I'm using Magical Record 2.3.0 beta 5 and I have troubles understanding how to get my NSManagedObjects for the current thread. I have a long running NSOperation where I need my PSPlayer (NSManagedObject).
When I init the NSOperation, I keep an id of my PSPlayer and re-fetch the same object in the operation's main method. According to Apple that the way to do it.
#implementation TAPlayerUpdateOperation
- (instancetype)initWithPlayer:(PSPlayer *)player;
{
self = [super init];
if (self) {
self.playerMD5Id = player.md5Id;
}
}
- (void)main
{
#autoreleasepool {
__block BOOL keepUpdating = YES;
PSPlayer *player = [[PSPlayer MR_findAllWithPredicate:[NSPredicate predicateWithFormat:#"md5Id == %#", self.playerMD5Id]] firstObject];
NSLog(#"player.md5Id = %#", player.md5Id);
// rest of my operation logic
}
}
#end
When I run my app with -com.apple.CoreData.ConcurrencyDebug 1, I get a crash when accessing the property in the NSLog statement.
What is the correct way to get my NSManagedObject so that it is safe for the current thread?
I've pinned the problem down to the following snippet where it crashes as well.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
PSPlayer *player =[[PSPlayer MR_findAll] firstObject];
NSLog(#"player = %#", player.name);
});
cheers,
Jan
You need to ensure that everything is saved and merged before the fetch would work. If you're using MR then it's better to take the managed object and call inContext: on it supplying the other context and have it do the work (it also avoids a predicate).
I expect the crash is because you use player.md5Id instead of self.playerMD5Id so you're accessinh the managed object on the wrong thread.
Related
I have something along the line of this :
#implementation ImageLoader
NSMutableDictionary *_tasks;
- (void) loadImageWithURL:(NSURL *)url callback:(SMLoaderCallback)callback {
NSMutableArray *taskList = [_tasks objectForKey:urlString];
if (taskList == nil) {
taskList = [[NSMutableArray alloc] initWithCapacity:5];
[taskList addObject:callback];
[_tasks setObject:taskList forKey:urlString];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *image = [[UIImage alloc] initWithData:data];
for (SMLoaderCallback cb in taskList) {
cb(image, nil);
}
[_tasks removeObjectForKey:url.absoluteString];
});
});
} else {
[taskList addObject:callback];
}
}
#end
In here, I'm trying to queue up image downloads to gain performance (as an exercise only). So I'm keeping a NSDictionary that maps an URL with an array of callbacks to be called whenever the data is downloaded.
Once the image is downloaded, I no longer need this array, so I remove it from the dictionary.
I would like to know if, in this code (with ARC enabled), my array along with the callbacks are correctly released when I call [_tasks removeObjectForKey:url.absoluteString].
If your project uses ARC and that dictionary is the only thing that is referencing to those values, yes. it will be remove permanently.
ARC keeps track of number of objects that is pointing to some other object and it will remove it as soon as the count reaches 0.
so adding to dictionary -> reference count += 1
removing from dictionary -> reference count -= 1
Somewhe
I'm trying to write a simple (toy) program that uses the NSFilePresenter and NSFileCoordinator methods to watch a file for changes.
The program consists of a text view that loads a (hardcoded) text file and a button that will save the file with any changes. The idea is that I have two instances running and saving in one instance will cause the other instance to reload the changed file.
Loading and saving the file works fine but the NSFilePresenter methods are never called. It is all based around a class called FileManager which implements the NSFilePresenter protocol. The code is as follows:
Interface:
#interface FileManager : NSObject <NSFilePresenter>
#property (unsafe_unretained) IBOutlet NSTextView *textView;
- (void) saveFile;
- (void) reloadFile;
#end
Implementation:
#implementation FileManager
{
NSOperationQueue* queue;
NSURL* fileURL;
}
- (id) init {
self = [super init];
if (self) {
self->queue = [NSOperationQueue new];
self->fileURL = [NSURL URLWithString:#"/Users/Jonathan/file.txt"];
[NSFileCoordinator addFilePresenter:self];
}
return self;
}
- (NSURL*) presentedItemURL {
NSLog(#"presentedItemURL");
return self->fileURL;
}
- (NSOperationQueue*) presentedItemOperationQueue {
NSLog(#"presentedItemOperationQueue");
return self->queue;
}
- (void) saveFile {
NSFileCoordinator* coordinator = [[NSFileCoordinator alloc] initWithFilePresenter:self];
NSError* error;
[coordinator coordinateWritingItemAtURL:self->fileURL options:NSFileCoordinatorWritingForMerging error:&error byAccessor:^(NSURL* url) {
NSString* content = [self.textView string];
[content writeToFile:[url path] atomically:YES encoding:NSUTF8StringEncoding error:NULL];
}];
}
- (void) reloadFile {
NSFileManager* fileManager = [NSFileManager defaultManager];
NSFileCoordinator* coordinator = [[NSFileCoordinator alloc] initWithFilePresenter:self];
NSError* error;
__block NSData* content;
[coordinator coordinateReadingItemAtURL:self->fileURL options:0 error:&error byAccessor:^(NSURL* url) {
if ([fileManager fileExistsAtPath:[url path]]) {
content = [fileManager contentsAtPath:[url path]];
}
}];
dispatch_async(dispatch_get_main_queue(), ^{
[self.textView setString:[[NSString alloc] initWithData:content encoding:NSUTF8StringEncoding]];
});
}
// After this I implement *every* method in the NSFilePresenter protocol. Each one
// simply logs its method name (so I can see it has been called) and calls reloadFile
// (not the correct implementation for all of them I know, but good enough for now).
#end
Note, reloadFile is called in applicationDidFinishLaunching and saveFile gets called every time the save button is click (via the app delegate).
The only NSFilePresenter method that ever gets called (going by the logs) is presentedItemURL (which gets called four times when the program starts and loads the file and three times whenever save is clicked. Clicking save in a second instance has no noticeable effect on the first instance.
Can anyone tell me what I'm doing wrong here?
I was struggling with this exact issue for quite a while. For me, the only method that would be called was -presentedSubitemDidChangeAtURL: (I was monitoring a directory rather than a file). I opened a technical support issue with Apple, and their response was that this is a bug, and the only thing we can do right now is to do everything through -presentedSubitemDidChangeAtURL: if you're monitoring a directory. Not sure what can be done when monitoring a file.
I would encourage anyone encountering this issue to file a bug (https://bugreport.apple.com) to encourage Apple to get this problem fixed as soon as possible.
(I realize that this is an old question, but... :) )
First of all, I notice you don't have [NSFileCoordinator removeFilePresenter:self]; anywhere (it should be in dealloc).
Secondly, you wrote:
// After this I implement *every* method in the NSFilePresenter protocol. Each one
// simply logs its method name (so I can see it has been called) and calls reloadFile
// (not the correct implementation for all of them I know, but good enough for now).
You're right: it's the incorrect implementation! And you're wrong: it's not good enough, because it's essential for methods like accommodatePresentedItemDeletionWithCompletionHandler: which take a completion block as a parameter, that you actually call this completion block whenever you implement them, e.g.
- (void) savePresentedItemChangesWithCompletionHandler:(void (^)(NSError * _Nullable))completionHandler
{
// implement your save routine here, but only if you need to!
if ( dataHasChanged ) [self save]; // <-- meta code
//
NSError * err = nil; // <-- = no error, in this simple implementation
completionHandler(err); // <-- essential!
}
I don't know whether this is the reason your protocol methods are not being called, but it's certainly a place to start. Well, assuming you haven't already worked out what was wrong in the past three years! :-)
This is my first question on Stack Overflow, so please excuse me if I'm breaking any etiquette. I'm also fairly new to Objective-C/app creation.
I have been following the CS193P Stanford course, in particular, the CoreData lectures/demos. In Paul Hegarty's Photomania app, he starts with a table view, and populates the data in the background, without any interruption to the UI flow. I have been creating an application which lists businesses in the local area (from an api that returns JSON data).
I have created the categories as per Paul's photo/photographer classes. The creation of the classes themselves is not an issue, it's where they are being created.
A simplified data structure:
- Section
- Sub-section
- business
- business
- business
- business
- business
- business
My application starts with a UIViewController with several buttons, each of which opens a tableview for the corresponding section (these all work fine, I'm trying to provide enough information so that my question makes sense). I call a helper method to create/open the URL for the UIManagedDocument, which was based on this question. This is called as soon as the application runs, and it loads up quickly.
I have a method very similar to Paul's fetchFlickrDataIntoDocument:
-(void)refreshBusinessesInDocument:(UIManagedDocument *)document
{
dispatch_queue_t refreshBusinessQ = dispatch_queue_create("Refresh Business Listing", NULL);
dispatch_async(refreshBusinessQ, ^{
// Get latest business listing
myFunctions *myFunctions = [[myFunctions alloc] init];
NSArray *businesses = [myFunctions arrayOfBusinesses];
// Run IN document's thread
[document.managedObjectContext performBlock:^{
// Loop through new businesses and insert
for (NSDictionary *businessData in businesses) {
[Business businessWithJSONInfo:businessData inManageObjectContext:document.managedObjectContext];
}
// Explicitly save the document.
[document saveToURL:document.fileURL
forSaveOperation:UIDocumentSaveForOverwriting
completionHandler:^(BOOL success){
if (!success) {
NSLog(#"Document save failed");
}
}];
NSLog(#"Inserted Businesses");
}];
});
dispatch_release(refreshBusinessQ);
}
[myFunctions arrayOfBusinesses] just parses the JSON data and returns an NSArray containing individual businessses.
I have run the code with an NSLog at the start and end of the business creation code. Each business is assigned a section, takes 0.006 seconds to create, and there are several hundred of these. The insert ends up taking about 2 seconds.
The Helper Method is here:
// The following typedef has been defined in the .h file
// typedef void (^completion_block_t)(UIManagedDocument *document);
#implementation ManagedDocumentHelper
+(void)openDocument:(NSString *)documentName UsingBlock:(completion_block_t)completionBlock
{
// Get URL for document -> "<Documents directory>/<documentName>"
NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
url = [url URLByAppendingPathComponent:documentName];
// Attempt retrieval of existing document
UIManagedDocument *doc = [managedDocumentDictionary objectForKey:documentName];
// If no UIManagedDocument, create
if (!doc)
{
// Create with document at URL
doc = [[UIManagedDocument alloc] initWithFileURL:url];
// Save in managedDocumentDictionary
[managedDocumentDictionary setObject:doc forKey:documentName];
}
// If the document exists on disk
if ([[NSFileManager defaultManager] fileExistsAtPath:[url path]])
{
[doc openWithCompletionHandler:^(BOOL success)
{
// Run completion block
completionBlock(doc);
} ];
}
else
{
// Save temporary document to documents directory
[doc saveToURL:url
forSaveOperation:UIDocumentSaveForCreating
completionHandler:^(BOOL success)
{
// Run compeltion block
completionBlock(doc);
}];
}
}
And is called in viewDidLoad:
if (!self.lgtbDatabase) {
[ManagedDocumentHelper openDocument:#"DefaultLGTBDatabase" UsingBlock:^(UIManagedDocument *document){
[self useDocument:document];
}];
}
useDocument just sets self.document to the provided document.
I would like to alter this code to so that the data is inserted in another thread, and the user can still click a button to view a section, without the data import hanging the UI.
Any help would be appreciated I have worked on this issue for a couple of days and not been able to solve it, even with the other similar questions on here. If there's any other information you require, please let me know!
Thank you
EDIT:
So far this question has received one down vote. If there is a way I could improve this question, or someone knows of a question I've not been able to find, could you please comment as to how or where? If there is another reason you are downvoting, please let me know, as I'm not able to understand the negativity, and would love to learn how to contribute better.
There are a couple of ways to this.
Since you are using UIManagedDocument you could take advantage of NSPrivateQueueConcurrencyType for initialize a new NSManagedObjectContext and use performBlock to do your stuff. For example:
// create a context with a private queue so access happens on a separate thread.
NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
// insert this context into the current context hierarchy
context.parentContext = parentContext;
// execute the block on the queue of the context
context.performBlock:^{
// do your stuff (e.g. a long import operation)
// save the context here
// with parent/child contexts saving a context push the changes out of the current context
NSError* error = nil;
[context save:&error];
}];
When you save from the context, data of the private context are pushed to the current context. The saving is only visible in memory, so you need to access the main context (the one linked to the UIDocument) and do a save there (take a look at does-a-core-data-parent-managedobjectcontext-need-to-share-a-concurrency-type-wi).
The other way (my favourite one) is to create a NSOperation subclass and do stuff there. For example, declare a NSOperation subclass like the following:
//.h
#interface MyOperation : NSOperation
- (id)initWithDocument:(UIManagedDocument*)document;
#end
//.m
#interface MyOperation()
#property (nonatomic, weak) UIManagedDocument *document;
#end
- (id)initWithDocument:(UIManagedDocument*)doc;
{
if (!(self = [super init])) return nil;
[self setDocument:doc];
return self;
}
- (void)main
{
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init];
[moc setParentContext:[[self document] managedObjectContext]];
// do the long stuff here...
NSError *error = nil;
[moc save:&error];
NSManagedObjectContext *mainMOC = [[self document] managedObjectContext];
[mainMOC performBlock:^{
NSError *error = nil;
[mainMOC save:&error];
}];
// maybe you want to notify the main thread you have finished to import data, if you post a notification remember to deal with it in the main thread...
}
Now in the main thread you can provide that operation to a queue like the following:
MyOperation *op = [[MyOperation alloc] initWithDocument:[self document]];
[[self someQueue] addOperation:op];
P.S. You cannot start an async operation in the main method of a NSOperation. When the main finishes, delegates linked with that operations will not be called. To say the the truth you can but this involves to deal with run loop or concurrent behaviour.
Hope that helps.
Initially I was just going to leave a comment, but I guess I don't have the privileges for it. I just wanted to point out the UIDocument, beyond the change count offers
- (void)autosaveWithCompletionHandler:(void (^)(BOOL success))completionHandler
Which shouldn't have the delay I've experienced with updating the change count as it waits for a "convenient moment".
I have a method in some class which performs some task using a block. When I execute that method using NSInvocationOperation then control never goes to the block. I tried logging inside the block but that is never called actually. But if I simply call that method with instance of that class then everything works as expected.
Don’t blocks run inside NSOperation?
NSInvocationOperation *op = [[NSInvocationOperation alloc] initWithTarget:myClassObj selector:#selector(myClassMethod:) object:obj1];
[[AppDelegate sharedOpQueue] addOperation:op];
[op release];
- (void)myClassMethod:(id)obj
{
AnotherClass *otherClass = [[AnotherClass allco] init]
[otherClass fetchXMLWithCompletionHandler:^(WACloudURLRequest* request, xmlDocPtr doc, NSError* error)
{
if(error){
if([_delegate respondsToSelector:#selector(handleFail:)]){
[_delegate handleFail:error];
}
return;
}
if([_delegate respondsToSelector:#selector(doSomeAction)]){
[_delegate doSomeAction];
}
}];
}
- (void) fetchXMLWithCompletionHandler:(WAFetchXMLHandler)block
{
_xmlBlock = [block copy];
[NSURLConnection connectionWithRequest:request delegate:self];
}
-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
if(_xmlBlock) {
const char *baseURL = NULL;
const char *encoding = NULL;
xmlDocPtr doc = xmlReadMemory([_data bytes], (int)[_data length], baseURL, encoding, (XML_PARSE_NOCDATA | XML_PARSE_NOBLANKS));
NSError* error = [WAXMLHelper checkForError:doc];
if(error){
_xmlBlock(self, nil, error);
} else {
_xmlBlock(self, doc, nil);
}
xmlFreeDoc(doc);
}
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
if(_xmlBlock) {
_xmlBlock(self, nil, error);
}
}
You are performing your NSConnection asynchronously (which you don't need to do in an NSOperation because you should already be on a background thread).
After your call to fetchXMLWithCompletionHandler, your method ends. This signals that the NSOperation is finished and it gets released and it's thread gets either reused for something else or, more likely, released as well. This means that by the time you get your callbacks, your initial object doesn't exist anymore!
There are two solutions :
1) Use NSURLConnection synchronously. This will wait in your myClassMethod until it has got a response.
2) Learn about NSOperations's concurrent mode. I don't know if this will work with NSInvocationOperation though :( And it's fairly complicated compared to option (1).
I would use method (1) - you have already created a background thread to perform your operation in, why bother creating another one to do your connection request?
There are two ways of fixing your problem:
The easy way out
is — as Dean suggests — using +[NSURLConnection sendSynchronousRequest:returningResponse:error:], as you already are on a different thread. This has you covered — I'd say — 80-90% of the time, is really simple to implement and Just Works™.
The other way
is only slightly more complicated and has you covered for all the cases where the first method does not suffice — by visiting the root of your problem:
NSURLConnection works in conjunction with the runloop — and the threads managed by NSOperationQueue don't necessarily use (or even have!) an associated runloop.
While calling +[NSURLConnection connectionWithRequest:delegate:] will implicitly create a runloop, if needed, it does not cause the runloop to actually run!
This is your responsibility, when the NSOperationQueue you use is not the queue associated with the main thread.
To do so, change your implementation of fetchXMLWithCompletionHandler: to look similar to the following:
- (void)fetchXMLWithCompletionHandler:(WAFetchXMLHandler)block
{
self.xmlHandler = block; // Declare a #property for the block with the copy attribute set
self.mutableXMLData = [NSMutableData data]; // again, you should have a property for this...
self.currentConnection = [NSURLConnection connectionWithRequest:request delegate:self]; // having a #property for the connection allows you to cancel it, if needed.
self.connectionShouldBeRunning = YES; // ...and have a BOOL like this one, setting it to NO in connectionDidFinishLoad: and connection:didFailWithError:
NSRunLoop *loop = [NSRunLoop currentRunLoop];
NSDate *neverExpire = [NSDate distantFuture];
BOOL runLoopDidIterateRegularly = YES;
while( self.connectionShouldBeRunning && runLoopDidIterateRegularly ) {
runLoopDidIterateRegularly = [loop runMode:NSDefaultRunLoopMode beforeDate:neverExpire];
}
}
With these small changes, you're good to go. Bonus: this is really flexible and (eventually) reusable throughout all your code — if you move the XML-parsing out of that class and make your handler simply take an NSData, an NSError and (optionally) an NSURLResponse.
Since you probably don't want the clients of your loader to see and possibly mess with the properties I just suggested you should add, you can declare them in a class continuation.
I'm contemplating how to offload the drawing of a very large Core Data tree structure to CATiledLayer. CATiledLayer seems to be awesome because it performs drawing on a background thread and then fades in tiles whenever they're drawn. However, because the information of the drawing comes from a Core Data context that is by design not thread safe, I'm running into race condition issues where the drawing code needs to access the CD context.
Normally, if I need to perform background tasks with Core Data, I create a new context in the background thread and reuse the existing model and persistent store coordinator, to prevent threading issues. But the CATiledLayer does all the threading internally, so I don't know when to create the context, and there needs to be some kind of context sharing, or I can't pass the right entities to the CATiledLayer to begin with.
Is there anyone with a suggestion how I can deal with this scenario?
Cheers,
Eric-Paul.
The easiest solution is to use the dispatch API to lock all of your data access onto a single thread, while still allowing the actual drawing to be multi-threaded.
If your existing managed object context can only be accessed on the main thread, then this is what you do:
- (void)drawInContext:(CGContextRef)context // I'm using a CATiledLayer subclass. You might be using a layer delegate instead
{
// fetch data from main thread
__block NSString *foo;
__block NSString *bar;
dispatch_sync(dispatch_get_main_queue(), ^{
NSManagedObject *record = self.managedObjecToDraw;
foo = record.foo;
bar = record.bar;
});
// do drawing here
}
This is a quick and easy solution, but it will lock your main thread while fetching the data, which is almost certainly going to create "hitches" whenever a new tile is loaded while scrolling around. To solve this, you need to perform all of your data access on a "serial" dispatch queue.
The queue needs to have it's own managed object context, and you need to keep this context in sync with the context on your main thread, which is (presumably) being updated by user actions. The easiest way to do this is to observe a notification that the context has changed, and throw out the one used for drawing.
Define an instance variable for the queue:
#interface MyClass
{
NSManagedObjectContext *layerDataAccessContext;
dispatch_queue_t layerDataAccessQueue;
}
#end
Create it in your init method:
- (id)init
{
layerDataAccessQueue = dispatch_queue_create("layer data access queue", DISPATCH_QUEUE_SERIAL);
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(contextDidChange:) name:NSManagedObjectContextDidSaveNotification object:nil]; // you might want to create your own notification here, which is only sent when data that's actually being drawn has changed
}
- (void)contextDidChange:(NSNotification *)notif
{
dispatch_sync(layerDataAccessQueue, ^{
[layerDataAccessContext release];
layerDataAccessContext = nil;
});
[self.layer setNeedsDisplay];
}
And access the context while drawing:
- (void)drawInContext:(CGContextRef)context
{
// fetch data from main thread
__block NSString *foo;
__block NSString *bar;
dispatch_sync(layerDataAccessQueue, ^{
NSManagedObject record = self.managedObjectToDraw;
foo = record.foo;
bar = record.bar;
});
// do drawing here
}
- (NSManagedObject *)managedObjectToDraw
{
if (!layerDataAccessContext) {
__block NSPersistentStoreCoordinator *coordinator;
dispatch_sync(dispatch_get_main_queue(), ^{
coordinator = [self persistentStoreCoordinator];
});
layerDataAccessContext = [[NSManagedObjectContext alloc] init];
[layerDataAccessContext setPersistentStoreCoordinator:coordinator];
}
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
NSEntityDescription *entity =
[NSEntityDescription entityForName:#"Employee"
inManagedObjectContext:layerDataAccessContext];
[request setEntity:entity];
NSPredicate *predicate =
[NSPredicate predicateWithFormat:#"self == %#", targetObject];
[request setPredicate:predicate];
NSError *error = nil;
NSArray *array = [layerDataAccessContext executeFetchRequest:request error:&error];
NSManagedObject *record;
if (array == nil || array.count == 0) {
// Deal with error.
}
return [array objectAtIndex:0];
}
I've given up trying to share managed object context instances between CATiledLayer draws and now just alloc/init a new context at every call of drawLayer:inContext: The performance hit is not noticable, for the drawing is already asynchronous.
If there's anyone out there with a better solution, please share!