I'm needing to quickly and simply parse an XML file in my app. The best solution for me seems to be something along the lines of an XML-to-NSDictionary converter which I can then dig through easier.
I chose to use this small class: http://troybrant.net/blog/2010/09/simple-xml-to-nsdictionary-converter/
However I'm confused about the returned NSDictionary* and its state in memory. The class method dictionaryForXMLData allocs and inits the class. objectWithData then goes off and does the heavy lifting returning the NSDictionary object which gets returned to the caller.
The confusing part for me is [reader release] before the return (I understand this must happen because of the way reader was created). My understanding of NSDictionaryis that all the objects will also be sent the release message which means the object that we're returning shouldn't have a retain count. Note that the object is created with a convenience method and only gets a retain count when it is added to the stack.
Should the return statement not be return [rootDictionary autorelease]. The problem I see with that is that there is no telling when rootDictionary will be released and depending on the size of the XML file it could sit there clogging memory? Or do I have it all wrong?
What they are doing is not correct. The object returned by objectWithData: is possibly simply a reference to something retained inside reader. And by releasing reader, you are potentially deallocating the rootDictionary you got earlier. Depending on the inner workings of the XMLReader class, it is possible that the thing they return from objectWithData: is retained and autoreleased; but that is an implementation detail, and you cannot depend on it (plus Cocoa memory management is local, so you should never care about what other functions do).
The correct thing to do would be to retain the rootDictionary you get, and then autorelease it (to balance the retain since you have to return it without ownership).
+ (NSDictionary *)dictionaryForXMLData:(NSData *)data error:(NSError **)error
{
XMLReader *reader = [[XMLReader alloc] initWithError:error];
NSDictionary *rootDictionary = [[[reader objectWithData:data] retain] autorelease];
[reader release];
return rootDictionary;
}
Related
I understand blocks are objective c objects and can be put in NSDictionary directly without Block_copy when using ARC. But I got EXC_BAD_ACCESS error with this code:
- (void)viewDidLoad
{
[super viewDidLoad];
[self method1:^(BOOL result){
NSLog(#"method1WithBlock finished %d", result);
}];
}
- (void) method1:(void (^)(BOOL))finish{
NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:^(NSData *rcb){
finish(YES);
}, #"success",
^(NSError *error){
finish(NO);
}, #"failure", nil];
[self method2:dict];
}
- (void) method2:(NSDictionary *)dict{
void (^success)(NSData *rcb) = [dict objectForKey:#"success"];
success(nil);
}
If I change method1: to this, no error raised.
- (void) method1:(void (^)(BOOL))finish{
void (^success)(NSData *) = ^(NSData *rcb){
finish(YES);
};
void (^failure)(NSError *error) = ^(NSError *error){
finish(NO);
};
NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:success, #"success",
failure, #"failure", nil];
[self method2:dict];
}
Can anybody explain why I have to use automatic variables to store the blocks before putting them to dictionary ?
I am using iOS SDK 6.1.
According to the "Transitioning to ARC Release Notes", you have to copy a block stored
in a dictionary (emphasis mine):
Blocks “just work” when you pass blocks up the stack in ARC mode, such
as in a return. You don’t have to call Block Copy any more.
You still need to use [^{} copy] when passing “down” the stack into
arrayWithObjects: and other methods that do a retain.
The second method works "by chance" because success and failure are a
__NSGlobalBlock__ and not a "stack based block" that needs to be copied to the heap.
I understand blocks are objective c objects and can be put in NSDictionary directly without Block_copy when using ARC.
No, they're not common objects. When you create a block it is on the stack, and it doesn't matter of what is it's retain count, when you exit form the function it will be popped from the stack. Copy it to make it stay alive.
You must copy blocks before passing them to a method when 1) the block will be stored for longer than the duration of the call, and 2) the parameter you are passing it to is a normal object pointer type (i.e. id or NSObject *) instead of a block type.
This is the case for your call. dictionaryWithObjectsAndKeys: stores the argument in the resulting dictionary, and it simply expects normal object pointer arguments (of type id), and does not know whether you are passing blocks or not.
The reason for the second condition I said is because if the method parameter already takes a block type (e.g. for any completion handler parameters), then that method is already aware of the special memory management requirements of blocks, and therefore will take responsibility for copying the block if it needs to store it. In that case the caller doesn't need to worry about it. However, in your case, you are passing it to a general method that doesn't know it's getting a block, and thus doesn't know to copy it. So the caller must do it.
Can anybody explain why I have to use automatic variables to store the
blocks before putting them to dictionary ?
About this, your second example happens to work because recent versions of the ARC compiler is super conservative about blocks and inserts copies whenever you assign it to a block type variable. However, this is not guaranteed by the ARC specification, and is not guaranteed to work in the future, or in another compiler. You should not rely on this behavior.
I have a class named "ServerDataLayer" that holds a NSURLConnection, and a NSMutableData that its writing the received HTTP data in to. When the connection finishes, it simply fires a delegate that my caller passed itself as a reference, the method looks like this:
-(void) serverDataLayerResponse:(id)entity
{
if ([entity isMemberOfClass:[LoginResponse class]])
{
LoginResponse *response = (LoginResponse*)entity;
NSLog(#"Error Code: %d", response.errorCode);
NSLog(#"Error Message: %#", response.errorMessage);
NSLog(#"Registered: %c", response.registered);
NSLog(#"AuthToken: %#", response.authToken);
[AppData shared].authToken = response.authToken;
ServerDataLayer *request = [[[ServerDataLayer alloc] initWithServer:_serverUrl delegate:self] autorelease];
[request getPlayerDetails];
//[_server getPlayerDetails];
}
}
Here's my problem...the internal _receivedData and _connection variables are currently in use whilst this delegate method is in progress. I wanted to use my same ServerDataLayer instance to fire another request off "[_server getPlayerDetails]", but the _connection and _receivedData variables internally were getting overwritten and I was getting in to a mess about when to retain/release at the right time.
So my work around was just to instantiate the ServerDataLayer each time I wanted to talk to the server. Now...in the example above, I'm instantiating the request with an 'alloc', and setting an 'autorelease' as I lose scope of this 2nd request. Will this 2nd request stay in memory whilst it's NSURLConnection is busy internally performing the request?
I'm getting a bit lost at this point on how to manage the object references for this kind of process. Any help would be appreciated.
An NSURLConnection, if used via the delegate methods will attach itself as an input to a run loop. However it won't retain its delegate. So your ServerDataLayer would be deallocated (and hopefully remember to cancel the connection). You could use object associations to give your object the same lifecycle as the URL connection, if you were suitably careful about the potential retain loop.
If you use sendAsynchronousRequest:... then you'll probably be fine anyway; assuming you reference self or any instance variable in the completion block then you'll be retained by the block and live for at least as long as it does.
There's really no need to confuse yourself over retain/release any more. Even if ARC isn't an option, you can just declare the relevant instance variables as retain properties within a class extension and use self.property notation to set new values. Retains and releases will be handled for you.
The only caveat is that you should never use dot notation in either your init or dealloc as a special case of the rule that it isn't safe to call methods on a class that's only half instantiated or is half destroyed.
Just use multiple ServerDataLayer instances.
I bet that I could find the answer of this question from reading similar threads here or by googling, but I would like "hear" it first hand so to speak because it's an anomaly in my understanding.
So here's the thing, I have some code which a previous employee wrote and I see a lot of a certain type of construct which looks rather odd to me and I just want to clarify what is "right and wrong".
For example
- (void) setWwanServiceId: (NSString *) newValue {
[wwanServiceId autorelease];
wwanServiceId = [newValue copy];
}
Here wwanServiceIdis a NSString member of the class, and to me this seem like a strange way to do it. As far as I understand it would begin by putting an autorelease on the object, basically saying: "whenever this object seem to not be used, release it for me I don't really care" then the copy will up the retain count +1 on.... wwanServiceId? or newValue? I guess the first.
Then to make me all more confused let's just quickly run through the life-cycle of the wwanServiceId-string..
basically the value will be set if we receive a notification which then from the notification-handler method will call the above -setWwanServiceId: method. other than that it will only ever be accessed for reading, so we are safe to say that it will at any given point:
put autorelease on object
retain a new string copy
Then there is one more quirk to this and this is where I am getting rather suspicious, namely in the -dealloc method which looks like this:
- (void) dealloc {
[[self wwanServiceId] release];
[super dealloc];
}
So what happens there? it will afaik release the wwanServiceId and as I said the only time it's memory management is being touched (if I haven't missed anything but I feel pretty sure) is to put an autorelease and retain it.
So to summarize:
Is the idea here that he thought that since he always retain a new copy after putting autorelease he need to release it in the end.. or well it's the only thing I can think about. Or just felt it would be safe to do an extra release in the end just in case..?
Because as far as I understand it, if this setter is called one time it will put an autorelease (-1 in future), do a retain (+1) and when destructor is called it will do the "final release" (-1).
Any ideas or suggestions helping me understand (if in fact I am wrong and the memory handling is correct as is) would be appreciated.
You wrote:
Here wwanServiceIdis a NSString member of the class, and to me this seem like a strange way to do it. As far as I understand it would begin by putting an autorelease on the object, basically saying: "whenever this object seem to not be used, release it for me I don't really care" then the copy will up the retain count +1 on.... wwanServiceId? or newValue? I guess the first.
This seems to point to the source of your confusion. wwanServiceId is a variable which can contain a reference to an object of type NSString. Variables do not have reference counts only objects do.
The previous employee wrote:
- (void) setWwanServiceId: (NSString *) newValue {
[wwanServiceId autorelease];
wwanServiceId = [newValue copy];
}
The expression [wwanServiceId autorelease] means: read the reference stored in wwanServiceId and autorelease the object that reference refers to - let's call that object A. Important: This does not delete object A; it will be released as some later stage and if at that time there are no remaining references object A will be deleted.
The expression [newValue copy] means: read the reference stored in newValue, use that to locate the object (call it object B), make a copy of that object to produce a new object (call it object C), and return a reference to the new object. This new object is owned by the caller of copy, so there is no need to retain it.
Finally the assignment stores the reference to object C into wwanServiceId.
So there is a maximum of three distinct objects involved:
A: the original object referenced by wwanServiceId, this is autoreleased to remove the ownership of wwanServiceId.
B: the object referenced by newValue, this is left untouched
C: a newly created copy of B owned through wwanServiceId
Why "autorelease" in the code and "maximum of three distinct" above?
The method could be called with newValue referencing object A, e.g. as in:
[self setWwanServiceId:[self wwanServiceId]]
If this was to occur and (a) release was used instead of autorelease and (b) there was no other reference to object A then the release would delete object A and then when [newValue copy] was evaluated newValue would be referencing a deleted object... The use of autorelease in this case delays the deletion until after the copy.
So what the previous employee wrote is not in any way "wrong", but as some of the other answer suggest it may be an unusual style. Another way you see this written is:
- (void) setWwanServiceId: (NSString *) newValue
{
NSString *oldValue = wwanServiceId;
wwanServiceId = [newValue copy];
[oldValue release];
}
which also ensures any deletion occurs after the copy.
HTH.
To be short and helpful:
This is wrong:
- (void) setWwanServiceId: (NSString *) newValue {
[wwanServiceId autorelease];
wwanServiceId = [newValue copy];
}
This is right:
- (void) setWwanServiceId: (NSString *) newValue {
if (newValue != wwanServiceId) {
[wwanServiceId release];
wwanServiceId = [newValue copy];
}
}
To explain in short:
[wwanServiceId autorelease]; is an unnecessary sent message, because autoreleasing an object will reduce the retain count at some unknown point in the future. And in the next line you are wwanServiceId = [newValue copy]; instantly setting the instance variable. So, in your memory you now have a to be autoreleased object and a new object. One of them is too much The new object is, where the pointer of your IVar is pointing to. The old one is swimming in your Memory Pool with probably no reference to it :-)
Autorelease as few as possible or use ARC.
Oh: And in the dealloc method, please do not send the message like this:
[[self wwanServiceId] release];
Better like this:
[wwanServiceId release];
as Apple recommends to work directly with instance methods in init and dealloc methods, instead of using getters and setters there.
Debug it and have a look.
[wwanServiceId autorelease];
wwanServiceId has an address. This statement does not change it. It does decrement the retain count of this object though.
Thats it.
wwanServiceId = [newValue copy];
This statement creates a new object. The new object is a copy of newValue. Compare the addresses of the objects and you will see, that the address inn wwanServiceId will vary from the address of newValue and it will vary from the address that wwanServiceId did have just before the statement was executed.
The retain, that is implicit in copy will affect wwanServiceId, but it affects the new object, that was just creaed with copy. It does not affect the wwanServiceId object which was autoreleased during the statement before.
At some point after the execution of setWwanServiceId had finished, the old and autoreleased object will dissappear. (Assuming that the retain count is 0 now. If it is >0 because it is still retained for other reasons or just for error, then it will remain and potentially cause a leak.)
Once you understood that you will not question any more what is happening in the dealloc method. wwanServiceId is released. Meaning its retain count is reduced by 1. In the event that it is 0 then it will be deallocated automatically.
You could even autorelease it there too. The diffrerence to autorelease is basically that an autoreleased object is still around and available while the current method is being executed. Its release comes into effect at some later point in time.
But there is no reason for autoreleasing the object in dealloc.
In the example given there is not even a good reason to autorelease the object in the setter setWwanServiceId. You may well release the object directly in both methods.
Assuming wwanServiceId is the private ivar mapped by the getter wwanServiceId and setter setWwanServiceId, I think it is not correct to autorelease the ivar in the setter.
I would have coded the following:
- (void) setWwanServiceId: (NSString *) newValue {
if (newValue != wwanServiceId) {
[wwanServiceId release];
wwanServiceId = [newValue copy];
}
}
What I mean is: it is not necessary to give ownership of your var to the autorelease pool (which it may be drained at the end of the application). Simply release the ivar. If someone is using it, no problem, it will have a strong reference to it. Otherwise it will be deallocated.
I ran into this problem while trying to fix a memory leak with the facebook-ios-sdk. How do i handle this situation when passing objects from no arc compiled classe to arc enabled classe?
This is the code inside the non arc compiled Facebook library: (i removed the unnecessary stuff which is not related to the problem) as you can see, result object is not autoreleased or released.
- (void)handleResponseData:(NSData *)data {
NSError* error = nil;
id result = [self parseJsonResponse:data error:&error];
self.error = error;
// Call the defined delegate wich is my AppDelegate didLoad method wich is arc enabled
[_delegate request:self didLoad:result];
}
- (id)parseJsonResponse:(NSData *)data error:(NSError **)error {
SBJSON *jsonParser = [[SBJSON alloc] init];
//gets the object wich leaks or gets overreleased
id result = [jsonParser objectWithString:responseString];
[jsonParser release];
return result;
}
Now if i try to add autorelease to the result object, i am facing a NSZombie when my arc code in my AppDelegate try's to release the object. However if i leave the code like this i'm facing memory leaks whit the result object which gets not released.
am i missing something basic? i can't get my head around this?
Thanx for any advice! Chris
The result returned from -parseJsonResponse:... is autoreleased already (see note at bottom).
Since the name of the -parseJson method doesn't begin with new, alloc, copy or mutableCopy, the compiler assumes that it returns an object with a +0 reference count, meaning it needs to be retained by the calling code if it is to be kept around, and doesn't need to be released if it's not being kept around. That's a long winded way of saying that it should neither leak nor cause a crash in your ARC code as written in your question.
Passing objects between ARC code and manual reference counting code doesn't require any special handling. You just need to make sure that methods' names match their memory management semantics in the non-ARC code. It certainly seems like you've done that in this case, although as you say, you didn't post your complete code.
Note: Presumably, objectWithString: returns an autoreleased object. If it doesn't it, it should (because it doesn't start with alloc, new, copy, mutableCopy).
I'm trying to retain a reference to a Block that's been passed in to my class by a method, to call at a later time. I'm having trouble, however, maintaining a reference to it.
The obvious way, I thought, was to add it to an ivar collection, all of which are supposed to maintain strong references to their contents. But when I try to pull it back out, it's nil.
The code is pretty simple:
typedef void (^DataControllerCallback)(id rslt);
#interface DataController : NSObject {
NSMutableArray* queue;
}
- (void) addBlock:(DataControllerCallback)callback;
- (void) functionToBeCalledLater;
#end
#implementation DataController
- (id) init {
self = [super init];
if (self != nil) {
queue = [NSMutableArray new];
}
return self;
}
- (void) addBlock:(DataControllerCallback)callback {
NSDictionary* toAdd = [NSDictionary dictionaryWithObjectsAndKeys:
[callback copy], #"callback",
#"some other data", #"data", nil];
[queue addObject:toAdd];
}
- (void) functionToBeCalledLater {
NSDictionary* dict = [queue lastObject];
NSLog(#"%#", [dict objectForKey:#"data"]; //works
DataControllerCallback callback = [dict objectForKey:#"callback"]; //this is nil
callback(#"an arguemnt"); //EXC_BAD_ACCESS
}
What's happening?
Update: I've tried it with [callback copy] and just callback inserting into the dictionary, neither works.
Update 2: If I just stick my block into an NSMutableSet, as long as I call copy, I'm fine. It works great. But if it's in an NSDictionary, it doesn't.
I've actually tested it by putting a breakpoint right after the NSDict is created and the callback never gets inserted. The description reads clearly "1 key-value pair", not two.
I'm currently getting around this with a specialised class that just acts as a container. The callback property is declared as strong; I don't even need to use copy.
The question still stands, though: why is this happening? Why won't an NSDictionary store a Block? Does it have something to do with the fact that I'm targeting iOS 4.3 and thus ARC must be built in as a static library?
Update 3: Ladies and gentleman: I am an idiot.
The code I presented here was obviously a simplified version of the actual code; most particularly, it was leaving some key/value pairs out of the dictionary.
If you're storing a value in an NSDictionary using [NSDictionary dictionaryWithObjectsAndKeys:], you had better be damn sure one of those values isn't nil.
One of them was.
ICYMI, it was causing an early termination of the argument list. I had a userInfo-type argument being passed into one of the "add to queue" methods, and you could, of course, pass in "nil". Then when I constructed the dictionary, chucking in that argument caused the constructor to think I had terminated the argument list. #"callback" was the last value in the dictionary constructor and it was never being stored.
Contrary to popular mis-conception, ARC does not automatically de-stackify Blocks passed as arguments to methods. It only de-stackify's automatically when a block is returned from a method/function.
I.e. this....
[dict setObject: ^{;} forKey: #"boom"];
... will crash if dict survives beyond the scope and you attempt to use the block (actually, it won't in this case because that is a static block, but that is a compiler detail that you can't rely on).
This is documented here:
How do blocks work in ARC?
Blocks “just work” when you pass blocks up the stack in ARC mode, such
as in a return. You don’t have to call Block Copy any more. You
still need to use [^{} copy] when passing “down” the stack into
arrayWithObjects: and other methods that do a retain.
The return value behavior could be automated because it is always correct to return a heap based block (and always an error to return a stack based block). In the case of a block-as-an-argument, it is impossible to automate the behavior in a way that would be both very efficient and always correct.
The analyzer likely should have warned about this use. If it didn't, file a bug.
(I derped a stack when I meant a heap. Sorry about that.)
The compiler doesn't automate blocks-as-parameters for a few reasons:
unnecessarily copying a block to the heap can be a significant performance penalty
multiple-copies of a block can multiply that performance penalty significantly.
I.e.:
doSomethingSynchronous(aBlock);
doSomethingSynchronous(aBlock);
doSomethingSynchronous(aBlock);
doSomethingSynchronous(aBlock);
If that were to imply four Block_copy() operations and aBlock contained a significant quantity of captured state, that'd be a huge potential hit.
• There are only so many hours in the day and automating the handling of parameters is rife with non-obvious edge cases. If this were handled automatically in the future, it could be done without breaking existing code and, thus, maybe it will be done in the future.
I.e. the compiler could generate:
aBlock = [aBlock copy];
doSomethingSynchronous(aBlock);
doSomethingSynchronous(aBlock);
doSomethingSynchronous(aBlock);
doSomethingSynchronous(aBlock);
[aBlock release];
Not only would this fix the problem of a block-as-param, but it would also only produce one copy of the block across all potential uses.
The question still stands, though: why is this happening? Why won't an
NSDictionary store a Block? Does it have something to do with the fact
that I'm targeting iOS 4.3 and thus ARC must be built in as a static
library?
Something bizarre is going on, then. Coincidentally, I've been using blocks-as-values in an ARC based application in the last week and it is working fine.
Do you have a minimal example handy?