I'm in a bit of a pickle. I know that calling [self methodName] from within a block will lead to a retain cycle.
However in this class due to multithreading I cannot allow execution of the method that the block is accessing from anywhere else other than the block, as it would potentially lead to serious problems.
Current code:
if (_getRunning==NO){
__weak SyncArrayMT *_weak_self = self;
_get_t = ^void (void){
_weak_self->_getRunning = YES;
NSArray *objects = [_weak_self get:getQuery
usingClassCtor:ctor
withAuthBlock:authBlock];
if (_weak_self.getBlockCb)
_weak_self.getBlockCb(objects);
_weak_self->_getRunning = NO;
};
}
Does exactly that, it calls [self getmethod]. While its ok for the dispatched block to run this method, I do not want anything outside this class calling this method.
So, would it be ok to override this inherited method as such:
- (NSArray *) get:(NSString *)getQuery usingClassCtor:(initBlock)initCb withAuthBlock:(authenticate)authBlock
{
NSLog(#"Direct call to get is not allowed - use the threaded method");
return nil;
}
And then change the block to this:
_get_t = ^void (void){
_weak_self->_getRunning = YES;
NSArray *objects = [super get:getQuery
usingClassCtor:ctor
withAuthBlock:authBlock];
if (_weak_self.getBlockCb)
_weak_self.getBlockCb(objects);
_weak_self->_getRunning = NO;
};
I have tried it and it works without doing a call to the [self getMethod], but will super be retained, properly released, etc? Yes I am using ARC. Would calling super within a block lead to any problem ? Is there a hack to get a __weak to super instead ?
Alternatively, how can I disallow direct calls to [self getMethod] (which is inherited) and only use it internally ?
I know that Objective-C doesn't exactly implement this, but I know there are tricks, such as declaring and implementing a method in the implementation file only.
EDIT#1:
I have tried with SEL & IMP and function pointers. Problem is that IMP and function pointers require as a parameter an instance, and this renders the hole point mute:
NSString * (*getFuncPtr)(id,SEL,id,id) = (NSString * (*)(id,SEL,id,id))[super methodForSelector:#selector(sendObjectsPassingTest:withAuthBlock:)];
NSString *reply = getFuncPtr(_weak_self,#selector(sendObjectsPassingTest:withAuthBlock:),predicate,authBlock);
This simply calls the inherited method. Trying to use it with super simply gives an error. At this point I will go ahead and simply use super within the block, and try and profile to see if it leads to any retain cycle.
EDIT#2:
Based on newacct's answer, this is what I ended up doing:
typedef NSArray * (* getFuncPtr)(id,SEL,id,id,id);
...
...
__weak SyncArrayMT *_weak_self = self;
_getMethod = (NSArray * (*)(id,SEL,id,id,id))[[[self class] superclass] instanceMethodForSelector:#selector(get:usingClassCtor:withAuthBlock:)];
_get_t = ^void (void){
NSArray *objects = _weak_self->_getMethod(_weak_self,#selector(get:usingClassCtor:withAuthBlock:),getQuery,ctor,authBlock);
}
I am hoping this should avoid any retain cycles, although I haven't actually profiled it yet.
I know that calling [self methodName] from within a block will lead to
a retain cycle.
That is not true in general. The block will retain self, yes. But there will only be a "retain cycle" if self somehow retains the block. In this case, it does.
but will super be retained
Yes, self will be retained (super is a call on self with a different method lookup pathway).
I have tried with SEL & IMP and function pointers. Problem is that IMP
and function pointers require as a parameter an instance, and this
renders the hole point mute:
NSString * (*getFuncPtr)(id,SEL,id,id) = (NSString * (*)(id,SEL,id,id))[super methodForSelector:#selector(sendObjectsPassingTest:withAuthBlock:)];
NSString *reply = getFuncPtr(_weak_self,#selector(sendObjectsPassingTest:withAuthBlock:),predicate,authBlock);
This simply calls the inherited method. Trying to use it with super simply gives an error. At this point I will go ahead and simply use super within the block, and try and profile to see if it leads to any retain cycle.
There are many wrong points here. First, as said above, super is a call on self (there is no such thing as a super object), so it would be sufficient to get the IMP for the method in the superclass, and call it on self.
BUT, [super methodForSelector:... does not get the method in the superclass. It actually gets the method in this class. The super in [super methodForSelector:... affects which methodForSelector: method is called. However, no class ever overrides methodForSelector:, so there is actually no difference between [super methodForSelector:... and [self methodForSelector:.... As said above, super calls the method on self, so it still finds the method based on the class of the current object.
You can get the right IMP by using the class method +instanceMethodForSelector::
NSString *(*getFuncPtr)(id,SEL,id,id) = (NSString * (*)(id,SEL,id,id))[[[self class] superclass] instanceMethodForSelector:#selector(sendObjectsPassingTest:withAuthBlock:)];
However, using the above will not work correctly if the current object is an instance of a subclass, because then [self class] will be the subclass. So to make sure it does what we want, we need to hard-code the name of our current class, or the superclass:
NSString *(*getFuncPtr)(id,SEL,id,id) = (NSString * (*)(id,SEL,id,id))[[SyncArrayMT superclass] instanceMethodForSelector:#selector(sendObjectsPassingTest:withAuthBlock:)];
NSString *reply = getFuncPtr(_weak_self,#selector(sendObjectsPassingTest:withAuthBlock:),predicate,authBlock);
It is also possible to do it using objc_msgSendSuper directly, but that function is not really that easy to use either. So I think you should stick with the IMP approach above.
Related
How can I avoid this warning in xcode. Here is the code snippet:
[player(AVPlayer object) addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(0.1, 100)
queue:nil usingBlock:^(CMTime time) {
current+=1;
if(current==60)
{
min+=(current/60);
current = 0;
}
[timerDisp(UILabel) setText:[NSString stringWithFormat:#"%02d:%02d",min,current]];///warning occurs in this line
}];
The capture of self here is coming in with your implicit property access of self.timerDisp - you can't refer to self or properties on self from within a block that will be strongly retained by self.
You can get around this by creating a weak reference to self before accessing timerDisp inside your block:
__weak typeof(self) weakSelf = self;
[player addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(0.1, 100)
queue:nil
usingBlock:^(CMTime time) {
current+=1;
if(current==60)
{
min+=(current/60);
current = 0;
}
[weakSelf.timerDisp setText:[NSString stringWithFormat:#"%02d:%02d",min,current]];
}];
__weak MyClass *self_ = self; // that's enough
self.loadingDidFinishHandler = ^(NSArray *receivedItems, NSError *error){
if (!error) {
[self_ showAlertWithError:error];
} else {
self_.items = [NSArray arrayWithArray:receivedItems];
[self_.tableView reloadData];
}
};
And one very important thing to remember:
do not use instance variables directly in block, use it as a properties of weak object, sample:
self.loadingDidFinishHandler = ^(NSArray *receivedItems, NSError *error){
if (!error) {
[self_ showAlertWithError:error];
} else {
self_.items = [NSArray arrayWithArray:receivedItems];
[_tableView reloadData]; // BAD! IT ALSO WILL BRING YOU TO RETAIN LOOP
}
};
and don't forget to do:
- (void)dealloc {
self.loadingCompletionHandler = NULL;
}
another issue can appear if you will pass weak copy of not retained by anybody object:
MyViewController *vcToGo = [[MyViewCOntroller alloc] init];
__weak MyViewController *vcToGo_ = vcToGo;
self.loadingCompletion = ^{
[vcToGo_ doSomePrecessing];
};
if vcToGo will be deallocated and then this block fired I believe you will get crash with unrecognized selector to a trash which is contains vcToGo_ variable now. Try to control it.
Better version
__strong typeof(self) strongSelf = weakSelf;
Create a strong reference to that weak version as the first line in your block. If self still exists when the block starts to execute and hasn’t fallen back to nil, this line ensures it persists throughout the block’s execution lifetime.
So the whole thing would be like this:
// Establish the weak self reference
__weak typeof(self) weakSelf = self;
[player addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(0.1, 100)
queue:nil
usingBlock:^(CMTime time) {
// Establish the strong self reference
__strong typeof(self) strongSelf = weakSelf;
if (strongSelf) {
[strongSelf.timerDisp setText:[NSString stringWithFormat:#"%02d:%02d",min,current]];
} else {
// self doesn't exist
}
}];
I have read this article many times. This is an excellent article by Erica Sadun on
How To Avoid Issues When Using Blocks And NSNotificationCenter
Swift update:
For example, in swift a simple method with success block would be:
func doSomeThingWithSuccessBlock(success: () -> ()) {
success()
}
When we call this method and need to use self in the success block. We'll be using the [weak self] and guard let features.
doSomeThingWithSuccessBlock { [weak self] () -> () in
guard let strongSelf = self else { return }
strongSelf.gridCollectionView.reloadData()
}
This so-called strong-weak dance is used by popular open source project Alamofire.
For more info check out swift-style-guide
In another answer, Tim said:
you can't refer to self or properties on self from within a block that will be strongly retained by self.
This isn’t quite true. It’s OK for you to do this so long as you break the cycle at some point. For example, let’s say you have a timer that fires that has a block that retains self and you also keep a strong reference to the timer in self. This is perfectly fine if you always know that you will destroy the timer at some point and break the cycle.
In my case just now, I had this warning for code that did:
[x setY:^{ [x doSomething]; }];
Now I happen to know that clang will only produce this warning if it detects the method starts with “set” (and one other special case that I won’t mention here). For me, I know there is no danger of there being a retain loop, so I changed the method name to “useY:” Of course, that might not be appropriate in all cases and usually you will want to use a weak reference, but I thought it worth noting my solution in case it helps others.
Many times, this is not actually a retain cycle.
If you know that it's not, you need not bring fruitless weakSelves into the world.
Apple even forces these warnings upon us with the API to their UIPageViewController, which includes a set method (which triggers these warnings–as mentioned elsewhere–thinking you are setting a value to an ivar that is a block) and a completion handler block (in which you'll undoubtedly refer to yourself).
Here's some compiler directives to remove the warning from that one line of code:
#pragma GCC diagnostic push
#pragma clang diagnostic ignored "-Warc-retain-cycles"
[self.pageViewController setViewControllers:#[newViewController] direction:navigationDirection animated:YES completion:^(BOOL finished) {
// this warning is caused because "setViewControllers" starts with "set…", it's not a problem
[self doTheThingsIGottaDo:finished touchThePuppetHead:YES];
}];
#pragma GCC diagnostic pop
Adding two cents on improving precision and style. In most cases you will only use one or a couple of members of self in this block, most likely just to update a slider. Casting self is overkill. Instead, it's better to be explicit and cast only the objects that you truly need inside the block. For example, if it's an instance of UISlider*, say, _timeSlider, just do the following before the block declaration:
UISlider* __weak slider = _timeSlider;
Then just use slider inside the block. Technically this is more precise as it narrows down the potential retain cycle to only the object that you need, not all the objects inside self.
Full example:
UISlider* __weak slider = _timeSlider;
[_embeddedPlayer addPeriodicTimeObserverForInterval:CMTimeMake(1, 1)
queue:nil
usingBlock:^(CMTime time){
slider.value = time.value/time.timescale;
}
];
Additionally, most likely the object being cast to a weak pointer is already a weak pointer inside self as well minimizing or eliminating completely the likelihood of a retain cycle. In the example above, _timeSlider is actually a property stored as a weak reference, e.g:
#property (nonatomic, weak) IBOutlet UISlider* timeSlider;
In terms of coding style, as with C and C++, variable declarations are better read from right to left. Declaring SomeType* __weak variable in this order reads more naturally from right to left as: variable is a weak pointer to SomeType.
I ran into this warning recently and wanted to understand it a bit better. After a bit of trial and error, I discovered that it originates from having a method start with either "add" or "save". Objective C treats method names starting with "new", "alloc", etc as returning a retained object but doesn't mention (that I can find) anything about "add" or "save". However, if I use a method name in this way:
[self addItemWithCompletionBlock:^(NSError *error) {
[self done]; }];
I will see the warning at the [self done] line. However, this will not:
[self itemWithCompletionBlock:^(NSError *error) {
[self done]; }];
I will go ahead and use the "__weak __typeof(self) weakSelf = self" way to reference my object but really don't like having to do so since it will confuse a future me and/or other dev. Of course, I could also not use "add" (or "save") but that's worse since it takes away the meaning of the method.
In objective c, suppose I have an object Obj stored in a NSMutableArray, and the array's pointer to it is the only strong pointer to Obj in the entire program. Now suppose I call a method on Obj and I run this method in another thread. In this method, if Obj sets the pointer for itself equal to nil will it essentially delete itself? (Because there will be no more strong pointers left) I suspect the answer is no, but why? If this does work, is it bad coding practice (I assume its not good coding, but is it actually bad?)
It is highly unlikely that an object would be in a position to cause its own release/deallocation if your code is designed properly. So yes, the situation you describe is indicative of bad coding practice, and can in fact cause the program to crash. Here is an example:
#interface Widget : NSObject
#property (retain) NSMutableArray *array;
#end
#implementation Widget
#synthesize array;
- (id)init
{
self = [super init];
if(self) {
array = [[NSMutableArray alloc] init];
[array addObject:self];
}
return self;
}
- (void)dealloc
{
NSLog(#"Deallocating!");
[array release];
[super dealloc];
}
- (void)removeSelf
{
NSLog(#"%d", [array count]);
[array removeObject:self];
NSLog(#"%d", [array count]);
}
#end
and then this code is in another class:
Widget *myWidget = [[Widget alloc] init];
[myWidget release]; // WHOOPS!
[myWidget removeSelf];
The second call to NSLog in removeSelf will cause an EXC_BAD_ACCESS due to the fact that array has been deallocated at that point and can't have methods called on it.
There are at least a couple mistakes here. The one that ultimately causes the crash is the fact that whatever class is creating and using the myWidget object releases it before it is finished using it (to call removeSelf). Without this mistake, the code would run fine. However, MyWidget shouldn't have an instance variable that creates a strong reference to itself in the first place, as this creates a retain cycle. If someone tried to release myWidget without first calling removeSelf, nothing would be deallocated and you'd probably have a memory leak.
If your back-pointer is weak (which it should be since a class should never try to own it's owner, you will end up with a retain-cycle) and you remove the strong pointer from the array the object will be removed from the heap. No strong pointers = removed from memory.
You can always test this.
If you need a class to bring to a situation where its deleted, the best practice is to first retain/autorelease it and then make the situation happen. In this case the class won't be deleted in a middle of its method, but only afterwards.
I think we can say it might be bad coding practice, depending on how you do it. There are ways you could arrange to do it safely, or probably safely.
So let's assume we have a global:
NSMutableArray *GlobalStore;
One approach is to remove yourself as your final action:
- (void) someMethod
{
...
[GlobalStore removeObject:self];
}
As this is the final action there should be no future uses of self and all should be well, probably...
Other options include scheduling the removal with a time delay of 0 - which means it will fire next time around the run loop (only works of course if you have a run loop, which in a thread you may not). This should always be safe.
You can also have an object keep a reference to itself, which produces a cycle and so will keep it alive. When its ready to die it can nil out its own reference, if there are no other references and that is a final action (or a scheduled action by another object) then the object is dead.
I'm from the C++ world so the notion of assigning this makes me shudder:
this = new Object; // Gah!
But in Objective-C there is a similar keyword, self, for which this is perfectly acceptable:
self = [super init]; // wait, what?
A lot of sample Objective-C code uses the above line in init routines. My questions:
1) Why does assignment to self make sense (answers like "because the language allows it" don't count)
2) What happens if I don't assign self in my init routine? Am I putting my instance in some kind of jeopardy?
3) When the following if statement fails, what does it mean and what should I do to recover from it:
- (id) init
{
self = [super init];
if (self)
{
self.my_foo = 42;
}
return self;
}
This is a topic that is frequently challenged by newcomers:
Wil Shipley: self = [stupid init];
Matt Gallagher: What does it mean when you assign [super init] to self?
Apple documentation: Implementing Initializers
Cocoa-Dev: self = [super init] debate
Basically, it stems from the idea that a superclass may have over-ridden the designated initializer to return a different object than the one returned from +alloc. If you didn't assign the return value of super's initializer into self, then you could potentially be dealing with a partially initialized object (because the object that super initialized isn't the same object that you're initializing).
On the whole, it's pretty rare for super to return something different, but it does happen in a couple of cases.
In Objective-C, initializers have the option of returning nil on failure or returning a completely different object than the one the initializer was called on (NSArray always does this, for example). If you don't capture the return value of init, the method might be executing in the context of a deallocated object.
Some people disagree about whether you should do the whole assign-to-self rigamarole if you don't expect to get something else back from the superclass initializer, but it's generally considered to be good defensive coding.
And yes, it looks weird.
It is true that init may return nil, if the initialization fails. But this is not the primary reason why you should assign to self when you implement your own initializers.
It has been mentioned before, but it is needed to stress even harder: the instance returned from an initializer may not be the same instance as the one you sent in, in fact it may not even be of the same class!
Some classes use this as a standard, for example all initializer to NSString and NSArray will always return a new instance of a different class. Initializers to UIColor will frequently return a different instance of a specialized class.
And you yourself can happely implement something like this if you want:
-(id)initWithName:(NSString*)name;
{
if ([name isEqualToString:#"Elvis"]) {
[self release];
self = [[TheKing alloc] init];
} else if (self = [super init]){
self.name = name;
}
return self;
}
This allows you to break out the implementation of some special case into a separate class, without requiring the clients of your API to care or even know about it.
All the other points here are valid, but it's important for you to understand as well that self is an implicit parameter to every Objective-C method (objc_msgSend() passes it) and can be written to, just like any other method parameter. (Writing to explicit parameters is generally frowned upon, unless they are out parameters.)
Typically, this is only done in the -init method, for the reasons others have stated. It only has any effect because self is returned from the method and used in the assignment id obj = [[NSObject alloc] init]; It also affects the implicit resolution of ivars, because, for example, if myVar is an ivar of my class, then accessing it in a method causes it to be implicitly resolved to self->myVar.
I'm still new to Objective C, but this post helped me in understanding this.
To sum it up, most init calls return the same object that self is already initialized to. If there is an error, then init will return nil. Also, some objects such as singletons or unique objects (like NSNumber 0) will return a different object than the one initialized (the singleton or a global 0 object). In these situations you need to have self reference that object. I'm by no means an expert in what is going on behind the scenes here, but it makes sense on the surface, to me.
If [super init] returns nil that means that you have been deallocated and your self parameter is now an invalid pointer. By blindly following the self = [super init] convention you will save you from potentially nasty bugs.
Consider the following non-typical initializer:
- (id)initWithParam:(id)param {
if (!param) {
// Bad param. Abort
self = [super init]; // What if [super init] returns nil?
[self release];
return nil;
}
else
{
// initialize with param.
...
}
}
Now what happens if my superclass decides to abort and return nil? I have been de-allocated and my self parameter is now invalid and [self release] will crash. By re-assigning self, I avoid that crash.
I've read in many places that you should always initialize Objective-C objects like so:
- (id) init {
if (self = [super init]) {
....
}
return self;
}
Because the super's init method may return a separate object from the current self.
Now I'm trying to do something like this, and I'm not sure if I have it right, vis-a-vis how retaining and releasing should work:
- (id) init:(int)idx {
id obj = [Cache findSelf:idx];
if (obj) {
[self release];
self = [obj retain];
} else {
self = [self doLoad];
}
return self;
}
I'm mostly curious if this is the correct way to do the retaining and releasing of self and obj. Is there a better way?
You're correct about the self = [super init] part, since some Cocoa classes actually do return a different object than the one that was allocated. However, this is the exception rather than the rule, and doing so in your own code should be exceedingly rare or not done at all. Although it may be tempting to intercept -init calls, you'd be going against the grain of established convention and what Objective-C programmers expect the code to do.
This type of -init method is generally a bad approach, since -init methods should be as straightforward as possible, and should really be concerned with initializing the object. I'd probably write a convenience method like this:
+ (id) instanceForIndex:(NSUInteger)index {
id obj = [Cache findSelf:index];
if (obj == nil) {
obj = [[self alloc] init];
// Add to cache
}
return [[object retain] autorelease];
}
Then call this method instead of -init. This will make the -init logic much cleaner.
Also, I'm not sure what your Cache class does, but it could be worth rethinking that implementation, and using a hidden static variable to store instances (for example, an NSMutableDictionary, where the key is an NSNumber created from the index). This SO question may be of use.
I agree with Quinn that you should use a convenience class method. Still, I think that your init method is mostly correct, except in your else clause you need to call the parent initializer, i.e. self = [super init].
I don't plan to write applications without IB, I'm just in the process of trying to learn more about programming.
How can I get a single instance of my AppController class at startup? (It's normally loaded from the nib.) And can you clear up the use of +initialize and -init? If I understand, +initialize is called on all classes at startup. How can I use this to create an instance of my AppController with instance variables that make up my interface?
Hope that makes sense, and thanks for any help.
+initalize is sent to a class the first time it or one of its subclasses receives a message for the first time. So, when you do:
instance = [[[YourClass alloc] init] autorelease];
That alloc message triggers initialize.
If you do the same thing with a subclass:
instance = [[[SubclassOfYourClass alloc] init] autorelease];
That alloc message will trigger +[YourClass initialize] the same way the other one did (prior to also triggering +[SubclassOfYourClass initialize]. But only one of these will do it—each class's initialize never gets called more than once. (Unless you call it yourself with [super initialize] or [SomeClass initialize]—so don't do that, because the method won't be expecting it.)
-init, on the other hand, initializes a new instance. In the expression [[YourClass alloc] init], you are personally sending the message directly to the instance. You may also call it indirectly, through another initializer ([[YourClass alloc] initWithSomethingElse:bar]) or a convenience factory ([YourClass instance]).
Unlike initialize, you should always send init (or another initializer, if appropriate) to your superclass. Most init methods look roughly like this:
- (id) init {
if ((self = [super init])) {
framistan = [[Framistan alloc] init];
}
return self;
}
Details differ (this method or the superclass's or both may take arguments, and some people prefer self = [super init] on its own line, and Wil Shipley doesn't assign to self at all), but the basic idea is the same: call [super init[WithSomething:…]], make sure it didn't return nil, set up the instance if it didn't, and return whatever the superclass returned.
This implies that you can return nil from init, and indeed you can. If you do this, you should [self release], so that you don't leak the failed object. (For detecting invalid argument values, an alternative is NSParameterAssert, which throws an exception if the assertion fails. The relative merits of each are beyond the scope of this question.)
How can I use this to create an instance of my AppController with instance variables that make up my interface?
The best way is to do it all in main:
int main(int argc, char **argv) {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
AppController *controller = [[[AppController alloc] init] autorelease];
[[NSApplication sharedApplication] setDelegate:controller]; //Assuming you want it as your app delegate, which is likely
int status = NSApplicationMain(argc, argv);
[pool drain];
return status;
}
You'll do any other set-up in your application delegate methods in AppController.
You already know this, but for anyone else who reads this: Nibs are your friend. Interface Builder is your friend. Don't fight the framework—work with it, and build your interface graphically, and your application will be better for it.
Another solution to the problem of launching an app without a nib.
Instead of allocing your own controller, just use the extra parameters in the NSApplicationMain() method:
int retVal = NSApplicationMain(argc, argv, #"UIApplication", #"MyAppDelegate");
This takes care of all the proper linking one would need.
Then, the only other thing you'd need to remember is to make your own window and set it to visible.
A set of NIBs seem to be an unsatisfactory answer, even when represented in XML (as a XIB), because there's no easy way to compare or merge them with any standard subversion or SCM-style tool. The encoded information is fragile and not intended to be edited by mere humans. How would changes be represented by a GUI? Would I step through each attribute of each control and visually check them?
If the app's behavior is written in code, however, there is a chance that I can figure out what's going in, even if I have to keep lots of details close at hand at the same time.
A proposed solution: use a top-level NIB that the main architect coded up, but then code the rest of the app explicitly.
Anybody got a better idea?