain or userInfo of that NSError instance that you created in the calling code.
- (NSArray*)executeFetchRequest:(Request *)request error:(NSError*)error {
...
error.domain = ... // not allowed!
error.userInfo = ... // not allowed!
}
It's effectively another return value. The error is not dominant by convention in Cocoa when there is a return value for the operation. When an error is encountered, it may be returned to you by this out parameter.
In the case of NSError, it works this way because NSError is not a mutable type - its fields are set at initialization and never mutated. Therefore, you cannot pass an NSError as usual and set the error code.
I am working on catching errors in my app, and I am looking into using NSError. I am slightly confused about how to use it, and how to populate it.
Could someone provide an example on how I populate then use NSError?
Well, what I usually do is have my methods that could error-out at runtime take a reference to a NSError pointer. If something does indeed go wrong in that method, I can populate the NSError reference with error data and return nil from the method.
Example:
- (id) endWorldHunger:(id)largeAmountsOfMonies error:(NSError**)error {
// begin feeding the world's children...
// it's all going well until....
if (ohNoImOutOfMonies) {
// sad, we can't solve world hunger, but we can let people know what went wrong!
// init dictionary to be used to populate error object
NSMutableDictionary* details = [NSMutableDictionary dictionary];
[details setValue:#"ran out of money" forKey:NSLocalizedDescriptionKey];
// populate the error object with the details
*error = [NSError errorWithDomain:#"world" code:200 userInfo:details];
// we couldn't feed the world's children...return nil..sniffle...sniffle
return nil;
}
// wohoo! We fed the world's children. The world is now in lots of debt. But who cares?
return YES;
}
We can then use the method like this. Don't even bother to inspect the error object unless the method returns nil:
// initialize NSError object
NSError* error = nil;
// try to feed the world
id yayOrNay = [self endWorldHunger:smallAmountsOfMonies error:&error];
if (!yayOrNay) {
// inspect error
NSLog(#"%#", [error localizedDescription]);
}
// otherwise the world has been fed. Wow, your code must rock.
We were able to access the error's localizedDescription because we set a value for NSLocalizedDescriptionKey.
The best place for more information is Apple's documentation. It really is good.
There is also a nice, simple tutorial on Cocoa Is My Girlfriend.
I would like to add some more suggestions based on my most recent implementation. I've looked at some code from Apple and I think my code behaves in much the same way.
The posts above already explain how to create NSError objects and return them, so I won't bother with that part. I'll just try to suggest a good way to integrate errors (codes, messages) in your own app.
I recommend creating 1 header that will be an overview of all the errors of your domain (i.e. app, library, etc..). My current header looks like this:
FSError.h
FOUNDATION_EXPORT NSString *const FSMyAppErrorDomain;
enum {
FSUserNotLoggedInError = 1000,
FSUserLogoutFailedError,
FSProfileParsingFailedError,
FSProfileBadLoginError,
FSFNIDParsingFailedError,
};
FSError.m
#import "FSError.h"
NSString *const FSMyAppErrorDomain = #"com.felis.myapp";
Now when using the above values for errors, Apple will create some basic standard error message for your app. An error could be created like the following:
+ (FSProfileInfo *)profileInfoWithData:(NSData *)data error:(NSError **)error
{
FSProfileInfo *profileInfo = [[FSProfileInfo alloc] init];
if (profileInfo)
{
/* ... lots of parsing code here ... */
if (profileInfo.username == nil)
{
*error = [NSError errorWithDomain:FSMyAppErrorDomain code:FSProfileParsingFailedError userInfo:nil];
return nil;
}
}
return profileInfo;
}
The standard Apple-generated error message (error.localizedDescription) for the above code will look like the following:
Error Domain=com.felis.myapp Code=1002 "The operation couldn’t be completed. (com.felis.myapp error 1002.)"
The above is already quite helpful for a developer, since the message displays the domain where the error occured and the corresponding error code. End users will have no clue what error code 1002 means though, so now we need to implement some nice messages for each code.
For the error messages we have to keep localisation in mind (even if we don't implement localized messages right away). I've used the following approach in my current project:
1) create a strings file that will contain the errors. Strings files are easily localizable. The file could look like the following:
FSError.strings
"1000" = "User not logged in.";
"1001" = "Logout failed.";
"1002" = "Parser failed.";
"1003" = "Incorrect username or password.";
"1004" = "Failed to parse FNID."
2) Add macros to convert integer codes to localized error messages. I've used 2 macros in my Constants+Macros.h file. I always include this file in the prefix header (MyApp-Prefix.pch) for convenience.
Constants+Macros.h
// error handling ...
#define FS_ERROR_KEY(code) [NSString stringWithFormat:#"%d", code]
#define FS_ERROR_LOCALIZED_DESCRIPTION(code) NSLocalizedStringFromTable(FS_ERROR_KEY(code), #"FSError", nil)
3) Now it's easy to show a user friendly error message based on an error code. An example:
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"Error"
message:FS_ERROR_LOCALIZED_DESCRIPTION(error.code)
delegate:nil
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[alert show];
Great answer Alex. One potential issue is the NULL dereference. Apple's reference on Creating and Returning NSError objects
...
[details setValue:#"ran out of money" forKey:NSLocalizedDescriptionKey];
if (error != NULL) {
// populate the error object with the details
*error = [NSError errorWithDomain:#"world" code:200 userInfo:details];
}
// we couldn't feed the world's children...return nil..sniffle...sniffle
return nil;
...
Objective-C
NSError *err = [NSError errorWithDomain:#"some_domain"
code:100
userInfo:#{
NSLocalizedDescriptionKey:#"Something went wrong"
}];
Swift 3
let error = NSError(domain: "some_domain",
code: 100,
userInfo: [NSLocalizedDescriptionKey: "Something went wrong"])
Please refer following tutorial
i hope it will helpful for you but prior you have to read documentation of NSError
This is very interesting link i found recently ErrorHandling
I'll try summarize the great answer by Alex and the jlmendezbonini's point, adding a modification that will make everything ARC compatible (so far it's not since ARC will complain since you should return id, which means "any object", but BOOL is not an object type).
- (BOOL) endWorldHunger:(id)largeAmountsOfMonies error:(NSError**)error {
// begin feeding the world's children...
// it's all going well until....
if (ohNoImOutOfMonies) {
// sad, we can't solve world hunger, but we can let people know what went wrong!
// init dictionary to be used to populate error object
NSMutableDictionary* details = [NSMutableDictionary dictionary];
[details setValue:#"ran out of money" forKey:NSLocalizedDescriptionKey];
// populate the error object with the details
if (error != NULL) {
// populate the error object with the details
*error = [NSError errorWithDomain:#"world" code:200 userInfo:details];
}
// we couldn't feed the world's children...return nil..sniffle...sniffle
return NO;
}
// wohoo! We fed the world's children. The world is now in lots of debt. But who cares?
return YES;
}
Now instead of checking for the return value of our method call, we check whether error is still nil. If it's not we have a problem.
// initialize NSError object
NSError* error = nil;
// try to feed the world
BOOL success = [self endWorldHunger:smallAmountsOfMonies error:&error];
if (!success) {
// inspect error
NSLog(#"%#", [error localizedDescription]);
}
// otherwise the world has been fed. Wow, your code must rock.
Another design pattern that I have seen involves using blocks, which is especially useful when a method is being run asynchronously.
Say we have the following error codes defined:
typedef NS_ENUM(NSInteger, MyErrorCodes) {
MyErrorCodesEmptyString = 500,
MyErrorCodesInvalidURL,
MyErrorCodesUnableToReachHost,
};
You would define your method that can raise an error like so:
- (void)getContentsOfURL:(NSString *)path success:(void(^)(NSString *html))success failure:(void(^)(NSError *error))failure {
if (path.length == 0) {
if (failure) {
failure([NSError errorWithDomain:#"com.example" code:MyErrorCodesEmptyString userInfo:nil]);
}
return;
}
NSString *htmlContents = #"";
// Exercise for the reader: get the contents at that URL or raise another error.
if (success) {
success(htmlContents);
}
}
And then when you call it, you don't need to worry about declaring the NSError object (code completion will do it for you), or checking the returning value. You can just supply two blocks: one that will get called when there is an exception, and one that gets called when it succeeds:
[self getContentsOfURL:#"http://google.com" success:^(NSString *html) {
NSLog(#"Contents: %#", html);
} failure:^(NSError *error) {
NSLog(#"Failed to get contents: %#", error);
if (error.code == MyErrorCodesEmptyString) { // make sure to check the domain too
NSLog(#"You must provide a non-empty string");
}
}];
extension NSError {
static func defaultError() -> NSError {
return NSError(domain: "com.app.error.domain", code: 0, userInfo: [NSLocalizedDescriptionKey: "Something went wrong."])
}
}
which I can use NSError.defaultError() whenever I don't have valid error object.
let error = NSError.defaultError()
print(error.localizedDescription) //Something went wrong.
Well it's a little bit out of question scope but in case you don't have an option for NSError you can always display the Low level error:
NSLog(#"Error = %# ",[NSString stringWithUTF8String:strerror(errno)]);
Two hopefully minor questions regarding CoreData that I've been unable to find answers to:
1) I have a faulted object. Accessing an attribute as a property is not firing the fault, accessing the same property via KVC IS firing the fault. Any idea why?
i.e. object.title returns nil and object is still faulted, but [object valueForKey:#"title"] returns the title and the object is no longer a fault.
2) Updates to existing records have stopped working. Add/Delete works. Add/Update share the same code path (one is passed the existing object, the other a newly inserted object). However Update wont work. The data in the updated object is correct and set to the new values and the save succeeds with no errors, but the record in the database remains unchanged. Any idea?
NB: There is only one NSManagedObjectContext
Cheers
couldn't tell much from your description without code.
however it looks like you have updated the object in ram but the update wasn't submitted to the database layer making the physical change.
EDIT:
Yes, "Add" and "Delete" is different from "edit/update" a record.
for performance reason mapped objects are saved in memory as entities when you doing manipulation against NSManagedObjectContext you are not coding against database entirely.
check the link below:
http://cocoawithlove.com/2010/02/differences-between-core-data-and.html
normal work flow:
load appropriate rows from a database
instantiate objects from these rows
make changes to the graph objects
that are now in memory
commit the changes back to the
database
This is my core data for saving.
AppDelegate *app = (AppDelegate*)[[UIApplication sharedApplication] delegate];
Tweet *newTweet = (Tweet *)[NSEntityDescription insertNewObjectForEntityForName:#"Tweet" inManagedObjectContext:app.managedObjectContext];
newTweet.status = status;
newTweet.post_date = theDate;
newTweet.post_id = post_id;
newTweet.sent_error = sent_error;
newTweet.sent_status = sent_status;
newTweet.screen_name = [Settings getActiveScreenName];
// SAVE
NSError* error = nil;
if (![app.managedObjectContext save:&error]) {NSLog(#"did this work?? = %# with userInfo = %#", error, [error userInfo]);}
I have this in my app delegate
- (void)applicationWillTerminate:(UIApplication *)application {
// need to check if TweetViewController is activel.
// User is writing a tweet.
UIViewController * topController = [navigationController visibleViewController];
if([topController isKindOfClass:[TweetViewController class]] ){
[Settings setObject:[(TweetViewController*)topController tweetText].text forKey:#"last_tweet_text"];
}
NSLog(#"good bye");
NSError *error;
if (managedObjectContext != nil) {
if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error]) {
// Handle error.
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
exit(-1); // Fail
}
}
}
and this as well in AppDelegate
/**
Returns the persistent store coordinator for the application.
If the coordinator doesn't already exist, it is created and the application's store added to it.
*/
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
if (persistentStoreCoordinator != nil) {
return persistentStoreCoordinator;
}
// ~/Library/Application Support/iPhone Simulator/User/
NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory] stringByAppendingPathComponent: #"tweetv12.sqlite"]];
NSError *error;
persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: [self managedObjectModel]];
if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:nil error:&error]) {
// Handle error
NSLog(#"cannot save data, change db name.");
}
return persistentStoreCoordinator;
}
I am also able to save delete and update data with this.