I have a wrapper class that is built around a network request.
lets call it 'WrapperNetworkRequest'
The reason I wrap it is because I process the JSON response once it comes back.
Then using the 'WrapperNetworkRequestDelegate' protocol, return the response to the delegate.
The problem I am having is that when using the 'WrapperNetworkRequest' class as an autoreleased object, it auto-releases before the network request can complete.
I came up with what I think is a horrible idea ([self retain]+[self release] at the appropriate times)
Any ideas on what is a proper way to handle this/What I am doing wrong?
Thanks
Instead of having your WrapperNetworkRequest retain itself in its NSURLConnectionDelegate implementation methods, have its delegate assume ownership. One possible way to do this:
#interface Foo : NSObject <WrapperNetworkRequestDelegate>
#property (nonatomic, retain) WrapperNetworkRequest * wrappedRequest;
#end
#implementation Foo
#synthesize wrappedRequest;
//....
- (void)bar
{
WrapperNetworkRequest * request = [WrapperNetworkRequest aNewAutoreleasedRequest];
request.delegate = self;
self.wrappedRequest = request; // Foo instance assumes ownership
[request goGetData];
}
//....
#end
Retaining self is a perfectly sane idea when you have to extend the object's lifetime. How exactly you do that is another matter: you can do [self retain] or you can add self to a shared retaining array keeping all objects of the given kind. Think NSOperationQueue. After you add an operation object to a queue, you can safely release the operation because by adding it to the queue you've transferred ownership of the operation to the queue.
Related
Can anyone explain the difference between setting someObject = someOtherObject; and self.someObject = someOtherObject; if someObject is a class property created with #property (nonatomic, retain) SomeType someObject;
To clarify I have something like:
#interface SomeClass : NSObject {
SomeType* someObject;
}
#property (nonatomic, retain) SomeType* someObject;
#end
I have noticed I get EXC_BAD ACCESS sometimes when I use the property without self and it seems quite random. When I use self my program acts as it should be. I don’t get any compiler errors or warnings when I skip self so I guess it is somehow valid syntax?
self.someObject = someOtherObject makes use of the property. Properties generate setters and getters for you. In your case, you gave the retain attribute to the property, which means that an object set via this property will automatically receive a retain message which increases its retain count by 1. Additionally, the old value of the member variable is sent a release message which decreases its retain count.
Obects are being deallocated, when their retain count reaches 0. You get an EXC_BAD_ACCESS ecxeption if you try to access a deallocated object (e.g. if you try to release it too often).
In your case:
SomeOtherObject *soo = [[SomeOtherObject alloc] init]; //retain count: 1
self.someObject = soo; //soo's retain count is now 2
[soo release]; //soo's retain count is 1 again, as self still uses it.
[self doSomethingWithSoo];
However, if you do not use the setter, you must not release soo.
SomeOtherObject *soo = [[SomeOtherObject alloc] init]; //retain count: 1
someObject = soo; //soo's retain count is still 1
[soo release]; //soo's retain count is 0, it will be deallocated
[self doSomethingWithSoo]; //will fail with an EXC_BAD_ACCESS exception, as soo does not exist anymore.
Properties are just a convenient way to access the data. So when you are declaring the property #property (nonatomic, retain) SomeType* someObject; this means that during access there would be synthesized 2 methods:
getter:
-(SomeType*) someObject {
return someObject;
}
setter
-(void) setSomeObject:(SomeType*) obj {
[someObject release];
someObject = [obj retain];
}
So the main difference between properties and ivars is that properties dynamically creating the setter/getter methods (and you can override them). But when you're writing someObject = new_val, you're just copying the reference to the memory location. No additional work is done there except one assembly instruction.
There is one more thing to mention: atomic and nonatomic.
With atomic, the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.
In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than atomic.
Edit: so if you have some variable, that is accessed from different threads or/and some additional work has to be done (e.g. retain, raise some flags ...), then your choice is property. But sometimes you have a variable, that is accessed very often and access via property can lead to a big overhead, because processor has to perform much more operations to synthesize and call method.
It's all about memory management.
Your class property someObject have generated accessors with annotation #property / #synthsize in your .h / .m files.
When you are accessing you property with someObject, you directly access the property. When you are accessing self.someObject you are calling your accessor [self someObject] whitch take care of memory management for you.
So when you need to assign a class property it's cleaner to do self.someObject = someOtherObject; because you use the setter and does not have to take care about releasing and retaining. When your setter is generated with #property (nonatomic, retain) so it will take care about retaining for you.
The difference between the two is:
1) when you do not use "self." you are assigning the result directly to the member variable.
2) when you are using "self." you are calling the setter method on that property. It is the same as [self setMyObject:...];
so in case of self.myobject, it keeps its retain, and in other case, (without self), if you are not using alloc, then it will be treated as autoreleased object.
In most cases you will find you want to use "self.", except during the initialization of the object.
By the way, you can also use self.someObject = [someOtherObject retain] to increase retain counter
I am newbie to iPhone programming. I have the following doubt which is stopping me to go ahead. Please consider the following code:
---------.h------
#interface myClass: UIViewController
{
UIImage *temp;
}
#property (nonatomic, retain) UIImage *temp;
---------.m------
#interface myClass
#synthesize temp;
-(void) dealloc
{
[temp release];
[super dealloc];
}
The above is the only program code. Thats it ... nothing else. Do I need to declare [temp release] in dealloc method even though I am not using the property accessor method in my program at all. What if I don't declare [temp release] in dealloc. Will that create memory leak as I am releasing something which I haven't retained as I am not calling property accessor method.
Also when i print retain count for temp why does it show 0 even though it is getting retained in #property.
Thanks in advance
If no value has ever been assigned to (an instance of) myClass.temp, then there won't be a leak. But you should release it in your dealloc.
#property is only a declaration that instance of myClass will have this property. You need to assign it a value before that value gets retained.
myClass *instance = [[myClass alloc] init];
// instance will now retain the value passed in
// and is therefore responsible for releasing it
instance.temp = [UIImage imageNamed:#"whatever"];
// if instance is not retained anywhere else,
// its dealloc will be called
[instance release];
On a sidenote, you should give your classes names that start with an uppercase
letter, i.e. MyClass. Not required, but makes things clearer.
You can also use self.temp = nil; in your dealloc You're sorta not supposed but it kinda works better and looks cleaner. It's a bit of an iffy subject...
What you are doing is correct. Scroll to the "dealloc" section of this Apple Doc: Declared Properties
Soon, however, these properties will be cleaned up automatically when you synthesize them (in the next Cocoa update) -- that being said, a convention I have personally began to follow so that my code works in the future is setting self.temp = nil; in dealloc instead of sending a release message (read the apple doc i posted, it explains this). The accessor method created at runtime releases the object first, so for me and quite a few other devs, this is a better/safer way of cleaning up declared properties in our dealloc.
Your code is correct.
The general rule is that, for all variables you declare in #interface, you must clean them up in -dealloc. Some variables will need to be released, others just need to be nil'd out, depending on how you've declared the #property.
In your example above, temp may never have been given a value explicitly by you, but the ObjC runtime will have initialized the value of temp to nil when an instance of your class gets allocated.
Sending a -release to a nil object is generally not a problem, so the [temp release] is fine. It's a no-op. When temp has a non-nil value in -dealloc, the [temp release] gets to do its job of freeing up the memory.
If you need temp to have a non-nil value on creation, you'll need to implement the -init method and make sure it gets some value. While your class is legitimate & functional without an -init method, you really should get in the habit including one in every custom class you design.
You'll need the default initializer at a minimum: -init. You may also want to design a more detailed initializer that could be used to give your temp ivar an a value, like -initWithImage:
Here's what you should also be including in your class:
#implementation MyClass
...
- (id) init {
self = [super init];
if (self != nil) {
// The minimal default initializer.
// temp will already have a value of nil, so you don't need necessarily
// need to do anything more, unless temp needs a real value on initialization.
}
return self;
}
- (void) dealloc {
...
}
#end
To implement a more detailed initializer, which would be known as the designated initializer, you would to something like this:
#implementation MyClass
...
- (id) initWithImage:(UIImage *)newImage {
self = [super init];
if (self != nil) {
temp = [newImage retain];
}
return self;
}
// Implement the default initializer using your more detailed initializer.
- (id) init {
// In this default initializer, every new instance comes with a temp image!
return [self initWithImage:[UIImage imageNamed:#"foobar"]];
}
- (void) dealloc {
...
}
#end
Here, the designated initializer -initWithImage: is the authoritative initializer. All other initializers, including -init, get implemented using -initWithImage:.
You get to exercise a lot of discretion over whether to implement any initializers beyond the minimal default initializer. Maybe -init is good enough for your purposes. That's fine. Sometimes more detailed initializers make using the class more convenient. Experience (and the Force) will be your guide.
Note that I didn't use the generated property accessor in either initializer method. If you aren't required by circumstances, you should generally avoid using property accessors in -init methods and -dealloc, primarily because of potential pain-in-the-ass issues with side effects of automatic key-value coding notifications.
The initializer and dealloc methods play a special role in a class. As the class designer, it is your responsibility to set and clean up instance variables in these methods. A good rule of thumb is to leave the use of synthesized property accessors for the callers of your class, and the implementation of other methods in the class.
When doing initialization of an instance, or deallocation, you can and should touch the ivars directly. They're yours. You declared them, so you can handle them directly. When implementing other methods in your class, you generally should use the property accessors.
JeremyP's link to the Cocoa Conceptual documentation on objects is a good one. You should definitely read the sections on Objects, and periodically re-read it as you gain more experience writing custom classes of your own. Eventually, it will all start making sense.
In my code, I use a singleton object as a central point in my app to load and cache images the app frequently needs, so I don't have to do resource-intensive memory allocation each time I load an image.
But there are times during the execution of my app where memory usage gets intense and I would like to release the cached image data. Currently, I'm just releasing the UIImage instances from my singleton when I get a memory warning.
I would prefer, however, to be able to release the entire singleton object. Is that possible? If so, how?
Of course it is. Although it's rather likely that the memory usage of this object is negligible compared to the images.
By the nature of a singleton, you need to have an accessor for it, where you will create it if it does not currently exist:
+ (MySingletonClass*) mySingleton
{
if ( mySingleton == nil )
{
mySingleton = [[MySingletonClass alloc] init];
}
return mySingleton;
}
You just need to add another that you call when you want to destroy it:
+ (void) destroyMySingleton
{
[mySingleton release];
mySingleton = nil;
}
If you keep references to it around elsewhere you'll have trouble; don't do that. If you access from multiple threads you'll need to synchronize. Otherwise, it's pretty straightforward -- the getter will recreate when you next need it.
Here's an example of a singleton accessor for the OpenAL code I'm using.
// Eric Wing. Singleton accessor. This is how you should ALWAYS get
// a reference to the sound controller. Never init your own.
+ (OpenALSoundController*) sharedController
{
static OpenALSoundController* shared_sound_controller;
#synchronized(self)
{
if (nil == shared_sound_controller)
{
shared_sound_controller = [[OpenALSoundController alloc] init];
}
}
return shared_sound_controller;
}
OpenAL takes a while to load up so keeping one instance around is exactly what I need. With more than one thread in play (not my situation currently but I want my code to be ported to situations where this is the case) I put a lock on self. #synchronized(self) does exactly that.
Now I allocated the memory so I'm responsible for releasing it. I could call [shared_sound_controller autorelease] in the +sharedController accessor method but this may release the controller early, particularly when I have more than one thread and I call the accessor for the first time in a thread that's not the main thread.
Any object you create you can just release at any time. (Presuming you create it and set it's properties.)
self.myObject = [[myObjectClass alloc] init];
// do something with the object
[self.myObject release]; // anytime that you are not using the object
self.myObject = nil; // will also work if you've set the #property (retain, nonatomic)
I'm doing the following:
- (void) accelerometer: (UIAccelerometer *)accelerometer didAccelerate: (UIAcceleration *)acceleration {
if (self.lastAcceleration) {
double i = self.lastAcceleration.x;
It works fine until I actually tilt the phone. Then I get EXC_BAD_ACCESS on the last line. lastAcceleration is a property with a retain. When I look at "x" in the debugger, it has a large negative value. Why would that throw a EXC_BAD_ACCESS exception only on tilt?
-- EDIT (Since this answer applies to responses below) --
I added this and now it works:
- (void)dealloc {
[lastAcceleration release];
Why would that matter? Also, should it be
[self.lastAcceleration release];
I wasn't previously releasing lastAcceleration anywhere. Here is the header declaration:
#interface MyViewController : UIViewController <UIAccelerometerDelegate> {
UIAcceleration *lastAcceleration;
}
#property(nonatomic, retain) UIAcceleration *lastAcceleration;
#end
My hunch is that the accelerometer API has nothing to do with the crash, the code you have shown smells like bad memory management, given that you're mixing ivar and property access I suspect you might be doing the same in other parts you're not showing.
Anyway a couple best practice things:
any object you have a pointer for in your class you should have retained, and conversely when you release it you should also zap the pointer so you don't risk accessing it after it has been deallocated (the exception to this rule are some patterns like the delegate object, where retaining the object would cause a retain cycle, but that's a whole other topic)
ivar setters and getters that are automatically generated via the #synthesized directive will retain and release the object for you for code that simply looks like it's assigning a pointer, so they're pretty handy, but property access (self.something = ...) and ivar access (something = ...) are not equivalent so you have to be careful
One easy way to make sure you don't mix the two up is to do something like this:
#interface MyObject : NSObject
{
SomethingObject *_something;
}
#property (nonatomic, retain) SomethingObject *something;
#end
#implementation MyObject
#synthesize something = _something;
#end
What we're doing here is making the ivar and property names slightly different, so that you are more aware of which one you're using, and the compiler will bark if you use don't use the bare something = ... syntax.
Now the #synthesize'd accessors are something like this:
- (void)setSomething:(SomethingObject *)newSomething
{
[newSomething retain];
[_something release];
_something = newSomething;
}
- (SomethingObject *)something
{
return _something;
}
With all that out of the way, [lastAcceleration release] is a bad thing to do because it isn't also setting the lastAcceleration pointer to nil, you are not guaranteed that it won't be deallocated and if you accidentally use it you are likely to crash.
[self.lastAcceleration release]; is incorrect because accessors take care of all the retain/release stuff for you.
The correct thing to do here is self.lastAcceleration = nil; that, if you look at the accessor code, will release and set the pointer to nil.
What is likely happening is that you are releasing lastAcceleration somewhere without also setting it to nil, and the if (self.lastAcceleration) { check is hitting a released object.
Main reason to have retained properties is to avoid explicit retain/release calls and memory management bugs associated with them. But in dealloc method either way is fine, since object will cease to exist soon.
[self.lastAcceleration release]; - not necessary.
[lastAcceleration release]; self.lastAcceleration = nil;
Both are fine if used in dealloc.
Outside of dealloc use only
self.lastAcceleration = nil;
EXC_BAD_ACCESS is raised when you access released memory. My guess would be that you somewhere released self.lastAcceleration but didn't set it to null.
Are you sure it is related to tilting?
I'm sorting out some memory issues with my iPhone app and I've just been thinking about some basics. If I setup an ivar and never end up using it in the lifespan of my object, when I call dealloc on it, will that cause a problem? E.g.
#interface testClass {
id myobject;
}
#property (nonatomic, retain) id myobject;
#end
#implementation testClass
#synthesize myobject;
- (id)init {
...
// Do I have to set myobject to nil here?
// So if myobject isn't used the dealloc call to nil
// will be okay? Or can you release the variable without
// having set every object to nil that you may may not use
...
}
...
// Somewhere in the code, myobject may be set to
// an instance of an object via self.myobject = [AnObject grabAnObject]
// but the object may be left alone
...
- (void)dealloc {
[myobject release];
[super dealloc];
}
#end
Instance variables are initialized to 0 before your initializer runs..
Yes, ivars are always initialized to 0/nil/NULL/NO/etc.
However, if it helps you understand what's going on, go for it. The performance impact is negligible. You don't need to do it, but it won't cause any problems if you do.
I find that it is good practice to always set those ivars to nil in the init method. That way, you are absolutely sure that your call to release in the destructor can not cause problems.
If it turns out that Objective-C does automatically set them to nil, and for some reason you find yourself with a speed bottleneck that can be improved upon by removing those assignments (highly unlikely), then you can worry about removing them. In the meantime, set them all to nil and sleep easier :)
update: BJ Homer and Chuck have pointed out that the ivars will automatically be set to zero, so now it comes down to a decision on style.