I tweaked Tom's approach slightly, to get the following design methodology: Adding IVars to a Class (and it's subclasses). A future post will explain in detail why I needed these extra parameters, but briefly, I want to extend NSManagedObjectContext, a class I do not own, with methods that short cut the work required to build and execute fetching from Core Data. Adding a little bit of state and/or default values to the context (instead of passing that data into/out of the method calls) simplifies matters. Making IVar access transparent eases the burden on the caller. Since I don't own the class definition, I needed another method to associate data and Tom's post pointed the way.
Example Code
CompactFetch.h - Category declaration
@interface NSManagedObjectContext (CompactFetch)
@property (assign,nonatomic) NSUInteger defaultFetchLimit;
@property (retain,nonatomic,readonly) NSError* lastError;
@end
@property (assign,nonatomic) NSUInteger defaultFetchLimit;
@property (retain,nonatomic,readonly) NSError* lastError;
@end
In our header, we create a category, CompactFetch, for NSManagedContext. Notice the two properties created. These property declarations do not use @synthesize for their implementations. The nice thing about making them properties, is that code can use dot-notation to set or get their values. In addition, the code is KVC compliant. Note, you can use objects here, just be careful with assign & retain.
CompactFetch.m - IVar class declaration
@interface CompactFetchIVars : NSObject
@property (assign,nonatomic) NSUInteger defaultFetchLimit;
@property (retain,nonatomic) NSError* lastError;
+ (CompactFetchIVars*)fetch:(NSManagedObjectContext*)moc;
@end
@property (assign,nonatomic) NSUInteger defaultFetchLimit;
@property (retain,nonatomic) NSError* lastError;
+ (CompactFetchIVars*)fetch:(NSManagedObjectContext*)moc;
@end
Next, we create a class, CompactFetchIVars, which mirrors the @property declarations on the Category. Again, be sure to match up the assign & retain annotations. Although the property names need not be the same, keeping them that way reduces confusion.
Look at the class method, +fetch:, the heart of our design. It takes an NSManagedObjectContext and returns an instance of our class. We use this class method to attach new ivars (new data) onto the Core Data class. By the way, this @interface resides in the .m file, there's no reason to expose the caller to this detail.
Now, for CompactFetchIVars's class definition:
CompactFetch.m - IVar class definition
@implementation CompactFetchIVars
@synthesize defaultFetchLimit, lastError;
+ (CompactFetchIVars*)fetch:(NSManagedObjectContext*)moc
{
static void *compactFetchIVarKey = &compactFetchIVarKey;
CompactFetchIVars *ivars = objc_getAssociatedObject(moc, &compactFetchIVarKey);
if (ivars == nil) {
ivars = [[CompactFetchIVars alloc] init];
objc_setAssociatedObject(moc, &compactFetchIVarKey, ivars, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
[ivars release];
}
return ivars;
}
- (id)init
{
self = [super init];
return self;
}
- (void)dealloc
{
self.lastError = nil;
[super dealloc];
}
@end
@synthesize defaultFetchLimit, lastError;
+ (CompactFetchIVars*)fetch:(NSManagedObjectContext*)moc
{
static void *compactFetchIVarKey = &compactFetchIVarKey;
CompactFetchIVars *ivars = objc_getAssociatedObject(moc, &compactFetchIVarKey);
if (ivars == nil) {
ivars = [[CompactFetchIVars alloc] init];
objc_setAssociatedObject(moc, &compactFetchIVarKey, ivars, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
[ivars release];
}
return ivars;
}
- (id)init
{
self = [super init];
return self;
}
- (void)dealloc
{
self.lastError = nil;
[super dealloc];
}
@end
A typical class definition, we have @synthesize and init/dealloc. Note, -dealloc silently releases lastError through the nil assignment (via the synthesized property).
What's +fetch: up to? objc_getAssociatedObject retrieves an object previously associated with the target class, our NSManagedObjectContext. If this is the first time through, there is no association and nil is returned. In that case, we must create and associate an instance of our class, CompactFetchIVars, with the context. objc_setAssociatedObject performs the association. Note that both the get & set association calls require a key (in this case, compactFetchIVarKey), and despite the documentation, that key need not be a string. The functions use the key's unique location in program memory (assigned by the compiler) to determine uniqueness of the key.
CompactFetch.m - Category definition
@implementation NSManagedObjectContext (CompactFetch)
- (NSUInteger)defaultFetchLimit
{
return [CompactFetchIVars fetch:self].defaultFetchLimit;
}
- (void)setDefaultFetchLimit:(NSUInteger)limit
{
[CompactFetchIVars fetch:self].defaultFetchLimit = limit;
}
- (NSError*)lastError
{
return [CompactFetchIVars fetch:self].lastError;
}
- (void)setLastError:(NSError*)error
{
[CompactFetchIVars fetch:self].lastError = error;
}
//...
- (NSUInteger)defaultFetchLimit
{
return [CompactFetchIVars fetch:self].defaultFetchLimit;
}
- (void)setDefaultFetchLimit:(NSUInteger)limit
{
[CompactFetchIVars fetch:self].defaultFetchLimit = limit;
}
- (NSError*)lastError
{
return [CompactFetchIVars fetch:self].lastError;
}
- (void)setLastError:(NSError*)error
{
[CompactFetchIVars fetch:self].lastError = error;
}
//...
Finally, we implement the get/set methods for the Category we attached to NSManagedObjectContext. In them, we use the +fetch: class method to retrieve (or create, associate and retrieve) the instance of CompactFetchIVars attached to the NSManagedObjectContext. Then, we get or set the parameter, as appropriate. Under the covers, this goes from a method call on the Core Data instance via the Category through to the attached IVar instance. The runtime knows to call -dealloc on both NSManagedObjectContext and CompactFetchIVars, when retain count goes to zero. DO NOT RETAIN a reference from the IVar class to the attached class; any back track should be assign only, or a retain cycle is created and memory leaks.
Thanks a lot for your help on this blog. I look forward to reading more articles from you!
ReplyDeleteAppium Training in Chennai
Mobile Appium Coaching in Chennai
Appium Training in OMR
JMeter Training Course
JMeter Course
core java training in chennai
C C++ Training in Chennai
javascript training in chennai
The development of artificial intelligence (AI) has propelled more programming architects, information scientists, and different experts to investigate the plausibility of a vocation in machine learning. Notwithstanding, a few newcomers will in general spotlight a lot on hypothesis and insufficient on commonsense application. machine learning projects for final year In case you will succeed, you have to begin building machine learning projects in the near future.
DeleteProjects assist you with improving your applied ML skills rapidly while allowing you to investigate an intriguing point. Furthermore, you can include projects into your portfolio, making it simpler to get a vocation, discover cool profession openings, and Final Year Project Centers in Chennai even arrange a more significant compensation.
Data analytics is the study of dissecting crude data so as to make decisions about that data. Data analytics advances and procedures are generally utilized in business ventures to empower associations to settle on progressively Python Training in Chennai educated business choices. In the present worldwide commercial center, it isn't sufficient to assemble data and do the math; you should realize how to apply that data to genuine situations such that will affect conduct. In the program you will initially gain proficiency with the specialized skills, including R and Python dialects most usually utilized in data analytics programming and usage; Python Training in Chennai at that point center around the commonsense application, in view of genuine business issues in a scope of industry segments, for example, wellbeing, promoting and account.
The Nodejs Projects Angular Training covers a wide range of topics including Components, Angular Directives, Angular Services, Pipes, security fundamentals, Routing, and Angular programmability. The new Angular TRaining will lay the foundation you need to specialise in Single Page Application developer. Angular Training
I would definitely thank the admin of this blog for sharing this information with us. Waiting for more updates from this blog admin.
ReplyDeleteccna Training center in Chennai
Best CCNA Training Institute in Chennai
ReactJS course
Ethical Hacking Course in Chennai
Best PHP training in chennai
gst classes in chennai
ccna Course in Chennai
web designing training in chennai
I have been reading for the past two days about your blogs and topics, still on fetching! Wondering about your words on each line was massively effective. Techno-based information has been fetched in each of your topics. Sure it will enhance and fill the queries of the public needs. Feeling so glad about your article. Thanks…!
ReplyDeletemagento training course in chennai
magento training institute in chennai
magento 2 training in chennai
magento development training
magento 2 course
magento developer training
best php training in chennai
ReplyDeletebest php developer institution chennai
best php training with placement in chennai
best php training center in chennai
best php course in chennai
I have been reading for the past two days about your blogs and topics, still on fetching! Wondering about your words on each line was massively effective.
ReplyDeletebest appium training in chennai
appium online training
appium training centres in chennai
appium training institutes in chennai
appium training institutes for appuim in chennai
appium training content
Innovative blog thanks for sharing this information.
ReplyDeleteSelenium Training in chennai | Selenium Training in annanagar | Selenium Training in omr | Selenium Training in porur | Selenium Training in tambaram | Selenium Training in velachery
Thanks for the informative article About Java. This is one of the best resources I have found in quite some time. Nicely written and great info. I really cannot thank you enough for sharing.good luck.
ReplyDeleteAi & Artificial Intelligence Course in Chennai
PHP Training in Chennai
Ethical Hacking Course in Chennai Blue Prism Training in Chennai
UiPath Training in Chennai
its an very nice.
ReplyDeleteRobotic Process Automation (RPA) Training in Chennai | Robotic Process Automation (RPA) Training in anna nagar | Robotic Process Automation (RPA) Training in omr | Robotic Process Automation (RPA) Training in porur | Robotic Process Automation (RPA) Training in tambaram | Robotic Process Automation (RPA) Training in velachery
I like the helpful info you provide in your articles. I’ll bookmark your weblog and check again here regularly. Good luck for the next!
ReplyDeleteangular js training in chennai
angular js online training in chennai
angular js training in bangalore
angular js training in hyderabad
angular js training in coimbatore
angular js training
angular js online training