ios NSKeyedArchiver releasing = bad access - ios

I have an noob problem and I would like you yo point me in the right direction. Basicly I have a custom class which implements the copying protocol. However when I save the class during execution the custom class i released and I get a bad access. I can see in instruments that the retain count is -2. I save the custom class with the following method:
-(void)storeDataInFile:(NSString*)dataFileName DataArray:(NSArray*)dataToStore
{
//Get the path
NSString *path = [self pathToDocumentsForDataFile:dataFileName];
//Archive the file
[NSKeyedArchiver archiveRootObject:dataToStore toFile:path];
}
Is I use the method sor saving a array with strings it works flawless. What should I look deeper into regarding my custom class?
Regards

I soved this issue, however I only provided the solution in a comment which apprantly has been deleted. So I just wanted to post the answer which indicates it was a noob mistake.
From an eralier test implementation I had the following method in the class
- (id)copyWithZone:(NSZone *)zone
{
return self;
}
//retain is counted up
- (id)retain {
return self;
}
- (unsigned)retainCount {
return UINT_MAX;
}
These methods ruined my retain count :)

Related

Looping through an array of dynamic objects

I'm not really sure exactly how to describe what I want to do - the best I can do is provide some code as an example:
- (void) doStuffInLoopForDataArray:(NSArray *)arr forObjectsOfClass:(NSString *)class
{
for ([class class] *obj in arr)
{
// Do stuff
}
}
So I might call this like
NSArray *arr = [NSArray arrayWithObjects:#"foo",#"bar", nil];
[self doStuffInLoopForDataArray:arr forObjectsOfClass:#"NSString"];
and I would expect the code to be executed as if I had wrote
- (void) doStuffInLoopForDataArrayOfStrings:(NSArray *)arr
{
for (NSString *obj in arr)
{
// Do KVC stuff
}
}
Is there a way to get this kind of behavior?
I don't see much point in passing the class to the method. Run your loop as:
for (id obj in arr) {
and check the methods you want to call exist. Passing the class is only really useful if you want to check that the objects in the array are actually of that class, but you couldn't then do much with that information.
Another approach would be to create a single superclass that all the classes I'd like to use this method for inherit from. I can then loop using that superclass.
So if I want to be able to loop for MyObject1 and MyObject2, I could create a BigSuperClass, where MyObject1 and MyObject2 are both subclasses of BigSuperClass.
- (void) doStuffInLoopForDataArray:(NSArray *)arr
{
for (BigSuperClass *obj in arr)
{
// Do stuff
}
}
This loop should work for arrays of MyObject1 objects, arrays of MyObject2 objects, or arrays of BigSuperClass objects.
The more I've been thinking about this, the more I'm leaning towards this being the best approach. Since I can setup my BigSuperClass with all the #propertys and methods I'd be interested in as part of my // Do Stuff, which means I won't have to check respondsToSelector as with the other answers. This way just doesn't feel quite as fragile.
I came up with an idea while I was typing up this question, figured I might as well finish it. I just need to change how I'm doing my loop, and I don't really need to send in the class.
- (void) doStuffInLoopForDataArray:(NSArray *)arr
{
for (int i=0; i < [arr count]; i++)
{
// Do stuff
}
}
I should note that part of my // Do stuff is checking to make sure if ([[arr objectAtIndex:i] respondsToSelector:...]) before I actually try to do anything with it - and from what I understand that should prevent any nasty crashes.

Is it ok not to invoke [super init] in a custom init method?

I have a MKPolyline subblass which I want to implement NSCoding, i.e.
#interface RSRoutePolyline : MKPolyline <NSCoding>
I asked a question on the best way to encode the c-array and got an excellent answer. However, there is no init method defined on MKPolyline, i.e. there is no other way to give it data other than its class method polylineWithPoints:points.
Is this code where my comment is ok?
- (void)encodeWithCoder:(NSCoder *)aCoder
{
MKMapPoint *points = self.points;
NSUInteger pointCount = self.pointCount;
NSData *pointData = [NSData dataWithBytes:points length:pointCount * sizeof(MKMapPoint)];
[aCoder encodeObject:pointData forKey:#"points"];
[aCoder encodeInteger:pointCount forKey:#"pointCount"];
}
- (id)initWithCoder:(NSCoder *)aDecoder
{
NSData* pointData = [aDecoder decodeObjectForKey:#"points"];
NSUInteger pointCount = [aDecoder decodeIntegerForKey:#"pointCount"];
// Edit here from #ughoavgfhw's comment
MKMapPoint* points = (MKMapPoint*)[pointData bytes];
// Is this line ok?
self = (RSRoutePolyline*)[MKPolyline polylineWithPoints:points count:pointCount];
return self;
}
You should call an init method on any subclass of NSObject. Since MKPolyline is an NSObject, you should init it.
But MKPolyline has no methods and no init. This is Objective C's was of telling you that you can't subclass it.
Instead, as WDUK suggested, define your own class. It keeps track of your list point points, and manages NSCoding to save and restore them as needed.
#interface RSPolyline: NSObject<NSCoding>
- (id) initWithPoints: (NSArray*) points;
- (id) initWithCoder:(NSCoder *)aDecoder;
- (void) encodeWithCoder:(NSCoder *)aCoder;
- (MKPolyline*) polyLine;
#end
Your class can generate a polyline on request, perhaps caching the result if performance is an issue.
As a rule, don't reach for inheritance first. When you want to extend and improve a class, think first of composition.
It's dirty not to call [super init], and it doesn't bode well with my idea of good programming. Without calling super yourself, it isn't a true subclass; just a bastardization of composition that relies on a side effect of calling a convenience constructor. Saying this, I believe your method described will work OK, but it goes against the grain of good Objective-C programming and its conventions.
What I would suggest is to use MKPolyLine as an MKPolyLine instance, and use a category to add the extra bells and whistles you need. As for adding extra instance variables and such, you can use associated objects. An introduction to this concept can be found here, and this SO question addresses the use of them with categories: How do I use objc_setAssociatedObject/objc_getAssociatedObject inside an object?
While it is generally allowed to create and return a different object in an init method, there are three problems with that line (explained below). Instead of this, I would suggest overriding the points and pointCount properties so that you can return values stored in an instance variable, and call the super implementation there if the instance variable is empty. Then, your initializer just sets these instance variables so that they will be used.
- (MKMapPoint *)points {
if(myPointsIvar == NULL) return [super points];
else return myPointsIvar;
}
// similarly for pointCount
The first problem is that you are creating a new object, but not releasing the old one, which means you are leaking it. You should store the result in a different variable, then release self, then return the result (you don't need to store it in self).
Second, polylineWithPoints:count: returns an autoreleased object, but initWithCoder: should return a retained one. Unless there is another retain on it, it could be deallocated while you are still using it.
If these were the only problems, you could solve both like this:
MKPolyline *result = [MKPolyline polylineWithPoints:points count:pointCount];
[self release];
return [result retain];
However, there is a third problem which cannot be solved so easily. polylineWithPoints:count: does not return a RSRoutePolyline object, and the object it returns may not be compatible with your subclass's methods (e.g. it probably won't support NSCoding). There really isn't a way to fix this, so you can't use polylineWithPoints:count:.

General design - Where do I put centrally accessed objects

I have my main app delegate
I have a few UIViewController derived instances driven by a Storyboard
Say I'd like to provide a centralized persistence layer for my application - perhaps Core Data of SQLite. Where would I put those objects? I'm missing some centrally accessible "Application" class you can access from all the UIViewController instances.
Is there a pattern to follow here?
you should check the singleton pattern:
In software engineering, the singleton pattern is a design pattern
that restricts the instantiation of a class to one object. This is
useful when exactly one object is needed to coordinate actions across
the system. The concept is sometimes generalized to systems that
operate more efficiently when only one object exists, or that restrict
the instantiation to a certain number of objects. The term comes from
the mathematical concept of a singleton.
here is a source for a example implementation: What should my Objective-C singleton look like?
and here is the direct link for the modern solution:
https://stackoverflow.com/a/145395/644629
What you're describing is your model layer. There are two main ways to manage the model:
At application startup, create the main model object and hand it to the first view controller.
Make the main model object a Singleton.
The "main model object" in both cases is generally some kind of object manager. It could be a document, or it could be a PersonManager if you have a bunch of Person objects. This object will vend model objects from your persistence store (generally Core Data).
The advantage of a Singleton here is that it's a little easier to implement and you don't have to pass around the manager. The advantage of a non-Singleton is that it's easier to have more than one (for a document-based system), and it's easier to test and reason about non-singletons than singletons. That said, probably 80% of my projects use a singleton model manager.
As a side note, that you appear to already understand: never store the model in the application delegate, and never use the application delegate as a "rendezvous point" to get to the model. That is, never have a sharedModel method on the application delegate. If you find yourself calling [[UIApplication sharedApplication] delegate] anywhere in your code, you're almost always doing something wrong. Hanging data on the application delegate makes code reuse extremely difficult.
Go with a singleton pattern, which has scope of application lifetime.
#interface DataManager ()
#end
#pragma mark -
#implementation DataManager
#pragma mark - Shared Instance
static DataManager* sharedInstance = nil;
#pragma mark - Singleton Methods
- (id)init
{
self = [super init];
if (self) {
// Initialization code here.
}
return self;
}
+ (DataManager*)sharedInstance
{
#synchronized([DataManager class])
{
if (!sharedInstance) {
//[[self alloc] init];
sharedInstance = [[DataManager alloc] init];
}
return sharedInstance;
}
return nil;
}
+ (id)alloc
{
#synchronized([DataManager class])
{
NSAssert(sharedInstance == nil, #"Attempted to allocate a second instance \
of a singleton.");
sharedInstance = [super alloc];
return sharedInstance;
}
return nil;
}
#end
Declare your properties in .h file and synthesize them here in .m file.
To use that property just call:
// set value
[[DataManager sharedInstance] setSharedProperty:#"ABC"]; // If its a string
// get Value
NSLog(#"value : %#", [[DataManager sharedInstance] sharedProperty]);
Hope this is what you required.
Enjoy Coding :)

Is it good approach to use [self release], [self retain]?

I created DownloadAndParseBook class. It will not autorelesed before it gеt any data or network error.
I used [self release], [self retain]. Is it good approach to use [self release], [self retain]? Is DownloadAndParseBook contain any potential bugs?
#implementation GetBooks
-(void) books
{
for(int i =0; i<10; i++)
{
DownloadAndParseBook *downloadAndParseBook =
[[[DownloadAndParseBook alloc] init]autorelease];
[downloadAndParseBook startLoadingBook];
}
}
#end
#implementation DownloadAndParseBook
- (id)initWithAbook:(int)bookID
{
if(self = [super init])
{
[self retain];
}
return self;
}
- (void)startLoadingBook
{
[NSURLConnection connectionWithRequest:request delegate:self];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
[self release];
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
[self saveResultToDatabase];
[self release];
}
#end
Self retaining is very occasionally an appropriate pattern. It's rare, but sometimes in certain kinds of multi-threaded code its important to make sure that you don't vanish in the middle of processing something. That said, this is not one of those times. I'm having trouble imagining a case where your current approach would be helpful. If someone creates your object and then never calls startLoadingBook, then it leaks. If someone calls startLoadingBook, then your object is retained anyway, because NSURLConnection retains its delegate until it finishes.
That said, I believe much of your problem is coming from the fact that your object model is wrong. Neither GetBooks nor DownloadAndParseBook make sense as classes. What you likely mean is BookManager (something to hold all the books) and BookDownloadController (something to manage the downloading of a single book). The BookManager should keep track of all the current BookDownloadControllers (in an NSSet or NSArray ivar). Each BookDownloadController should keep track of its NSURLConnection (in an ivar). You should not just create connections and have them "hang on themselves" (i.e. self-retain). This feels convenient, but it makes the code very hard to deal with later. You have no way to control how many connections you're making. You have no way to cancel connections. It becomes a mess really quickly.
No it is not a best practice.
Retaining / releasing your object should be done by the "owner" of your object.
For your particular example, the owner of your DownloadAndParseBook object is the object that does the alloc/init. That should be the oen retaining/releasing your DownloadAndParseBook instance.
Best practice here would be alloc/init for DownloadAndParseBook, retain done by the owner, all your download/parse logic, then sending a callback to the owner that all the operations are done (through a delegate for example), at which point, the ower sends a release message to your object.
The question would be: Why does an object require to retain itself? You may want to implement your class like a singleton.
Unlike the other responders I would say that your pattern might work. See also Is calling [self release] allowed to control object lifetime?
There are some other issues in your code however:
In -(void) books I guess you want to send the startLoadingBook message to downloadAndParseBook and not to self
If you create a initWithAbook method it will not be called when you init your book with the standard init method. In the current code above [self retain] will be never called
In your code above bookID will not be saved
I would not use "init" pattern here, but everything in a static function thus the caller can not make mistake with the ownership of the class.
Code:
- (id) initWithId:(int)bookId {
self = [super init];
if (self) {
// save bookId here
}
return self;
}
+ (void) startLoadingBookWithID:(int)bookId {
DownloadAndParseBook* book = [[DownloadAndParseBook alloc] initWithId:bookId];
[NSURLConnection connectionWithRequest:request delegate:book];
}
// release self when it finished the operation
// and document well that its behaviour
If you think well, NSURLConnection itself should work exactly the same way: when you don't release an NSURLConnection when it finished its work, it does it itself. However in the connectionWithRequest it also can not autorelease itself since it has to be alive until the request is served. So the only way it can work is the pattern described above
Never use [self release]. The only possible exception would be in an singleton class/object. The methods release and retain should only be sent by the owner of an object. This usually means, whichever object created the object in question, should also be the one to release it.

Does the iOS SDK provide queues and stacks?

I'm writing an iPhone app, and I'm surprised that there seem to be no NSQueue or NSStack classes in Apple's Foundation Framework. I see that it would be quite easy to roll my own, starting with an NSMutableArray, so I'll do that unless I've missed something. Have I missed something?
Here's my Stack class, in case it's useful to those who come after me. As you can see, the pop method involves enough code that you'd want to factor it out.
Stack.h:
#import <Foundation/Foundation.h>
#interface Stack : NSObject {
NSMutableArray *contents;
}
- (void)push:(id)object;
- (id)pop;
#end
Stack.m
#import "Stack.h"
#implementation Stack
// superclass overrides
- (id)init {
if (self = [super init]) {
contents = [[NSMutableArray alloc] init];
}
return self;
}
- (void)dealloc {
[contents release];
[super dealloc];
}
// Stack methods
- (void)push:(id)object {
[contents addObject:object];
}
- (id)pop {
id returnObject = [[contents lastObject] retain];
if (returnObject) {
[contents removeLastObject];
}
return [returnObject autorelease];
}
#end
as far as I know there is no generic class avaialbe. Try using the NSMutableArray, add via addObject and get first/last via objectAtIndex and removeObjectAtIndex.
Another easy way would be to extend NSMutableArray's capabilities by making use of Objective C's categories. You can do that by adding two files to your project:
NSMutableArray+Stack.h
#interface NSMutableArray (StackExtension)
- (void)push:(id)object;
- (id)pop;
#end
NSMutableArray+Stack.m
#import "NSMutableArray+Stack.h"
#implementation NSMutableArray (StackExtension)
- (void)push:(id)object {
[self addObject:object];
}
- (id)pop {
id lastObject = [self lastObject];
[self removeLastObject];
return lastObject;
}
#end
Now you can use a regular NSMutableArray in every other file of your project like a stack and call push or pop on that object. Don't forget to #import NSMutableArray+Stack.h in those files. Here is some sample code how you can use your new NSMutableArray as a stack:
NSMutableArray *myStack = [[NSMutableArray alloc] init]; // stack size = 0
NSString *aString = #"hello world";
[myStack push:myString]; // stack size = 1
NSString *anotherString = #"hello universe";
[myStack push:anotherString]; // stack size = 2
NSString *topMostStackObject;
topMostStackObject = [myStack pop]; // stack size = 1
NSLog("%#",topMostStackObject);
topMostStackObject = [myStack pop]; // stack size = 0
NSLog("%#",topMostStackObject);
The log output will be:
hello universe
hello world
I'm a bit late to this party, but are you aware of CHDataStructures?
http://cocoaheads.byu.edu/code/CHDataStructures
I have put a working iOS Objective C queue object on GitHub. The code was taken from various posts and by no means is owned by me.
https://github.com/esromneb/ios-queue-object/
If you see any problems please fork, and make a pull request!
Yes, an NSMutableArray doubles as a stack or queue. (It would be slightly inefficient as a queue.)
You could also use C++'s stack and queue adapter, but it makes memory management a bit messy if you want to store Objective-C objects with it.
ObjectiveSugar is a very popular CocoaPod that provides, among a bunch of other great stuff, push and pop API calls on NSMutableArray. Sure, it's not in the iOS SDK, but I'm sharing it here because I was looking for the same thing, and this was the solution I went with (and it certainly didn't hurt that we were already using this CocoaPod in our codebase).
No. You missed nothing. That's all. Objective-C is higher level language look like C. Low level control is not required.
Cocoa classes are designed for easier use than efficiency. If you want to deal with performance, you have an option of raw C (or C++) implementation. Otherwise, just use easy way. Of course, early-optimization is evil.
If you want a kind of encapsulation, just make a new class which contains NSMutableArray within it. Hide inner NSMutableArray and just expose what you want. But you'll realize this is unnecessary.

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